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R E P O R T 166-1

Enhanced Residential Heat Pumps

Wisconsin Case Study Results

September 1997

Prepared for

595 Science Drive Madison, WI 53711-1076

Phone: 608.238.4601 Fax: 608.238.8733

Email: [email protected] www.ecw.org

Copyright © 1997 Energy Center of Wisconsin All rights reserved

This report was prepared as an account of work sponsored by the Energy Center of Wisconsin (ECW). Neither ECW, participants in ECW, the organization(s) listed below, nor any person on behalf of any of the organizations mentioned above:

(a) makes any warranty, expressed or implied, with respect to the use of any information, apparatus, method, or process disclosed in this report or that such use may not infringe privately owned rights; or

(b) assumes any liability with respect to the use of, or damages resulting from the use of, any information, apparatus, method, or process disclosed in this report.

Project Manager

Craig Schepp Energy Center of Wisconsin

Research & Report Preparation

Steven W. Carlson, P.E. CDH Energy Corporation P.O. Box 641 Cazenovia, NY 13035 (315) 655-1063

Contents Abstract...............................................................................................................................i

Report Summary ..............................................................................................................iii

Introduction

Overview........................................................................................................................1

Background....................................................................................................................1

Project Objectives .........................................................................................................2

Market Background......................................................................................................3

Method

Demonstration Site Selection......................................................................................5

Installation Notes ..................................................................................................11

Approach .....................................................................................................................18

Monitored Data Points-Geothermal Heat Pumps .............................................18

Monitored Data Points-Gas Engine Driven Heat Pump ..................................21

Data Collection ......................................................................................................24

Results

Performance .................................................................................................................25

Efficiency................................................................................................................25

Energy use..............................................................................................................28

Loop Performance .................................................................................................32

Gas Engine Heat Pump .........................................................................................36

Comfort .........................................................................................................................37

Heating....................................................................................................................37

Cooling....................................................................................................................38

Economics ....................................................................................................................42

Operating Costs.....................................................................................................42

Initial Cost..............................................................................................................44

Utility perspective.................................................................................................47

Societal perspective..............................................................................................48

Discussion .......................................................................................................................49

References........................................................................................................................51

Appendix: Enhanced Heat Pumps Data Summary ............................ follows page 52

Andrea

Appendix: Enhanced Heat Pumps Data Summary (download 166-2.pdf)

http://www.ecw.org/prod/166-2.pdf

Tables and Figures Table 1 Summary of existing Wisconsin housing stock heating fuel...............3

Table 2 New single-unit housing building permits ..............................................4

Table 3 Manufacturer activity in Wisconsin ........................................................5

Table 4 National and Wisconsin market equipment summary ...........................6

Table 5 Systems configuration summary...............................................................7

Table 6 Ground heat exchanger options summary ...............................................8

Table 7 System design characteristics .................................................................10

Table 8 Continuously monitored data points .....................................................21

Table 9 Continuously monitored data points .....................................................23

Table 10 Unit operating mode identification—gas engine heat pump ..............23

Table 11 General data acquisition specifications .................................................24

Table 12 Heating operation energy flows..............................................................26

Table 13 Cooling operation energy flows..............................................................28

Table 14 Demonstration site energy prices ...........................................................42

Table 15 Operating cost comparisons at demonstration sites ...........................44

Table 16 Simple system payback summary ...........................................................46

Figure 1 Demonstration site location map ..............................................................9 Figure 2 Site One drilling rig—Oxford ...................................................................11 Figure 3 Site Four loop layout in trench—Eau Claire..........................................12 Figure 4 Site Five loop layout in trench—Merrill ................................................13 Figure 5 Site Six loop layout in trench—Holcombe.............................................14 Figure 6 Hydronic hybrid system schematic ........................................................15 Figure 7 Site Nine horizontal drilling rig ................................................................16 Figure 8 Site Three gas engine heat pump ............................................................17 Figure 9 Typical system schematic with data points ..........................................20 Figure 10 System schematic with data points ........................................................22 Figure 11 Heat pump heating efficiency variation with outdoor

temperature.................................................................................................25 Figure 12 Seasonal system energy flows ................................................................27 Figure 13 1995/96 heating season electricity use...................................................29 Figure 14 Cooling season electricity use................................................................30 Figure 15 Desuperheater impact on water heating energy use............................31 Figure 16 Loop fluid temperature distribution for the 1995/96 heating

season .........................................................................................................33 Figure 17 Seasonal ground heat exchanger performance trend...........................35 Figure 18 Gas engine heat pump supply air temperature......................................37 Figure 19 GHP supply air temp erature relationship to outdoor temperature .....38 Figure 20 Comparison of indoor and outdoor humidity levels ............................40 Figure 21 Space humidity and temperature levels compared to ASHRAE

comfort region............................................................................................41 Figure 22 Operating cost comparisons....................................................................43 Figure 23 Operating cost comparisons at demonstration sites ...........................44 Figure 24 Rate of return sensitivity to investment time frame and energy

price.............................................................................................................47 Figure 25 Source energy comparison example between fossil fuels and

GHPs............................................................................................................48

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Abstract This study documents the on-site performance of residential heat pumps in Wisconsin. Seven geothermal heat pumps and one gas engine heat pump were monitored during heating and cooling seasons. The installations employed a variety of modern, cost-saving heat exchanger designs including vertical bore, two-level horizontal, single-level horizontal, flat slinky, and hybrid slinky loop configurations. All of the heat pumps met comfort requirements and demonstrated reliable operation. The economics and performance of the geothermal heat pumps suggest that the technology is economically viable and technically feasible in many residential settings in Wisconsin.

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Report Summary Enhanced heat pump technologies offer improved efficiency and lower operating costs than conventional fossil fuel or electric heating and cooling equipment. The relative inexperience with the equipment in the Wisconsin market coupled with the higher initial cost of the systems are barriers to market acceptance. This study demonstrates the performance of seven geothermal heat pump systems at seven residences and one gas engine heat pump at one residence.

This report documents the performance of both conventional and custom ground heat exchanger designs. These include vertical bore holes, with single and dual pipes per hole, “6-pipe” two-level horizontal, single-level horizontal, flat slinky, and hybrid slinky configurations. Horizontal designs ranged form 600 ft/ton to 900 ft/ton of loop with the shorter heat exchangers operating about 5ºF cooler through the heating season. Loop temperatures ranged from the lower 20s to mid-40s °F across the sites. Seasonal heating efficiencies ranged from gross COPs of 2.6 to 2.9 for single-speed split systems and over 3.2 for two-speed and oversized units. Heat pumps generally met 95 percent of the heating load.

Cooling loads were about five to 10 percent of the heating loads with loop temperatures remaining in the 60s and 70s °F. The oversized heat pump in cooling removed adequate amounts of moisture to maintain comfort.

Energy prices vary among the utilities, with the largest variation due to a radio load control program offered by the cooperatives. Their half-priced electricity for the geothermal heat pump generated annual savings of $500 to $700 per year for loads in excess of 115 million Btu. The incremental cost for the four-ton systems with highest savings was $6000. These economics amount to a seven percent return over 20 years with propane at $0.75/gallon or a 12 percent return with propane at $0.95/gallon. Some initial cost reduction of about 10 percent is possible by using smaller capacity heat pumps and relying on more auxiliary heat. However the corresponding operating cost increase results in only a 50 basis point increase in the investment return.

The gas engine heat pump maintained the space conditions comfortably, but relied on the auxiliary boiler to meet 35 percent of the heating load. Its heating capacity was limited by the outdoor air temperature. Alone the heat pump COP was near 1.0, but combined with auxiliary boiler the system COP dropped to 0.8. A larger capacity unit would have been needed and less thermostat setback could have been employed to increase the system efficiency. The similar operating cost to conventional high efficiency gas furnace / air-conditioners makes the added cost of at least $2800 difficult to justify in the Wisconsin market.

The project demonstrates the technical feasibility of enhanced heat pumps in Wisconsin. The variety of ground heat exchanger designs make GHP system available in most residential settings. The larger initial investment requires a

E n h a n c e d H e a t P u m p C a s e S t u d i e s

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longer time horizon. These long term economics along with unique system characteristics will have to be valued by the consumer before the technologies are accepted in the marketplace.

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Introduction

Overview

This document presents the results from the Enhanced Heat Pump Applications project. The project sought to determine how the Wisconsin heat pump market could be improved and to develop technical information to support market development. The report contains results from demonstration sites of heat pump system performance and economic analyses. The Appendix contains detailed data summaries from each demonstration site.

Background

Enhanced residential heat pump technologies offer paths to meeting two broad objectives. For utilities they offer the means to meet load shape objectives. For society they offer improved energy efficiency and the means to meet reduced emission goals. Reduced reliance on electric resistance supplemental heat reduces electric utility winter peaks and improves efficiency. The availability of gas cooling options reduces summer electric peaks and improves the local distribution company’s load factor by increasing summer gas use. Increased energy efficiency reduces emissions and improved load factors improve cost-effectiveness. For these reasons enhanced heat pump technologies have been receiving ongoing support and promotion from government and utilities.

While electric air-source heat pump performance has been improving, appli-cability and interest in cold climates have remained limited. The current emphasis in technology and market improvements to overcome the limitation of electric air-source heat pumps is focused in two areas: ground-source heat pumps—now called geothermal heat pumps (GHPs)—and gas driven heat pumps. GHPs are an established technology with annual shipments of 40,000 units per year. The less extreme source and sink temperatures of ground coupled heat pumps reduce the performance penalty during the hottest and coldest days of the year. Gas-driven engine heat pumps have only recently been commercialized and gas generator-absorber-heat exchange technology (GAX) is under prototype development. The gas technologies offer lower cost supplemental heat along with improved performance. Advances in performance and installation practices as well as positive GHP experiences in nearby regions lead to this reassessment of the Wisconsin heat pump market.

DOE and EPA are undertaking market development efforts on a national level. The promo tional efforts in their National Earth Comfort Program seek to increase ground-source heat pump shipments to 400,000 units per year in an effort to reduce greenhouse gas emissions drastically by the year 2001. The program plans provide coordination of marketing efforts and development of technology


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demonstrations and research into installation and performance improvements. A collaboration between manufacturers, utilities, government and trade allies known as the Geothermal Heat Pump Consortium is implementing much of this program.

The current Wisconsin market for GHPs is very small and limited to areas with utility rebates or reduced electricity rates. There are general perceptions that heat pumps are not well suited for cold climate operations and that the higher initial cost hinders acceptance. Contractors generally have no motivation to promote new or unfamiliar technologies. Purchasers’ preference for low first cost often precludes GHP consideration. Low electricity and gas prices in Wisconsin diminish the motivation for investing in efficiency. The regulatory environment focuses on peak shaving, load shifting, and conservation, while perceiving electric heat pumps as adding load when not displacing electric resistance heat. A better understanding of heat pump technologies by both customer and contractor as well as a reevaluation of the impact of emerging technologies on utility systems might make heat pumps more acceptable. A better understanding of market and infrastructure issues will help the utilities and regulators determine the needs of the customers and trade allies.

Project Objectives

This project was intended to help Wisconsin utilities acquire experience in heat pump technology design, installation techniques, technology performance, and market penetration issues. The overriding goal was to determine how to improve the Wisconsin heat pump market and gather the necessary information and experience to make those improvements. To that end the project consisted of two concurrent phases: market assessment and technical case studies. Both phases supported the ultimate goal of developing technical and market information vital for improving the heat pump market in Wisconsin.

The market assessment (Energy Center of Wisconsin, 1996) identified the Wisconsin customer’s needs and infrastructure in contrast to nearby regions with market success. It investigated currently available technologies and installation techniques as well as identified upcoming improvements.

The case study demonstrations focused on the technology issues. They demonstrated various installation techniques and provided first-hand perfor-mance results for various configurations across the state. This information both builds contractor confidence in their ground heat exchanger designs and sizing practices as well as documented performance for utility program policy makers. This report presents the demonstration sites’ performance results.

I n t r o d u c t i o n

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Market Background

The Wisconsin residential space conditioning energy needs are dominated by heating. Typical seasonal heating loads range from the 80 million Btu to 120 million Btu with cooling loads around 12 million Btu. Temperature ranges are extreme with winter design temperatures around -15ºF and summer design temperatures over 90ºF. During the 1995/96 monitoring we recorded temperatures as low as -33ºF and as high as 104ºF.

All of the investor owned utilities either supply both gas and electricity or are owned by a holding company supplying both energy forms. Cooperatives handle rural areas with a mix of generation and purchased power from investor owned utilities. Propane and oil are widespread in rural areas. Of the 1.8 million single family homes in the state, 61 percent heat with natural gas as shown in Table 1 based on 1990 census data. It is most likely that the remaining houses do not have natural gas available. About 34 percent or 630,000 houses have drilled water wells, consistent with rural locations without natural gas availability.

Table 1: Summary of existing Wisconsin housing stock heating fuel

Nationally new construction represents the majority of the GHP market. Table 2 summarizes single unit new house building permits from the Construction Statistics Division of the Census Bureau. About 12,000 new houses are built each year in Wisconsin. Only several hundred GHP systems are installed each year. Many of these systems use open wells.

House heating fuel Housing units percent

Utility gas 1,111,733 61

Bottled gas 152,823 8

Electricity 168,615 9

Fuel oil 265,600 15

Wood 107,239 6

Other 16,108 1

Total 1,822,118 100


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Table 2: New single-unit housing building permits

The large investor owned utilities have limited experience with GHPs. They have had small promotional programs or pilot marketing programs. Some efforts were limited to retaining electric heat customers while others sought to compete with oil and propane in areas where natural gas was unavailable. Heat pumps don’t play a significant role in their plans partly because all offer gas service so they have the heating market either way. There is less emphasis on the residential sector. Marketing efforts focus more on industrial and large commercial customers who might have alternative electricity supply options available in a more competitive market. Furthermore, regulatory policy prohibits electricity promotion activity with rate payer funds due to studies that concluded that sales promotion would ultimately require added capacity and increase rates.

The most aggressive GHP promotion programs are supported by the rural cooperatives. They are free to promote electricity sales as they wish. They promote the technology, develop infrastructure, and offer rebates and financing. Their goal is to build electric heat market share to improve their seasonal load shape. Some use heat pumps in conjunction with propane backup systems to build controllable load. They see the GHP option as allowing them to compete for other end uses such as water heating, clothes drying and cooking, since fuel choice is often made based on the heating system.

Several thousand GHPs are installed in the state—most are open loop systems, given the high quality and abundance of ground water. Nearly all of the closed-loop installations are in areas with promotion from the electric utility. Besides rebates and financing, some utilities offer free electric hot water heaters and half prices electricity for the heat pump and water heater. The relatively high initial cost and lack of awareness about the technology by consumers and most contractors has limited its acceptance. There was very limited exposure to air-source heat pumps through past promotions. These experiences developed the market’s view that heat pumps don’t work well in northern climates—a stigma carried over to GHPs. There were only three residential gas engine heat pumps installed in the state.

Year Permits issued

1990 9856

1991 10,045

1992 12,478

1993 12,546

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Method

Demonstration Site Selection

Criteria for selection of demonstration sites primarily sought diversity in ground heat exchanger designs. The small size of the market limited options with other characteristics such as manufacturer, equipment type, and system configuration.

We identified candidate sites mainly from contractors. Both utilities and manufacturers supplied list of active contractors in Wisconsin. Beside site information we also obtained a significant information about installation practices and costs from the contractors in support of the market assessment efforts.

The initial market assessment in 1994/95 identified 18 manufacturers of which 10 had contacts in Wisconsin. We found three manufacturers with relatively significant activity in Wisconsin. WaterFurnace and TETCO each have over 500 sites installed. Econar had over 70 sites operating. Table 3 summarizes manufacturer activity. Some of the other brands had up to a half-dozen past installations per contractor. All but one of the demonstration sites were WaterFurnace installations. The other site used Econar equipment. Tetco units were used mainly in open well configurations.

Table 3: Manufacturer activity in Wisconsin

Manufacturer Activity Notes

Addison - nothing found

Bard - one dealer, but little activity

Climate Master / Carrier - nothing found

Command-Aire / Trane - one distributor, but little activity

Econar X active in center of state (35 sites/year)

Florida Heat Pump - one dealer, but little activity

Hydro Delta - two distributors, but little activity

Mammoth - two dealers, no activity

TETCO X active in multiple areas (50+ sites/year)

WaterFurnace X active in multiple areas (50+ sites/year)

Andrea

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The predominate system configuration is a water to air heat pump. However, we found some sites using hydronic heat distribution. Some systems were configured to supply hot water to tubes buried in the slab in a “radiant” heat application. Some of these systems also include a water to air coil for condi-tioning the second floor and supplying cooling. We included one demonstration site with a water-to-water system including a hybrid slab and air distribution system.

Table 5: Systems configuration summary

Including various ground heat exchanger designs was the highest priority in the demonstration project for several reasons: The ground heat exchanger is most of the incremental cost of the system, there are several main designs with multiple variations in use, and theoretical prediction of ground heat exchanger design depends on many assumptions. The choice to use a particular design depends on the contractor’s experience, type of excavation equipment available and site conditions.

In Wisconsin open loop systems using water from the house’s water well have been popular because of the low first cost. The heat pump uses the existing well so no excavation for a loop is required. On a national level regulations, water discharge concerns, higher pumping power, and water quality have produced a trend toward closed loops. The water temperature remains fairly constant resulting in consistent performance. No open well systems were included in the demonstration because there are no loop design issues impacting performance.

Closed loop configurations are varied. Vertical loops require little land, but often cost more to install. Horizontal loops require excavation to a depth of six feet or more and space enough to accommodate several thousand feet of pipe. Many possible trench/loop configurations exist to try to optimize the amount of soil contact for a given soil condition, within the constraints of minimizing labor and material costs.

System Configuration Applications

Forced air—DX most popular in national and Wisconsin market

Hydronic—slab systems stress comfort and storage aspects, multiple Wisconsin contractors are installing these

Hydronic—baseboard nothing specifically noted about Wisconsin sites

Combination (slab/forced air w/ water coil)

installed for comfort, cooling and storage (TOU rates)

Andrea

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Figure 1: Demonstration site location map

1

2

3

4

5, 5b

6

7

9

Site selection for a gas engine heat pump site was limited to the Wisconsin Public Services’ Power House demonstration site. This Site had one of only three gas engine heat pump installations in the state at the time of site selection. The gas engine heat pump market is just beginning with only a few thousand total units installed nationally.

Table 7 summarizes the earth heat exchanger designs at the demonstration sites. Loops vary from conventional horizontal and vertical layouts to modified designs conceived by the contractor. Some contractors perform the excavation themselves. Usually these are conventional “6-pipe” designs installed with a backhoe. Others subcontract the excavation because they are vertical or rely on wider trenches that are more economically dug with a larger tracked excavator. Excavation times typically ranged from one to two days depending on the skill of the excavator and soil conditions.


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Table 7: System design characteristics*

Site # Location Loop type Design Pipe length (ft)

Unit size (tons)

1 Oxford Vertical Four bore holes, two contain two loops and two contain single loops—two holes at 150 ft depth and two at 100 ft depth

800 4 packaged

2 Sun Prairie Vertical Two 150 ft bore holes with a single loop in each 300 2 packaged

3 Green Bay NA Gas Engine Heat Pump—Variable speed, auxiliary gas heater

NA 3

4 Eau Claire Flat Slinky Two 100 ft trenches, 7ft deep and 7 ft wide, each contain two slinky coils of 600 ft pipe

2400 4 split

5 Merrill System #1

Hybrid Slinky

Two 100 ft trenches, 6 ft deep with three 600 ft slinky coils in each trench. One slinky lies flat. The others are upright on the trench walls.

3600 4 split

System #2 “6-pipe” Horizontal

Two 300 ft trenches, 6 ft deep with three 600 ft conventional “6-pipe” loops in each.

3600 4 split

6 Holcombe Flat Horizontal

One 250 ft trench, 6 ft deep, 8 ft wide with four 600 ft loops of pipe going out and returning at the same depth.

2400 4 hydronic hybrid

7 Merrill “6-pipe” Horizontal

Two 300 ft trenches, 10 ft deep, 3 ft wide with three 600 ft loops of pipe out at 10 ft and returning at 8 feet

3600 5 2-speed

9 Prescott “2-pipe” Horizontal

Five 500 ft loop each in single trenches, 7 ft deep, 2 ft wide. Last loop is in two 8” trenches at 5 ft depth. Tried horizontal boring unsuccessfully and slit trenching.

2500 4 split

* All loop pipe is 3/4-inch polyethylene with 25% methanol or propylene glycol anti-freeze solution. A pond loop at site 8 was never installed.

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Installation Notes

The first two sites used vertical heat exchangers. These systems required drilling of multiple bore holes. Typical designs use one, six inch, 150 foot deep bore hole per ton of heat pump capacity with a single loop of 3/4” polyethylene tubing in each hole. Usually 15 feet separate the holes, so relatively little surface area is required for the loop field. The holes are sealed with bentonite grout (a type of clay based cement) to keep surface water from contaminating ground water. The conditions at site one were mainly clay allowing lower cost drilling. At site two the drillers hit rock at a 40 depth, a more typical situation.

Figure 2: Site One drilling rig—Oxford


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The design of the ground heat exchanger at site four came about because of the limited lot size. The house is in a sub division on a partially wooded lot. The loop was a slinky installed flat at the bottom of two wide 100 foot long trenches. The trenches were angled to fit onto the lot. An excavator dug the trenches using a four foot bucket. The contractor usually can have trenches dug this way and have the loop installed in under a day. At this site the confining space and the relative lack of experience of the excavator operator added a second day to the excavation. The contractor precoiled the slinky and transported the loops to the site. This proved to be labor intensive. With the cooler temperatures in April when the installation occurred, the tubing was stiff and difficult to work with.

Figure 3: Site Four loop layout in trench—Eau Claire

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Site five was the contractor’s house. He had plenty of land for a conventional loop, but he wanted to try a slinky loop to gain experience. The ground heat exchanger consisted of two 100 foot long four foot wide trenches. These trenches were dug with a two-foot-wide bucket on a the contractor’s own backhoe. It took one day for each trench to dig and install the loop. The six slinky loops were formed on site in several hours. A loop was installed flat on the bottom of the trench and two loops were laid along the walls of the trench. Back filling proved to be challenging. Difficulty came in trying to keep the upright loops from bending over as the trenches were filled. This challenge was overcome by filling from the end of the trench rather than pushing the dirt back in along the sides.

Figure 4: Site Five loop layout in trench—Merrill

A second system conditions the shop. The ground heat exchanger on this sys-tem used a conventional 6-pipe design. Three 600 foot loops were installed in two foot wide 300 foot long trenches. The loops were rolled out at a the bottom of the six to seven foot deep trench. One to two feet of back fill were installed before the remaining loops were uncoiled on top of the backfill. This design typically takes at least two days to install. The loop at site seven used the same 6-pipe design, but with deeper (10 feet) and wider (3 feet) trenches. The home owner at this site also used his own excavation equipment to install the loop.


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The ground heat exchanger at site six used one 250 foot long, ten foot wide trench. Four 600 ft rolls of tubing were uncoiled at the bottom of the trench. An excavator with a four foot bucket dug the trench. Original plans called for the excavator to dig the trench when the foundation was dug, however, the limited space on site would have required the cement delivery truck to drive on the newly excavated and filled trench. The trench was dug after the foundation was poured. The wooded lot limited the trench length to 250 feet. The trench would have normally been 300 feet, allowing the 600 ft coils of pipe to be uncoiled to the end of the trench. With the shorter trench some of the tubing had to be crowed and coiled back on itself at the end of the trench.

Figure 5: Site Six loop layout in trench—Holcombe

Site six also featured hydronic slab heat distribution. The water-to-water heat pump warmed water that was circulated to four zones of tubing contained within the poured slab of the basement and garage floors. The water also circulates to a water-to-air coil to supply heat to the upper floor. A 40-gallon tank acts as a buffer to reduce the cycling of the heat pump as individual zones turn on and off. For air conditioning, controls disable the zone circulation pump so water can only circulate to the air coil.

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Figure 6: Hydronic hybrid system schematic

• SZP

• THL• THE

• SRV

• SAH

• WU• SC

• TWL

Heat Pump Unit

GroundLoopPump

• TWE

• TG6• TG3

• FH

Ground Heat ExchangerWater Buffer Tank Hydronic Floor Zones

• SBBlower

SupplyAir

ReturnAir

• TAR• RHR

• TAS

Condensate

• FC

Water Coil

Propane Heater

T’Stat controlledzone valves Water

LoopPump

Zone Pump

• TAO• RHO

120V

240V


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The Prescott site (site nine) was originally intended to be installed with hori-zontal boring equipment. The loops would have been assembled with ‘U’ couplings like vertical loops and then pulled through the bore hole. Drilling conditions proved too rocky for the size of the boring equipment available. One loop was installed with a slit trencher with each leg in separate trenches. The remaining four loops were installed in 250 foot trenches with the supply and return leg separated vertically by two feet of backfill as in a 6-pipe design.

Figure 7: Site Nine horizontal drilling rig

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In comparison to GHP installations the gas engine heat pump was a more standard installation with and outdoor and indoor unit. The outdoor unit contain the engine, condenser section and auxiliary boiler. Refrigerant and auxiliary boiler line connect to the indoor blower unit. Most of the added cost of this system is the equipment rather than the on-site installation as with GHPs.

Figure 8: Site Three gas engine heat pump


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Approach

The goal of the field monitoring at the demonstration sites was to document system performance for a variety of closed loop configurations. Specifically the objectives of the monitoring were to:

• Measure the capacity and efficiency of the enhanced heat pump systems in Wisconsin, and compare these data to the expected performance.

• Compare the operating costs of the enhanced heat pump systems to other alternate heating and cooling systems and evaluate the cost-effectiveness of the system.

• Evaluate the performance of the ground loop heat exchanger and compare the performance and cost of various designs.

These project goals called for the monitoring to include energy use, heating and cooling load, and equipment operating statuses.

Monitored Data Points-Geothermal Heat Pumps

The GHP systems have up to three main components: the heat pump, the ground heat exchanger, and the hot water system. We measured the heating and cooling loads; heat pump energy use; performance of the ground loop and its impact on the heat pump operation; and the contribution of the heat pump to the hot water system.

Energy use of the compressor circuit was submetered. The energy use of fixed power components such fans and pumps was derived from status data and one-time measurement of their power consumption.

The heating and cooling loads were derived based on the water-side conditions. An energy balance on the unit, considering the electric input, ground loop water mass flow and temperature difference, and desuperheater water mass flow and temperature difference determined the total unit capacity.

We also measured the capacity from the air-side conditions. The dry-bulb temperature of the supply and return air along with the relative humidity of the return air and condensate were measured primarily to determine the operating conditions of the heat pump (Entering coil dry-bulb and wet-bulb temperature). The supply air temperature is of interest because the delivery temperature is one aspect setting comfort levels. Condensate was measured to determine the latent capacity so concerns about limited moisture removal abilities on units sized for heating could be addressed.

Since the air flow rate was constant or varied among a limited number of fixed speeds, we did not measure air flow directly. We quantified the air flow when the

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unit was instrumented for comparison to design and to support later energy balance checks on the data. The status of the blower indicated air flow.

The entering and leaving ground loop temp eratures showed the performance of the ground heat exchanger. The entering water temperature drives the capacity and efficiency of the heat pump. Since the loop flow rate remained relatively constant, we measured the flow with non-intrusive ultrasonic flow metering equipment when the system was instrumented and used the pump status as an indication of flow. The far-field ground temperature at the loop depth were measured to show the theoretical limit on the loop temperature. The ground temperature at another shallower depth was used as a backup sensor and to show the temperature variation by depth.

The entering and leaving desuperheater water temperatures were used to determine the total energy supplied to the hot water system and for an energy balance on the heat pump. The desuperheater water flow rate was constant, allowing us to use the pump status as an indication of flow. The hot water use was measured directly since it is intermittent and varies.

The outdoor dry-bulb temperature and relative humidity are the predominate factor driving residential heating and cooling loads. They were measured directly at each site.

Figure 9 shows the data points schematically for a typical system. Only one system had hot water monitoring. The schematic depicts a forced air system. The hydronic system added the water temperature produced from the heat pump. Specific site details are contained in a separate site detail document. Table 8 lists the continuously monitored data. Data were collected at both 15 minute intervals and in records that summarize each operating cycle. This event triggered data showed the cycling and control of the unit. The two speed unit required additional status points to distinguish the different speeds.


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Figure 9: Typical system schematic with data points

Cold WaterInlet

• SRV

Blower

Refrig.Coil

Condensate

SupplyAir

• TAO• RHO

ReturnAir

• TAR• RHR

• FC

• TAS

• WU• SC

• TWL

Auxiliary Heater

Hot Water Supply

Hot WaterTank

Heat Pump Unit

Loop Pump

• TWE

• SDP

• TG6• TG3

• TDL

• TDE

• SHW

• FHW

• SB

Ground Heat ExchangerHot Water

• SAH

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Table 8: Continuously monitored data points

Name Description Name Description

TAO Outdoor air temperature TG6 Far-field ground temperature at 6 feet

RHO Outdoor relative humidity FC Condensate flow

TAS Supply air temperature FHW Hot water flow

TAR Return air temperature WU Total unit electricity use

RHR Return air relative humidity SC Compressor status

TWE Ground water loop temperature entering unit SB Blower status

TWL Ground water loop temperature leaving unit SRV Reversing valve status

TDL Hot water leaving desuperheater SDP Desuperheater pump status

TDE Hot water entering desuperheater SAH Auxiliary heat status

TG3 Far-field ground temperature at 3 feet TG3 Far-field ground temperature at 3 feet

Monitored Data Points-Gas Engine Driven Heat Pump

Electric use of the indoor blower and outdoor units were measured separately. The blower was variable speed so we needed to separately meter it to distinguish its electricity use from the outdoor fan, glycol pump, auxiliary heater blower, and controls. These other items use electricity at fixed levels were able to derive their contribution to the total electricity from the operating mode.

Two components use natural gas, the engine and the auxiliary boiler. Since the auxiliary boiler had a fixed firing rate we derived its gas use from the operating mode. This firing rate was also confirmed when the unit operated without the engine (emergency heat mode and when the ambient temperature was below -5ºF).

The heating and cooling load was measured based on the air-side conditions. These include the dry-bulb temperature and relative humidity of the supply and return air. In order to separate the contribution of the auxiliary heater from the heat pump, we measured an intermediate air temperature between the refrigerant and glycol coil.

The air flow rate was not constant in the unit, so we installed an averaging flow station. It consisted of a flow straightening section and multiple pressure ports. A differential pressure transducer connected to the flow station linearized the signal to make it proportional to the air flow rate.

The latent cooling capacity was determined by measuring the condensate from the refrigerant coil. We also compared this result to relative humidity measurements across the coil.


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The outdoor dry-bulb temperature and relative humidity were included for two reasons. The weather is a key performance factor in the equipment operation, setting the source or sink temperature and defrost needs. The weather is also the predominate factor driving residential heating and cooling loads.

Figure 10: System schematic with data points

• FGU

Coolant Valve

Ck. Valve

• ST1

• SRV

Outdoor Coolant Coil(Summer)

Outdoor RefrigerantCoil

2-Speed Fan

Indoor Coolant Coil(Winter)

AuxiliaryHeater

Blower

EngineExhaust

AuxiliaryHeater

Exhaust

EngineCoolant

Recuperator

EngineCompressor

Rev. Valve

Engine DrivenCoolant Pump

(cooling) (heating)

VariableSpeedBlower

IndoorRefrigerant

Coil

Condensate

ReturnAir

SupplyAir

INDOOR UNITOUTDOOR UNIT

NaturalGas

ElectricallyDriven

CoolantPump

• SCV

• ST3

• TAO• RHO

• TAR• RHR

• FC

• TAS• RHS• FAS• WU

• TAU• WBI

CoolantT’Stat

Sump

• SB

Figure 10 shows the data points schematically. Table 9 lists the continuously monitored data that were collected at 15-minute time intervals and for each operating cycle of the heat pump.

M e t h o d

23

Table 9: Continuously monitored data points

Name Description Name Description

TAO Outdoor air temperature FGU Total unit natural gas use

RHO Outdoor relative humidity WU Outdoor unit electricity use

TAS Supply air temperature WBI Indoor blower electricity use

TAR Return air temperature SB Blower status

TAU Unit air temperature between coils SRV Reversing valve status (on = cooling/defrost)

RHS Supply air relative humidity SCU Coolant valve status (on = cooling)

RHR Return air relative humidity ST1 Thermostat stage 1 status

FAS Supply air flow rate ST3 Thermostat stage 3 status—auxiliary heater

FC Condensate flow

In addition to the 15 minute fixed time step data, event triggered data were collected for each operating cycle of the heat pump. The data collection interval was variable. Data were collected for intervals corresponding to each of the operating modes listed in Table 10. The length of each record ranged from a few minutes (defrost) to several hours (off). The unit’s operating mode was evaluated every scan based on the status points (ST1, ST3, SCV, SRV). A record was written each time the mode changes, summarizing the data during that period of operation. Table 10 lists the six operating modes of the unit along with the identifying status points.

Table 10: Unit operating mode identification—gas engine heat pump

Data Collection

Table 11 presents a general overview of how the data listed in Table 8 and Table 9 were measured and collected.

Operation Mode Mode # ST1 ST3 SCV SRV

OFF 0 --- --- --- --

Cooling 1 ON --- ON ON

Heating 2 ON --- --- ---

Auxiliary heating 3 ON ON --- ---

Emergency heating 4 --- ON --- ---

Defrost 5 ON ON --- ON


24

Table 11: General data acquisition specifications

Measurement Equipment

Data logger Campbell Scientific 21x data logger is completely programmable allowing fixed and event triggered data records, data filtering, and real time energy calculations.

Temperature Factory assembled special limits thermocouples are a cost effective way to measure temperature differences accurately. Low mass electrically isolated surface mount sensors were installed on copper alloy fittings.

Relative humidity Thin-film resistance sensors with a nominal 2% accuracy range.

Electric power Power transducers with current transducers incorporate the actual power factor with accuracy within 2%.

Natural gas A magnetically coupled pulse initiator attached to the gas meter face-plate provides low cost gas flow measurement, and can produce up to 10 pulses per dial rotation, increasing data resolution.

Condensate A count of condensate pump cycles measured moisture removal based on the volume capacity of the pump sump. The resolution is low, but the data represent actual moisture removal over a longer time period.

Air flow At the GHP sites air flow was measured once with a hot wire anemometer duct traverse or heat balance methods at fixed blower speeds. The blower status was used as a proxy for air flow. At the gas engine heat pump site a differential air flow station was installed to measure air flow directly.

Water flow Water flow was measured once with ultrasonic equipment. Domestic hot water use was measured with an in-line turbine meter.

Status High impedance optically isolated relays measured status points from control relays and thermostat signals without interfering with the equipment operation. Clamp on current switches were substituted on larger loads.

A data logger collected several types of data records simultaneously. The data logger scanned each data channel every second. Depending on the type of data point, data were averaged or summed over the record interval. Internal filtering processed data only for periods when equipment was operating (e.g. temperatures were only averaged when there was flow across the sensor). Data records were retrieved nightly over phone lines.

Each day as data was added to a database, it was checked for errors. These checks included verifying that values were within expected ranges, data showed consistency between data points, and trends in data and performance were within expectations. Each month we summarized the data and prepared performance summaries for each system.

25

Results

Performance

Efficiency

The efficiency of a heat pump depends on the temperature difference it works between. For an air-source heat pump this temperature difference is between the outdoor air and the indoor air. For a geothermal heat pump the temperature difference is between the ground loop fluid temperature and the indoor air. As the temperature difference increases the capacity of the heat pump to move heat decreases and it’s efficiency decreases. Therefore as Figure 11 indicates, the ability of a heat pump to heat drops as more heat is required with the drop in outdoor temperature. The drop is less severe with geothermal heat pumps because the ground loop fluid temperature remains in the upper 20s to lower 30s even on the coldest days. This example illustrates the benefit of GHPs versus air-source heat pumps: they can require little to no auxiliary heat.

Figure 11: Heat pump heating efficiency variation with outdoor temperature

Outdor Air Temperature

Heating Capacity

Air-Source Capacity

Ground-Source Capacity

Heating Load

AuxiliaryHeatNeeds

The seasonal efficiency of a GHP depends on the amount of time the heat pump operates at the various ground loop fluid temperatures found throughout the season. Heating COPs ranged from 2.5 to 2.9 with one exception at 3.8. The unit with the highest performance was half the size of the other units. Its relative sizing of the heat pump was larger resulting in less heat pump operation and a warmer ground loop heat exchanger temperature. The cooling COPs ranged from the summer of 1995 ranged from 3.1 to 5.4.

R e s u l t s

27

The values in Table 12 corresponds to elements in the schematic. For example at site four 61.4 million BTUs was extracted from the earth. The loop pump added 4.2 million BTUs and the compressor added 28.3 million BTUs. The sum of these energies was the total heat pump output of 93.9 million BTUs. The blower added 19.2 million BTUs and the auxiliary heat added 3.7 million BTUs to meet the total space load of 116.8 million BTUs. The efficiency in the Table (COP) is the gross heat pump output divided by the compressor and pump energy input. It does not take any credit for the blower heat, nor does it include any blower electricity use.

About two-thirds of the heat supplied by the heat pump came directly from the earth. Figure 12 summarizes the average site energy flows pictorially. In cooling about three-quarters of the heat rejected to the ground was extracted directly from the house. The balance of the heat was from the systems components: blower, compressor, and loop pump.

Figure 12: Seasonal system energy flows

Blower 5%Loop Pump 5%

Compressor 30%

Ground 60%

Blower 4%

Loop Pump 4%

Compressor 17% House 75%

Heating Cooling

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29

Figure 13: 1995/96 heating season electricity use

-

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

1 2 4 5 6

Site

kWh

Auxiliary

Aux. Pumps

Blower

Loop Pump

Compressor

The totals for site five include a partial season from January when the heat pump became operational. The site six data started at the beginning of December. The zone circulating pump at site six (Aux, Pumps) should be included with the blower energy as it circulates heat to the house. The blower electricity use at site four is large because the blower ran continuously.

Electricity use for cooling was about one tenth of that used for heating. The component electricity use distributed in the same fraction as the heating data.


30

Figure 14: Cooling season electricity use

0

500

1000

1500

2000

2500

1 2 4 5 6 7

Site

kWh

Blower

Loop PumpCompressor

Only one of the sites used the heat pump’s ability to supply hot water. The outlet of the compressor produces temperatures high enough to heat water for domestic purposes. A pump circulates water from the hot water tank, through a desuperheater heat exchanger on the outlet of the compressor, and back to the hot water tank. The effectiveness of this arrangement depends on the configuration of the system, the temperature setpoint of the hot water heater and the run time of the heat pump.

Figure 15 shows the relationship of daily water heater energy use to the amount of hot water consumed for several different scenarios. Initially the electric hot water heater thermostat was set to 150ºF. The data cluster near the top of the plot are for this period. The ability of the heat pump to supply hot water is limited by the water temperature. Around 150ºF the heat pump can add very little heat to the water. After September 1995 the water temperature set point was lowered to 130ºF. This lower temperature increased the ability of the desuperheater to heat water. In January of 1996 the desuperheater pump stopped operating. The middle data cluster represent days when the desuperheater did not operate. The lower cluster are for days when the desuperheater pump ran for more than one hour.

The results from a regression analysis are printed at the top of the plot. The first value indicates the daily standby loss of the tank. The 1.683 kWh is the amount of heat that was lost through the tank shell each day. The next value of 0.141 kWh/gallon represents the amount of energy used to heat each gallon of water. This amount corresponds to a temperature rise of 58ºF. The final term of 0.459 kWh/hour is the amount of energy the desuperheater dis placed each day per hour of desuperheater run time. The results of the equation are bounded by the


32

summer and 1300 kWh of savings occurred in the winter. To displace the 1300 kWh of hot water heater energy use in the winter the heat pump had to use 450 kWh (1300 kWh ? 2.9 COP = 450 kWh). The net annual savings from the desuperheater was 1050 kWh (1500 kWh - 450 kWh). At $0.057/kWh this $60 savings is a 22 percent operating cost reduction over a standalone electric resistance water heater.

The other sites did not use the desuperheater option because they either had fossil fuel water heaters or received a free electric water heater and half-priced electricity. The installed incremental cost for a desuperheater was estimated by one contractor to be between $500 and $700. The desuperheater option included a small circulating pump, heat exchanger, additional controls, piping to the hot water tank, and connection to the hot water tank—usually a concentric tube inserted through the drain port.

Loop Performance

The performance of a GHP depends on the temperature of the fluid in the ground heat exchanger. Figure 16 shows the heating season distribution of loop temperatures. The different temperature ranges among the sites were due to the different loop configurations and size of the heating load. At any site the temperatures span only about 10ºF while most of the t ime the temperature is within 5ºF of the average.


34

temperature. Shallower depths lead to temperatures more closely affected by the air temperature. As the depth increases the influence of the air temperature diminishes and the ground temperature becomes constant at a level near the yearly average air temperature. To one extreme the vertical loops have minimal exposure to the surface and could ideally supply temperatures close to the constant ground water temperature. In practice cost limits the length of the loop. The length of the loop determines how close the loop temperature can approach the ground temperature. Soil moisture is another factor setting the loop performance. Damp soil improves the heat transfer of the soil. During the winter the moisture freezes thereby maintaining the temperature near freezing for a long period. During the summer moisture percolating through the soil takes heat away with it.

The loops with the coldest temperatures were the flat slinky installed in 200 feet of trench (site 4) and the flat horizontal installed in 250 feet of trench (site 6). Both loops used 600 feet of pipe per ton of heat pump capacity (2400 feet total). These were the shortest horizontal loops with the least amount of trench and had the largest heat extraction rate for the horizontal loops of around 10 Btu/hr-ft. Even with the coldest loop temperatures these systems needed less than five percent of the heating load from the auxiliary heat source. Most of the auxiliary heat was due to the utility radio load control that disabled the heat pump during peak utility periods.

The warmest loop temperatures were from the small vertical system at site 2. Both vertical systems used about 150 feet of pipe per ton of heat pump capacity. The system at site 2 was relatively oversized when compared to the other sites. All of the systems except the one at site 2 ran continuously on the coldest days. The system at site 2 ran for only 60 percent of the time on the coldest day. This relative oversizing of the heat pump and loop caused the warmer temperatures.

The other sites used the hybrid slinky (one loop flat at the bottom of the trench and two loops on each side of the trench) and standard 6-pipe designs. All three of these sites used 900 feet of pipe per ton of heat pump capacity (3600 feet total). These systems maintained the loop temperature in the low to mid 30s. The heat extraction rate was about five Btu/hr-ft, or half of the heat extraction rate as the shorter loops at sites four and six.

The seasonal variation in temperature was minimal. Most of the variation occurred during the fall as the loop temperature slowly cooled. Figure 17 illustrates this trend at site four. It shows the daily average loop temperature entering the heat pump, the daily heat transfer rate with the earth, and the undisturbed ground temperature at the loop depth. The inset plot shows the heat pump run time fraction related to daily average outdoor temperature. This plot shows the relative sizing of the heat pump for the temperature extremes.

The loop temperatures reached a quasi-steady state mode of operation after several months in the heating mode. The trend roughly follows the undis turbed ground temperature trend. The heat extraction rate remains relatively constant


36

Gas Engine Heat Pump

The gas engine heat pump is an air source heat pump that uses the variable speed capability of a natural gas engine to modulate the heating and cooling capacity. It extracts heat from the outdoor air instead of the ground. Since the air reaches colder temperatures than the ground, there is less heat available during the coldest weather. During the cold periods auxiliary heat was provided by a natural gas boiler. The auxiliary heat ran for 139 hours during the 1995/96 heating season.

Auxiliary heat was also needed during defrost cycles. With an air source heat pump frost forms on the outdoor heat exchanger coil when the coil temperature is below freezing. Periodically the heat pump operates in the cooling mode, supplying heat to the outdoor coil to melt the ice. During these defrost cycles the auxiliary boiler supplied heat to the indoor air. The unit ran in defrost for 79 hours.

During the coldest weather, below -5ºF, the engine would shut down since there was little heat it could extract from the air. The auxiliary heat met all of the heating needs during these periods, which amounted to 86 hours.

The engine ran for 2641 hours with the heat pump supplying 65 percent of the 57 million BTU of heat during the season. The remaining 35 percent of the heat was supplied from the auxiliary boiler and engine jacket coolant. The system used a total of 70.8 million Btu of natural gas at a seasonal gas efficiency of 81 percent. With the auxiliary boiler at about 60 percent efficiency the heat pump seasonal COP was just under 1.0. The unit also used 913 kWh of electricity for the condenser fan and circulating pumps.

The seasonal system heating efficiency is below what it might have been. The large amount of auxiliary heat use lowered the overall efficiency. Some of this auxiliary heat use was caused when the unit tried to warm the house in the morning after the thermostat had been set back during the night. The limited heat pump capacity during the coldest weather reduced the heat pump’s ability to recover from night setback. Instead, the system had to rely on auxiliary heat during the recovery period. As with any heat pump, night setback should be minimized or eliminated to reduce the need for auxiliary heat.

The engine ran for 564 hours in cooling meeting a load of 10.7 million BTU. The engine used 9.3 million BTU of gas and 160 kWh of electricity for the condenser fan and circulating pumps. The seasonal COP based solely on gas use was 1.2.

Natural gas prices varied monthly from a summer low of $0.36/therm to a winter high of $0.59/therm. The heating season cost was $406 for gas, $55 for electricity to run the outdoor unit, and $52 to operate the indoor blower. Cooling costs were $38 for gas, and $10 for each of the indoor and outdoor sections. A condensing gas furnace with a seasonal efficiency of 90 percent would cost $450 to operate at $58.5/therm. This cost is about equal to the cost of operating the gas engine

R e s u l t s

39

In northern climates heat pumps are sized to meet the heating load. This nec-essary practice results in the heat pump being oversized for the cooling load. When heat pump capacity is larger than necessary it runs less and therefore has less opportunity to remove moisture. This logic has led to the concern that undesirable damp conditions might be created during the cooling season. Manufacturers have responded by providing units with multispeed blowers and two-speed or dual compressor units. These options provide a smaller first stage of cooling that would run for longer periods and remove more moisture.

Only two of the demonstration sites had special features to enhance dehu-midification. The other sites had simple single speed units. The gas engine heat pump had a variable speed blower and compressor that was able to match the cooling capacity to the load while minimizing the need to cycle the unit off. While there was no explicit humidity control, the longer run time of the unit promotes moisture removal. One of the geothermal heat pump sites used a two-speed compressor to allow better capacity control. Again the longer run time in low speed promotes dehumidification.

Humidity and temperature data from the demonstration sites are useful in assessing the dehumidification performance. All of the sites maintained humidity levels below outdoor levels. Figure 20 compares the outdoor humidity level and indoor humidity level while the heat pump was running. The first five sets are from the 1995 cooling season and the remaining sets are from 1996. Site three used the gas-engine driven heat pump with variable capacity. Site 7 used a two-speed heat pump.


40

Figure 20: Comparison of indoor and outdoor humidity levels

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 1 2 3 4 5 6 7

Site

Indoor

Outdoor

1995 1996

Assessing the level of comfort is subjective. The American Society of Heating Refrigeration and Air-Conditioning Engineers has developed a criteria for comfort based on space conditions. It is partially bounded by a minimum temperature of 74ºF and a maximum relative humidity of 60 percent. Figure 21 shows the comfort region on a psychrometric chart. The chart relates the air temperature and relative humidity and absolute mois ture levels. The seasonal average space conditions are superimposed on the chart to show their relationship to the comfort region. Each site is denoted by its number. There are duplicate numbers for sites with multiple seasons of cooling data.


42

Economics

Operating Costs

Operating costs depend on equipment efficiency, energy prices and loading. These factors can vary considerably making savings estimates somewhat subjective. The following discussion identifies typical values and shows the sensitivity of savings to assumptions.

Table 14: Demonstration site energy prices

Site Electric Utility Electric Price Gas Utility Gas Price

1 Adams-Columbia Electric Cooperative

5.7¢/kWh no natural gas available

2 Sun Prairie Water and Light 5.3¢/kWh Wisconsin Gas 57.5¢/therm winter average

3 Wisconsin Public Service 6.0¢/kWh WPS 47.4¢/therm

4 Eau Claire Electric Cooperative 3.6¢/kWh interruptible, 6.9¢/kWh normally

no natural gas available

5 Wisconsin Public Service 6.0¢/kWh no natural gas available

6 Jump River Electric cooperative 3.7¢/kWh interruptible, 7.8¢/kWh normally


7 Wisconsin Public Service 6.0¢/kWh WPS 47.4¢/therm

9 Peppin Electric Cooperative 2.8¢/kWh interruptible, 6.3¢/kWh normally


Number 2 fuel oil 10/96 was $1.02/gallon, earlier $0.85/gallon; Propane 1996 $0.89/gallon, 1995 $0.95/gallon, earlier $0.75/gallon

Energy prices not only vary throughout the state, but also vary from year to year. Propane prices have had large swings with a recent low of 48¢/gallon to a high of over $1.30/gallon. Oil has ranged from 74¢/gallon to $1.00/gallon. Energy prices of propane and oil are more volatile than natural gas and electricity, but there are significant variations in electricity pricing depending on the local utility and rate. Residential rates are as low as 2.8¢/kWh in the winter on a dual-fuel interruptible rate, as high as 7.9¢/kWh on a flat rate, or as high as 13.5¢/kWh on-peak with a time-of-use rate. Winter natural gas rates range from 47¢/therm to 64¢/therm. With these wide ranges of energy prices operating costs are very sensitive to location and the utility serving the area. This variability is predominate in the current GHP market since most of the market is in rural locations serviced by local energy providers.

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43

Figure 22: Operating cost comparisons

$ -

$200

$400

$600

$800

$ 1000

$1200

GHP Add-on(2.8-3.2

COP)$0.035/kWh

GHP (2.8-3.2 COP)

$0.07/kWh

Natural GasFurnace

(80%-90%)

Fuel OilFurnace

(65%-80%)

LP Furnace(80%-90%)

$0.07/kWh, $0.65/therm, $0.75/gallon LP, $0.85/gallon oil100 MMBTU Heating, 10 MMBTU Cooling

$100 $180 $300

$340 $440 $520 $640

Savings

Savings

The operating cost comparisons in Figure 22 show several scenarios with a range of electricity prices for the GHP and a range in efficiency for all of the technologies. The lines on the top of the bars indicate the sensitivity of the costs to the level of efficiency. The first GHP bar is for the low cost dual-fuel interruptible electricity rate. With half-price electricity savings more than double. The second GHP bar represents the operating cost with full-price electricity. Several of the demonstration sites had lower cost electricity at or below 6¢/kWh. This reduction adds $100 to the savings levels shown in the chart. The two electricity price scenarios bounds the GHP savings for variations in electricity pricing. The cooling portion of the savings amount to only $10 to $30 since the total cost of cooling with a standard 10 SEER air-conditioner is only $70.

Actual site results in Figure 23 mirror the general operating cost comparisons of Figure 22. Site one had seasonal loads similar to the general case and a lower electricity price of 5.7¢/kWh. Site two had significantly smaller loads due to the house size and construction design. Site four was on an interruptible dual-fuel electricity rate at 3.5¢/kWh resulting in the largest absolute savings. Natural gas was not available at sites one or four. Table 15 presents the numerical values from Figure 23.


44

Figure 23: Operating cost comparisons at demonstration sites

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Site 1 (4-ton) Site 2 (2-ton) Site 4 (4-ton) Site 6 (4-ton)

GHP NG OIL LP

Table 15: Operating cost comparisons at demonstration sites

For most of the market where natural gas is unavailable the savings are in the $200 to $300 range. Where the utilities offer special electricity pricing, savings reach $500 to $700 annually. Recent propane price spikes in December of 1996 of over 1.37$/gallon dramatically increase the annual GHP savings over a 90 percent efficient furnace from at least $530 to $1230 at site four.

Initial Cost

The operating cost savings came at a price. There are two main elements increasing the price of GHPs: equipment cost and loop cost. The heat pump equipment costs about $1000 more than a furnace with and air-conditioner. The ground heat exchanger loop cost adds $3000 to $4000 to the project. Furthermore, the sites with the lowest price electricity on the dual-fuel interruptible rate require the purchase of a stand-alone propane furnace.

Site Annual Cost GHP Total Savings ($) Cooling Portion

($)

NG OIL LP ($)

1 850 30-120 60-230 200-320 10

2 190 80-100 90-140 130-160 35

4 400 360-450 400-560 530-640 15

6 540 440-640 435-640 610-740 5

R e s u l t s

45

Site 1

The cost for the complete 4-ton vertical system was $8850 for the heat pump and loop. There was only minor duct work at the unit since this was a retrofit installation. The loop was subcontracted for $4200. The cost of the heat pump was $3345, leaving $1300 for labor and other components. The cost of an oil furnace with air conditioner would have been at least $4000 leading to an incremental cost of no more than $4800.

Site 2

The cost for the complete 2-ton vertical system was $7900 for the heat pump, loop and duct work. The loop was subcontracted for $2400. Duct work is typ-ically between 1.0 to 1.25 $/ft2 of floor area or about $1600 for this site. A complete gas furnace with air conditioner system would have cost under $6000 leading to an incremental cost of at least $1900.

Site 3

The cost of the gas engine heat pump was $6600. A condensing natural gas furnace with high efficiency electric air conditioner would have been $4250. The incremental cost of the gas engine heat pump was $2350.

Site 4

The 4-ton horizontal system also included a condensing propane furnace for auxiliary heat when the utility exercised its radio load control. Based on quotes for both the actual system and an alternative propane furnace with electric air conditioning, the incremental cost was $5975.

Site 5

This site was the contractor’s house. The heat pump was already in place using well water. He installed a hybrid slinky loop for the system. Based on quotes for other jobs the incremental cost of the horizontal GHP system would have been about $4000

Site 6

The hybrid hydronic system had additional costs for the in floor slab heating system. The basement hydronic floor slab heat added about $3500 above the cost of a standard GHP air-only system. With the exception of the heat pump being a water-to-water unit, the GHP system is similar to the dual-fuel system at site four. The added cost over a standard propane furnace system would include the heat pump and loop for about $6000.


46

Site 7

The contractor from site five helped provided the two-speed heat pump out of inventory for reduced cost. The homeowner installed the loop himself with his own equipment.

The summary of Table 16 is based on the mid-point of the savings range for the type of heating system most likely to have been installed. This summary does not include any incentives other than the half price electricity load control program at site four and six.

Table 16: Simple system payback summary

With simple paybacks of at least eight years, either creative financing or a long timeline is required to broaden the GHP market. Figure 24 illustrates the sensitivity of life-cycle cost to the energy price and investment life at site four. This result is typical for the site on the half-priced interruptible rate. Sites with full-priced electricity had paybacks approaching 20 years, making the rate of return just break even after 20 years.

Site Incremental Cost Annual Savings Simple Payback

1 4800 150 32.0

2 1900 90 21.1

4 5975 590 10.1

6 6000 675 8.9

R e s u l t s

47

Figure 24: Rate of return sensitivity to investment time frame and energy price

-16.0%

-14.0%

-12.0%

-10.0%

-8.0%

-6.0%

-4.0%

-2.0%

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

12.0%

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Year

$0.75/gallon LP

$0.95/gallon LP

Dual-Fuel Rate $5975 Increment

$552 & $779 Savings

The economics are best for sites with the half-priced electricity. The electric utilities participation makes these systems cost effective. Not surprisingly most of the closed loop heat pumps are installed in areas with this load controlled incentive rate. At the other sites savings are in the $200 to $300 range, but incremental costs were close to $4000.

Most of the system equipment sizes were large enough to meet over 95 percent of the heating load. Reducing the size of the equipment reduces the initial cost, but requires more auxiliary heat use. All but one site used 4-ton equipment. In a split system arrangement a reduction in size from 4-tons to 3-tons saves $360 of heat pump cost and $240 or loop pipe cost. This 25 percent reduction in capacity would increase the auxiliary heat use by 100 hours or 4.0 million BTU at site four. With propane at 75¢/gallon the added annual cost would be about $40. The smaller size only decreases the simple payback by less than half a year. The annual rate of return in Figure 24 increases by 40 to 60 basis points on a 20 year time frame depending on the propane fuel cost. This analysis favors a smaller heat pump unit, but small change in economics indicate that the four-ton heat pump size is near the optimal economic size. If the auxiliary heat were electric resistance the incremental annual operating cost would nearly double and negate most of the economic advantage to smaller sized equipment.

Utility perspective

The utility load control program drives most of the closed loop geothermal heat pump installations in Wisconsin. These cooperatives offer these rates to improve load factor and gain heating market share from propane and oil. They gain a sheddable weather dependent load with a lower peak than electric resistance.


48

The investor owned utilities have not been as aggressive with GHP promo tion. Their markets are different. They typically serve large populations having natural gas available. The investor owned utilities also are under more regulation that does not allow the promotion of electric heating for load growth.

Societal perspective

The societal perspective is generally reflected through the regulatory priorities and restrictions placed on the utilities. Any choice related to public policy and the environment represents a compromise among a set of priorities of diverse interests.

The most notable societal benefit of GHPs is the elimination of multiple point-sources of pollution. The use of centrally produce electricity allows pollution to be managed at the generation facility rather than at the consumers’ locations and leaves the fuel choice decision to the utility. The overall pollution levels between various heating fuel choices depends on the generation dispatch. A detailed analysis is beyond the scope of this study, but Figure 25 shows a simple comparison between generating electricity with a fossil fuel for use in a GHP or burning a fuel directly. A GHP with a seasonal heating COP of 3.0 is about equivalent on a source energy basis with a fossil fuel furnace with a 90 percent seasonal heating efficiency. Obviously significant generating components of nuclear or hydroelectric power would reduce the amount of fossil fuel and thus the level of pollutants for the GHP. On a longer term, however, other issues such a nuclear waste disposal, or land use become concerns.

Figure 25: Source energy comparison example between fossil fuels and GHPs

100 MMBTU

100 MMBTU

Generation33%

Transmission& Distribution

90% GHP 300%

Furnace 90%111 MMBTU

112 MMBTU

Input OutputComponent Efficiencies

49

Discussion Both the geothermal heat pump and the gas engine heat pump add to consumer choice. They both meet comfort requirements with reliable operation.

Geothermal Heat Pump economics start to become attractive in rural setting where natural gas is unavailable, especially in those areas offering half-priced electricity for the heat pump. Other issues also make the GHP technology attractive—comfort, no on-site fuel storage, stable electricity prices relative to propane and oil, and centralized pollution management. Recent increases in propane prices from 75¢/gallon to 1.30¢/gallon illustrate the advantage of stable electricity prices. Annual savings more than double from $600 to over $1200, in turn reducing the payback from ten years to under five years.

Gas Engine Heat Pumps advance natural gas heating and bring residential natural gas cooling to market. The Wisconsin climate caused heavy reliance on the lower efficiency auxiliary boiler, reducing the total system efficiency. With operating costs nearly equivalent to conventional high efficiency equipment, the economics don’t support the added first cost. There are continuing efforts to reduce initial cost, such as eliminating the auxiliary gas boiler. The current cost premium and low energy prices make it difficult to justify the GEHP solely on economics in Wisconsin.

Some of the varieties of ground heat exchanger designs currently in use in Wisconsin have shortened both total length and installation time when compared to typical practice. A loop can be installed in as little as one day, and it is possible to install horizontal loops on mid-sized subdivision lots using a slinky configuration. Demonstrated loop lengths between 600 and 900 ft/ton showed the longer loops performing with warmer temperatures (as much as 5ºF). However, the shorter loops allow placement on smaller lots and reduce the loop material cost by at least $250.

Minimal auxiliary heat was required at most of the sites (less than five percent), indicating that smaller capacity units could be used to reduce the initial cost slightly. The operating cost would increase slightly (10 percent at site four) as the use of auxiliary heat would increase. Increasing electric resis tance heat use would also have a negative impact on the utility load factor. Sharply higher electric peaks would be reached in the coldest weather. For the site using fossil fuel auxiliary heat, the utility impact of smaller capacity equipment would be slightly reduced electric revenues.

Comfort levels were maintained with the heat pumps. Even with the lower supply air temperatures than with fossil furnaces people were satisfied with the comfort levels. The more frequent operation of the unit keeps the air mixed throughout the house, improving the distribution of heat. The oversized cooling capacity was not a problem. It reduced the interior humidity levels and maintained them within the industry recommended comfort levels.


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This project demonstrated the technical feasibility of enhanced heat pumps in Wisconsin. Although the gas engine heat pump system performance suffered under the constraint of being air-source, the geothermal heat pump benefited from the ground-coupling. The variety of ground heat exchanger designs available make GHP systems possible in most residential settings. Economically both systems incur a larger initial investment requiring longer time horizons for them to become cost effective. Ultimately the longer term economics along with unique system characteristics will have to be valued by the consumer before they are accepted in the marketplace.

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References Energy Center of Wisconsin. 1996. Market Assessment of New Heat Pump Technologies. Madison, WI: ECW.

ENERGY CENTEROF WISCONSIN

595 Science Drive

Madison, WI 53711

Phone: 608.238.4601

Fax: 608.238.8733

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www.ecw.org

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