Energy and the New Reality, Volume 1:

Energy Efficiency and the Demand for Energy Services

Chapter 9: Community-Integrated Energy Systems

L. D. Danny Harveyharvey@geog.utoronto.ca

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Community-Integrated Energy Systems consist of one or more of:

• A network of insulated underground pipes to distribute heat from various heat sources to where it is needed

• A network of insulated underground pipes to distribute cold water from a central chilling facility or natural source of cold water to where it is needed

• Cogeneration or a central heating plant• Trigeneration or a central cooling plant• Seasonal underground storage of heat or coldness

Figure 9.1 Pre-insulated district heating pipes in Copenhagen

Source: www.logstor.com

The world’s largest district heating systems are in:

• St Petersburg 237 PJ/yr• Moscow 150 PJ/yr• Prague 54 PJ/yr• Warsaw 38 PJ/yr• Bucharest 37 PJ/yr• Seoul 36 PJ/yr• Berlin 33 PJ/yr• Copenhagen 30 PJ/yr• New York 28 PJ/yr • Stockholm 27 PJ/yr

Figure 9.2 Percent of total space heating requirements met by district heating

0 20 40 60

Finland

Denmark

Sweden

Germany

France

USA

Norway

Netherlands

Canada

Percent of Space Heating

Source: Maker and Penny (1999, Heating Communities with Renewable Fuels: The Municipal Guide to Biomass District Energy, Natural Resources Canada and USA Department of Energy, www.nrcan.gc.ca)

District Heating

• Highly worthwhile if waste heat from electricity generation can be captured and used (cogeneration)

• This will reduce GHG emissions only if fossil fuel electricity production elsewhere is displaced, or if biomass is used

• If the absence of cogeneration, district heating can save energy through the more efficient operation of central boilers than of individual boilers (usually at part load) in individual buildings, although this advantage will not exist if new, condensing boilers are used in individual buildings

Other sources (besides electricity production) for heat for district heating are

• Sewage water (16oC in February in Tokyo)• Steam from incineration plants powering heat

pumps to extract heat from the exhaust of the incineration plants (done in Sweden)

• Low-grade geothermal heat in the return branch of a geothermal heating system, upgraded with heat pumps

Heat can be distributed either as steam or as hot water. Hot water provides a number of efficiency advantages due to its lower temperature than steam: • Less heat loss to the surroundings• Less sacrificed electricity production• Lower temperature heat sources can be used (in the absence of a heat pump,

the heat source has to be warmer than the temperature at which heat is distributed)

The lower the temperature at which heat is distributed, the better from an efficiency point of view

Figure 9.3 Variation in electricity generation efficiency, overall efficiency and electricity-to-useful heat ratio in a gas combined-cycle system as the temperature of the heat supplied to the district heating system varies.

0.0

0.5

1.0

1.5

2.0

0.00

0.25

0.50

0.75

1.00

40 90 140

Po

we

r:H

ea

t R

ati

o

Eff

icie

nc

y

District Heating Supply Temperature (oC)

Electrical Efficiency

Total Efficiency

Power:Heat Ratio

Source: Spurr and Larsson (1996)

Strategies for reducing the required supply temperature:

• Upgrade building thermal envelopes so that radiators do not need to be as hot as otherwise while still providing sufficient heat, and (in new developments) use radiant floor heating (which reduces the required temperature to 30oC or so)

• Use larger and more effective heat exchangers between the district heating hot water flow and the building hot water flow, so that there is less of a temperature drop between the two

• Increase the flow rate and/or use a peaking plant to boost the supply temperature during the coldest month

Upgrading old District Heating Systems

In many countries of Eastern Europe and the Former

Soviet Union

• Distribution systems are steam-based and lose up to 30 per cent of the heat that they carry

• The systems are also over-sized and so tend to operate at partial capacity (which lower efficiency)

• The buildings that they serve often lack thermostatic controls and are poorly insulated

Thus, in these countries the large potential energy savings is not in the expansion of district heating with cogeneration, but in the improvement of the efficiency of the existing system and of the buildings that they serve

Summary for District Heating

• Comparing modern district heating systems with modern on-site heating (using condensing boilers), there is little if any efficiency gain if generating heat centrally

• What little gain there is could be offset by distribution losses

• Reducing the losses in existing systems can yield large savings

• Taking heat from the generation of electricity with fossil fuels (cogeneration) will not reduce fossil fuel use if the alternative for electricity generation is to use renewable energy sources (such as hydro-electric power)

DH Summary (continued)

District heating can still provide a number of energy advantages over on-site heating

• Easier opportunities to store heat (within the network and in insulated thermal reservoirs or underground)

• Easier to switch fuels for heating (potentially important if we make the transition to hydrogen in the future)

• Easier to directly use renewable energy- biomass for cogeneration- electric heat pumps to use excess wind-generated electricity when it is available- geothermal heat

Non-Energy Advantages of District Heating

• Quiet and vibration-free heating at building sites• No need for emission of exhaust gases at the

building site• Reduced upfront costs for the building

developed• Savings in space, maintenance and insurance

costs

District Cooling

• Centralized operation of multiple chillers, each running at the fraction of full load that maximizes overall efficiency, can yield large energy savings compared to operation of equal sized chillers in individual buildings

• The use of larger chillers in centralized plants yields further savings, as larger chillers are more efficient than small chillers

• Savings from these two factors of up to 45% have been achieved

Non-energy advantages of district cooling

• Quiet and vibration-free cooling at building sites• Savings in space, maintenance and insurance costs• Reduced upfront cost for the building developer• No need for cooling towers on individual buildings

(reducing on-site costs and risks related to legionnaires disease)

• Frees up roof space for roof-top gardens or solar collectors (PV or thermal)

Societal Cost of District Cooling Systems Compared to

On-site Chillers

The total cost of district cooling can be less than for cooling of individual buildings, in spite of the cost of installing a district cooling pipe network, because

• Less total cooling capacity is needed (the peak loads of individual buildings do not all occur at the same time, so the system peak is less than the sum of the peaks of all the buildings in the system)

• Less total backup capacity is needed (it is not uncommon to have 2 equal-sized boilers or chillers in a given building, with one serving as backup. In a centralized system with 10-12 heating or cooling units, there might be only 1-2 backup units)

• Unit purchase costs are less for larger boilers and chillers

• Smaller operation and maintenance costs per unit of heating or cooling capacity

Cogeneration and Trigeneration

Cogeneration Sizing Options

• For one case study, energy savings is maximized if the cogeneration unit is about half the size that maximizes the area under the load duration curve

• Net energy savings initially increases as hours of operation increases and some heat is thrown away, because more less-efficiently generated electricity is displaced with more hours of operation

• As more heat is thrown away, the effective (marginal) efficiency of electricity generation decreases. When the marginal efficiency drops to that of the central powerplant x transmission efficiency, a further increase in the hours of operation will increase the overall energy use

Figure 9.4a,b Cogeneration sizing strategies: (a) maximizing the useful heat delivered (by maximizing the area of the rectangle under the load duration curve), (b) reducing the size of the cogeneration unit but maximizing the

number of hours that it operates per year.

heat demand

heat cogeneration “maximum rectangle”with thermal power and annual use Q U

load duration

Q

U

hea

th

eat

load duration

Source: Voorspools and D’haeseller (2006, Applied Thermal Engineering 26, 1972–1981, http://www.sciencedirect.com/science/journal/13594311)

(a)

(b)

Figure 9.4c,d Cogeneration sizing strategies: (c) Sizing the cogeneration unit as in (a) but with longer operation and some wasted heat; (d) using

multiple units.

heat demand

heatheat not usedh

eat

hea

t

load duration

heat demand

cogeneration 2

cogeneration 1

cogeneration 3

hea

t

load duration

Source: Voorspools and D’haeseller (2006, Applied Thermal Engineering 26, 1972–1981, http://www.sciencedirect.com/science/journal/13594311)

(c)

(d)

Trigeneration

• Involves taking heat from a steam turbine and using it to drive an absorption chiller

• Taking heat results in reduced electricity output• The sacrificed electricity could instead by used in an

electric chiller to produce chilled water for cooling purposes

• An absorption chiller, while not needing electricity for the basic cooling operation, requires greater auxiliary electricity use than an electric chiller for the operation of the cooling tower

• The net result is that it is often better to maximize electricity production, even though this requires throwing away some waste heat, and using the extra electricity in large and efficient electric chillers

Pollutant emission with cogeneration

These can go up or down compared to separate production of heat and electricity, depending on the efficiencies and emission factors for the cogeneration facility and the alternative systems

Differences in human exposure to emissions, related to the location and altitude of the emission sources, must also be considered.

Cost savings with decentralized electricity production through cogeneration

Costs with decentralized production will tend to be lower due to

• Reduced transmission and distribution costs (from an average of $1300/kW to $100-200/kW)

• Less needed powerplant capacity due to lower transmission and distribution losses

• Less needed backup capacity (4-5% instead of 15%) due to having many smaller units subject to random failure

• Revenue obtained by sale of heat produced through cogeneration