Harnessing Dorm Dryer Heat

Problem Statement:Our team addressed the problem of heat waste from dryer exhaust. Currently, there are seventy-two dryers for the dorm residents. As required by building codes, dryer emissions have to be exhausted to the outside of the building. Unfortunately, this exhaust contains a lot of heat that is lost to the atmosphere. This heat is energy that can be harnessed and used while still exhausting the emissions by use of a heat exchanger. As there are more than 3500 students using these dryers, the heat lost is substantial. This has a significant environmental footprint because natural gas is being consumed to heat these dryers and then excess heat is just wasted. Reusing the heat for another purpose will increase the amount of useful energy extracted from the gas and offset that cost. Natural gas is a fossil fuel, and burning it leads to GHG emissions like CO2. Even if electricity was used, the power plants that provide the campus with electricity are most likely burning fossil fuels. Reducing our fuel consumption on campus will decrease overall emissions.

Project Summary/Background: There are several laundry rooms on campus that have dryers running throughout the day. By using either an air-to-air or air-to-water heat exchanger, we hope to capture the heat lost from the dryers to recycle that heat back into the building, either to preheat the water supply or heat the building itself. In doing this, we hope to reduce the energy lost to the environment by adding it to the dorms energy supply.The goal of our project is to quantify how much energy we can actually harness from the dryer exhaust and the cost savings associated with it. To achieve this, our team collected temperature and mass air flow data from one of the laundry rooms. These values were then used to figure out how many therms of energy were in the exhaust and how much energy would be available taking into account the heat exchangers efficiency. Based on the current infrastructure, there are minimal environmental trade-offs associated with harnessing dorm dryer heat. Although energy and resources are used in the production and installation of the heat exchanger and ducts, these tradeoffs can ultimately be offset with the environmental benefits from energy efficiency and conservation from implementing the heat exchanger. This project is based off a very similar experiment done by students at WIT. They found the cost savings from harnessing the heat exhaust coming from the dryers and using it to preheat the water that feeds into the boilers. From their analysis, they found that the average water temperature that enters the boilers is only 4.4C. After several passes through the heat extractor series, the water temperature would raise to 33C. This significant increase in inlet temperature allows for the boiler to require less energy to bring the water to the necessary output temperature. Overall, the students calculated that their campus could save $3.90 for every 300 gallons drawn to the boiler. Similar to the WIT study, our team also captured dryer exhaust temperature and mass flow rate. However, our team decided that a friendly user interface for figuring out cost savings would be beneficial and make calculating energy savings from dryer exhaust easier and more accessible. This innovative approach will allow other universities and commercial dryer complexes to easily find their cost savings and decide if it is economical as well as environmentally beneficial to recover the heat.Finding a way to reuse this energy can pave the way for other schools to do the same. With enough universities and commercial dryer complexes recovering dryer heat, we will reduce the amount of natural gas extracted by fracking. Becoming more efficient with energy will help lead the world to become better at self-sufficiency and reduce the need for non-renewable resources.

Relationship to Sustainability:Sustainability can be defined as the ability to maintain the essentials of society today and for the future. This includes using natural resources efficiently. By recycling the heat energy lost from the dryers in the dorms, we can make the dorm buildings more efficient by reducing the amount of energy they need. By successfully reusing this heat, we can help reduce the amount of energy used in heating other elements of the building. This can reduce the amount of natural gas needed in the building, thus reducing costs for the school and its dependency on fossil fuels. Our campus experiences cold, long winters, so any way to help heat the buildings can become very useful in the long run.This project relates to the three pillars of sustainability, economic, environmental, and social, by promoting energy efficiency. Capturing and reusing exhaust heat will replace heat that would otherwise be generated from burning natural gas. This will have a positive impact on the local environment due to the potential to reduce the use of fossil fuels and in turn reduce the campuses carbon emissions. The reuse of dryer heat has the economic benefit of offsetting campus natural gas costs.

Materials and Methods:In the dorm laundry rooms, the dryers are stacked on top of each other, but each dryer is separately exhausted to the outside of the building. As seen in Figure 1, we assigned a number to each dryer for reference throughout the project. Temperature and velocity data were collected in order to complete the energy savings analysis of adding a heat exchanger (either air to air, or air to water) to the system. We collected data in one of the laundry rooms used by dorm residents. In order to collect data and learn more about the dryers on campus, we worked with a staff member who provided tours, equipment manuals, and access to restricted areas for data collection. Furthermore, our advisor and the mechanical engineering department provided our team with thermocouples and an anemometer to aid in data collection.

Figure 1: Dryer Set-up

We completed two control experiments (one with dryers 1,4,5,8, the other with dryers 2,3,6,7) in which we placed one towel in each dryer at high heat for 30 minutes and collected the exhaust temperature from each vent. The mechanical engineering department lent our team four thermocouples for the project. We inserted a thermocouple into the vent of each dryer; they were attached to the USB data loggers to collect temperature data that was later put in Excel. Furthermore, we inserted a USB data logger into one of the dryer exhaust vents to record temperature data every 30 seconds for a week. The USB data logger was hung to the middle of a wire in the center of the vent to achieve the most reliable reading. This allowed our team to obtain data to approximate an average cycle time and capacity factor for the overall dryers. It was also necessary to calculate the flow rate of the exhaust air. We obtained velocity data for dryers 3, 5, and 6 using a platinum wire anemometer provided by our advisor. An anemometer was inserted in a four inch exhaust duct. The averaged mass flow rate was taken at half inch intervals to create a profile. For this study, relative humidity data was not collected and was assumed to be 60%.Some of our teams major milestones include finalizing the data collection procedure and obtaining the necessary tools from the grant money and on campus labs. This marked the completion of our project planning and research phase. The next major milestone was reached when we successfully obtained accurate data that was available for analysis. Our final milestone was the completion of the analysis and generation of final conclusions. Results:Assumptions: Neglect variability in clothing moisture content All dryers operate at an equal amount of time (average cycle time used as capacity factor) Exhaust temperatures are constant in all dryers (average used) Neglect moisture phase change, aka at steady state Density of air is 1.225 kg/m^3 Specific Heat of air is 10006 J/kg-K Ground temperature of water on campus is 8 C Average air temperature in Rochester in the winter is -7 C Heat exchanger has a 70% emissivity Relative Humidity at 60%

Table 1: Reference ValuesFrom the control procedure explained above, thermocouples were used to collect temperature data. We calculated the temperature for each dryer and used the average temperature of 53.6C as our dryer exhaust temperature (Graph 1).

Graph 1: Control Temp DataDryerTemp [C]

154.6

257.2

358.0

456.5

557.4

656.5

741.6

847.2

Average53.6

Table 2: Temp DataThe graph below shows the data for the temperature of the exhaust air from Dryer 1. The data was broken up into each dryer cycle. Based on the different cycles, we were able to calculate the average cycle time, which was 75.35 minutes. It was determined that the capacity factor of the dryer was 0.36. This was concluded by taking the sum of the lengths of all the cycles (2.5 days) and dividing it by the total time interval we collected data (6.93 days).

Graph 2: USB Logger DataUsing the anemometer, multiple trials to find the mass flow rate were implemented. We assumed parabolic flow; however, our data showed that this wasnt the case. We came to the conclusion that air traveling from the outside enters the ventilation and changes the parabolic streamline of flow. We averaged our values from our flow profiles and trials to get a mass flow rate of .04kg/s. To find the specific capacity of humid dryer exhaust air, Equation 1 was used.

Equation 1To find the amount of savings from the heat exchanger, we first calculated the max amount of energy that would be saved if all the heat from the dryer could be transferred to the water (or air) running through the heat exchanger. We then multiplied the max energy ( ) by the emissivity () for a typical heat exchanger to obtain the actual amount of energy that would be transferred (). Equation 2 Equation 3 Equation 4These equations were then used to calculate the energy savings for an air to water heat exchanger and an air to air heat exchanger. We compared our measured data with the values provided by the dryer manual. The results are shown in Table 3 below.

Table 3: CalculationsAs seen from Table 3, the campus would save $1085 per year if an air to water heat exchanger was installed for the eight dryers in this laundry room, and $1442 for an air to air heat exchanger. When calculating the savings with the theoretical data provided in the manufacturers manual, the estimated annual savings would be four times higher. However, due to the age of the dryers, our collected values are more realistic.

ConclusionOverall, the project proved that harnessing the dorm dryer heat would lead to energy and cost savings for the university. Looking at the other dorm laundry rooms, if heat is harnessed from all seventy-two dryers the cost will be $9766 and $12978 respectively for water and air. In the future, our team could further investigate relative humidity by collecting our own RH values. We would also analyze the environmental impacts of adding a timer to the dryers, as currently there is a great variability in dryer cycle time from different users. Our data indicates that the cycle times range from 30 minutes to 120 minutes. A possible future expansion would be to see if we could reduce energy consumption by reducing the amount of time the dryers run when not needed (ie the clothes are already dry but the dryer continues to run). While this project may be implemented in the future, it is not likely to be implemented in the near future. The university has indicated that they are looking to upgrade the entire dryer system in the a few years, as the current system is getting old and the current maintenance costs have increased. It would be best economically to install these heat exchangers when the new dryers are installed. The results of this project have shown that harnessing dryer heat is environmentally, economically, and socially feasible and beneficial.

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