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 exchanger’s 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 4.4°C. After several passes through the heat extractor
series, the water temperature would raise to 33°C. 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 team’s 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%
Reference Values
Vent Radius
r
Vent Area
A
Specific Heat BDA
Cp
Specific Heat Water
Cpw
Inlet Water Temp
Inlet Air Temp
Room Temp
Exhaust Temp
Capacity Factor
CF
Emissivity
Ɛ
Cost per Therm
Time
Δt
Number of Dryers
N
Satur. Vapor Pressure
Atmospheric Pressure
Mole Fraction of Water
Partial Pressure Water
Mass Fraction of Water
0.0508
m
0.008107
m^2
1006
J/kg*K
4182
J/kg*K
8
C
-7
C
24
C
53.6
C
0.36
0.7
1.3
$/therm
210
days
8
dryers
112.5
torr
760
torr
0.0888
67.5
torr
0.055117
Table 1: Reference Values
From 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 Data
Dryer
1
2
3
4
5
6
7
8
Average
Temp
[°C]
54.6
57.2
58.0
56.5
57.4
56.5
41.6
47.2
53.6
Table 2: Temp Data
The 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 Data
Using the anemometer, multiple trials to find the mass flow rate were implemented. We assumed
parabolic flow; however, our data showed that this wasn’t 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.
%𝑅𝐻 = 𝑃
𝑃𝑤𝑎𝑡𝑒𝑟
𝑤𝑎𝑡𝑒𝑟
[53.6C]
Equation 1
To 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 (𝑄
) by the
emissivity (Ɛ) for a typical heat exchanger to obtain the actual amount of energy that would be
transferred (𝑄).
𝜀=𝑄
𝑄
Equation 2
𝑚𝑎𝑥
𝑄
= 𝐶𝑝 𝑀(
−
/
)
Equation 3
𝑀 = 𝛴 ̇ ∆𝑡 = 𝛴𝑁𝑗 ̇ 𝑗 ∆𝑡 × 𝐶𝐹 Equation 4
These 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.
Notation
qfuel
air flow
air vel
mass flow
Exhaust Temp
Total Mass per yr
Actual Energy
Energy Savings Water
Energy Savings Air
̇
M
Q
Units
J/s
m^3/s
m/s
kg/s
C
kg/yr
J
$
$
Manual Dryer Average
-7033.71
-1330.87
0.10
0.04
12.81
4.50
0.24
0.04
57.16
12342980 2335450.82
3.96E+11
7.50E+10
4775
1085
6307
1442
Table 3: Calculations
As 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 manufacturer’s manual,
the estimated annual savings would be four times higher. However, due to the age of the dryers,
our collected values are more realistic.
Conclusion
Overall, 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.
References
"Specific Heat of Dry Air." Specific Heat of Dry Air. N.p., n.d. Web.
<http://www.engineeringtoolbox.com/air-specific-heat-capacity-d_705.html>.
Wetter, Michael. "Simulation Model Air-to-Air Plate Heat Exchanger.",
<http://simulationresearch.lbl.gov/dirpubs/42354.pdf>.
Baggett, John, Javon Campbell, Richard Lamothe, and Ryan Oppel. "Heat Extractor." N.p., 29
Apr. 2011. Web. <http://myweb.wit.edu/campbellj/Microsoft%20Word%20%20ReportTemplate429.pdf>.
"Average Shallow GroundWater Temperatures." EPA. Environmental Protection Agency, n.d.
Web. <http://www.epa.gov/athens/learn2model/part-two/onsite/ex/jne_henrys_map.html>.