Final Report: Heating Attic Air Using Solar Thermal Energy for Space Heating and Drying Applications

EPA Grant Number: SU836024
Title: Heating Attic Air Using Solar Thermal Energy for Space Heating and Drying Applications
Investigators: Chau, Kenny , Agbonwaneten, Etinosa J. , Lozano, Jesse S. , Matsumoto, Mark , Nguyen, Nhat , Opot, Stephen R. , Tam, Kawai , Villanueva, Ariana E.
Institution: University of California - Riverside
EPA Project Officer: Hahn, Intaek
Phase: I
Project Period: August 15, 2011 through August 14, 2012
Project Amount: $15,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2011) RFA Text |  Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Built Environment , P3 Challenge Area - Energy , P3 Awards , Sustainability


Using a solar-powered attic fan, harnessed solar thermal heat from a rooftop solar heat collector and heated air from the attic will be diverted through a solar thermal closet and air ducts throughout the home to provide both clothes drying, and space heating applications as needed. This system will be tested in a community in Moreno Valley, California. Suitable temperatures for both applications will be achievable with the use of airflow valves; allowing for space heating during colder months and venting of hot air to the exterior during hot summer months. Since the solar thermal closet replaces gas and electricity with solar thermal energy, this results in net-zero energy consumption. Although capital costs make up the majority of the total costs to incorporate this system, replacing conventional drying and space heating units in homes via solar heating addresses the challenge to sustainability. This project was part of a senior design project with the participation of members from the Engineers Without Borders-UCR student chapter in collaboration with the developer of the Victory Gardens (VG) community in Moreno Valley, California. We will voluntarily hold classes to educate the community residents in regards to topics such as water and energy conservation, and its contribution to sustainability.

Energy consumption from air heating units such as clothes dryers and space heating (e.g. forced air) make up 15.9% of the total energy demand in the residential sector. An innovative design that combines heated air found in home attics and the use of solar thermal energy to heat ambient air for these air-heating home appliances and space heating needs can sustainably reduce the overall energy contribution from this sector.

Solar collectors comprising of aluminum will be installed on the roof of a model home and evaluated to determine the effective time for drying clothing. Thermocouple and humidity sensors will be installed at selected locations of intake and exhaust ducts to characterize the heat transfer capacities and will be used to design the space heating capabilities for the home.

Contribution to Pollution Prevention: The processes for generating electricity and combustive fuels for gas to power these drying and space heating applications uses produce particulate matter and greenhouse gases (GHG) that do not meet the standards regulated by the Clean Air Act, which enforces regulations on emissions with adverse effects on human health. Switching to alternate space heating and drying methods will decrease the household dependence on these energy sources and consequently reduce air pollution output from these appliances. Thus, this design of a solar-thermal closet used as an alternative to clothes drying with the option to be used for space heating will decrease the overall energy consumption of a home.

Summary/Accomplishments (Outputs/Outcomes):

The outputs and outcomes focused on four components: design of the solar thermal collector, effectiveness of drying, space heating potential, and an economic assessment of the complete working system. The design of the thermal capacity of the combinatorial solar thermal closet, space heater, and humidification system for the VG development was dependent on the meteorological conditions of Moreno Valley, CA. The monthly maximum and minimum temperatures from 2008 to 2010 were assessed to establish the potential extreme lows and highs of the inlet air temperatures that were heated in the solar thermal collector. During this period of time, the lowest temperature was 32.7°F and the highest was 107.1°F. This dictated the size of the solar thermal collector which accounts for the volume of air passing through the pipes to achieve the desired temperature of 155°F, which is comparable to the average temperature of conventional clothes dryers1. An Excel software program was made to optimize the solar thermal collector design by varying the operating parameters such as the ambient temperature and air flowrate. We determined that the solar collector with a pipe diameter of 4 inches and pipe length of 7 feet would require a total of six passes to achieve 155°F.

Our design of the solar collector was constructed primarily with rigid, aluminum metal sheets that were purchased and cut at a local metal shop, and aluminum ducting that is commonly used for conventional dryers. The solar collector was installed on the roof of a home with thermocouple sensors monitoring the ambient air temperature and produced exit air temperature. Throughout the month of February, our designed system was able to produce exit temperatures up to 137°F throughout peak hours (ranging from 12 to 2 pm) of the day from an ambient temperature of about 84°F. Insulated ducting was used to direct the heated air into the thermal closet.

An additional thermocouple probe was placed in the entrance of the thermal-closet to determine the transfer efficiency of the exit air as it was diverted from the collector into the closet via insulated ducts. Based on the exit temperature from the solar collector, we determined the length of time required to achieve drying of clothes to be approximately 1.5 hours. Before wash, clothes comprising of various types of fabric were initially weighed and recorded. We characterized the dryness of the clothing by measuring the initial and final weight over time. Complete dryness was determined when the final weight over time in the thermal-closet approximately reaches the initial weight recorded. We presented our data during the community meetings held by the developer and received hopeful responses from the residents regarding the clothes drying aspect; however, they were interested in the space heating capability of our design as the homes get relatively cold during the winter months.

As we are proposing that the heated air will be vented into the homes after clothes drying, we have monitored the relative humidity (RH) of the air during the clothes drying experiments. It was recognized that the humidity in the thermal closet reached an upward of 45%. Housing regulations require indoor humidity to be lower than 50% to prevent health related issues such as molding and mildew in the homes. An automatic ventilation system that is controlled by a humidistat controller will be required to ensure that air containing excess moisture greater than 50% is properly vented out of the homes as well as to reduce the amount of stagnant air in the living space. In turn, this will reduce the risk of the exposure of higher concentration of micro pollutants that may collect in the homes.

To demonstrate the economic feasibility, evaluations of profitability were done to compare the installation of the proposed system to installation of a conventional dryer and space heating system in the VG community. The criteria to evaluate profitability include the benefit or worth of the project at the end of its lifetime, the time to recover the money invested in the system, and the rate at which money was made on the investment. The cost-to-benefit ratio was determined to be approximately 4, proving the project is economically acceptable. It was determined that it would take approximately 5.3 years for the solar thermal energy system to recover the money used to install the system. The average annual net profit was determined using profit from energy conservation over the lifetime of 20 years and the fixed capital investment to install the system. The rate of return on investment was determined to be 23.4%.


We have achieved exiting air temperatures up to nearly 140°F from the solar collector based on an ambient temperature of 85°F during peak hours of the day (12 to 2 pm). Our clothes drying experiments have indicated that approximately 1.5 hours is required to achieve complete drying of the clothes. However, these experiments were evaluated over various days throughout the month of February, which is considered as one of the colder months. We expect the solar collector to produce a higher exit temperature over warmer months, which in turn would reduce the time required in the closet to achieve complete drying of the laundered clothes. During the clothes drying experiments, the air that was vented from the thermal closet reached a maximum 40% relative humidity. We are able to provide the residents with heating and humidification that is produced from drying their laundered clothes in the thermal closet as the optimal conditions for in-home relative humidity ranges from 30 to 50%. The use of heating and humidification is beneficial especially during the colder months as the air gets relatively dry with less than 30% relative humidity.

Supplemental Keywords:

alternatives, residential, home improvement, green construction, innovative technology, pollution prevention, ambient air, renewable, public good, cost-benefit

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P3 Phase II:

Heating Attic Air for Space Heating and Dryer Applications Using Solar Thermal