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6. Mechanical system
In accordance with the modularity concept pursued throughout the design of ElementHouse, custom mini-split heat pumps were installed in each module of the building. The heat pumps were designed and built by the team. The interior heat exchangers of the mini-split heat pump systems were composed of a drop ceiling of wire-on-tube heat exchangers of the type used in static domestic refrigeration condensers. The heat exchangers used radiation and natural convection to provide sensible heating or cooling to the space. Whether they provided heating or cooling depended on the setting of the valves of the heat pumps.
The heat pumps use R-22 as refrigerant and their compressors are electrically powered. The mini-split heat pumps handle the sensible loads of the house. An off-the-shelf dehumidifier handled latent loads. By separating the sensible and latent load conditioning, the team acquired increased flexibility and better comfort conditioning control. Ventilation air was provided with a separate whole-house heat recovery ventilator (HRV) system. During the competition, the team did not use the HRV and instead relied on natural ventilation by opening doors. For water heating, during the initial stage of simulation through the simplified model, an economic comparison was conducted between using an electrically-powered heat pump water heater and using solar thermal technology. It is widely accepted that solar thermal is a highly feasible option for water heating in solar homes.
However, for this application, the use of a heat pump system could provide some benefits including the simplicity of using a single solar array, eliminating additional piping, and the flexibility of hot water supply. The economic comparison yielded similar life cycle costs for both options, with the solar thermal system having a slightly lower cost. Therefore, because of the above-mentioned advantages of using a heat pump and the relative economic feasibility, it was decided to develop a custom heat pump water heater system. In order to showcase the use of green refrigerants, carbon dioxide was used in the final prototype. Efficiency of this system was less than optimal, with coefficient of performance (COP) of 1.86 over the desired heating range. System malfunctions affected the performance of the system during the competition.
The final design was modelled for three locations with different climatic conditions: Springfield (Illinois), Sterling (Virginia), and Phoenix (Arizona). Photovoltaic electricity generation and heating/ cooling loads obviously change between these locations. Results for Springfield and Sterling were relatively similar, with balanced heating and cooling energy. Phoenix, on the other hand, had much higher cooling energy requirements with virtually no heating. This location also had more solar energy potential. As noted in Figs. 20–25, the PV design accounted for a significant portion of the energy being used for the electric car, which was part of the competition. If the house had not required a solar car, the size of the PV system could have been reduced.
7. Achievement of expected building performance and discussion
Excellent cooperation between engineering and architecture at different design stages is the key for delivering a comfortable and energy-efficient solar house. The team was awarded first prize in the Market Viability contest. ElementHouse drew considerable public attention to net-zero-energy houses. One valuable lesson the team gained was how to use indoor space smartly and identify all the design details which would optimize energy saving. Although ElementHouse has a very small footprint, visitors have commented that they did not feel cramped, as the space is well designed to be compatible with various activities. All functional spaces are over-lapped and expand into one another while affording visual privacy. The interior space is also naturally merged with the exterior space and decks to enlarge the living space. Visitors commented that occupants can live with daylight, sunlight, cross ventilation and good views in each area of the house. The size of the house was normally over estimated by house visitors.