Technology
The primary goal of our engineering systems was to balance the effects of efficiency, cost, sustainability, and marketability. In the pursuit of this goal, we have developed a fascinating system that splits energy into two types and uses each for their most efficient purposes.
TWO KINDS OF ENERGY
We realized early on that there is a significant difference in the collection efficiencies of solar electric and solar thermal energy. We discovered that, depending on the season, around 65% of the total energy load in our house was thermal in nature - including space heating, cooling, and domestic water heating. Since solar thermal collectors are substantially more efficient than solar photovoltaic collectors we decided to address all of the thermal loads with thermal energy collection and only the electric loads with photovoltaic collection. This clearly results in increased thermal efficiency for heating loads. To address the cooling loads we have implemented an absorption chiller. At this time absorption chillers are very uncommon in residential settings, so our system is more complicated and expensive than future installations need to be. Nevertheless, we fell it is important to show that absorption chilling is a viable residential technology that should be explored.
Our electrical system is fairly similar to many others. We chose energy efficient appliances and light fixtures to minimize electrical consumption while maintaining a traditional level of functionality.
BIPV AND ELECTRICITY
The PV panels we are using are cleverly integrated into the construction of the roof itself. The dimensions of the Sunpower panels match exactly the width of our house sections when three are put side by side. In the same way that we have a rain screen separated a couple inches from the exterior walls, the PVs are lifted 3 inches off the waterproof roof layer, both protecting the roof and providing ventilation to the back sides of the PVs.
Due to the integration with construction and transportation restrictions, the PVs are not at an ideal angle, but we are confident that they will supply sufficient electricity for our needs.
We have installed a battery array to store power during the time of the competition. Batteries are becoming less important to practical PV installations in general because of the wide availability of net metering and the high cost of maintaining a battery bank and batter interactive inverters. The batteries are just a minor detail needed to meet the requirements of the competition but are not a critical long-term component of most residential PV systems.
Inverters, on the other hand, are an important linkage in the photovoltaic system. We chose Sunny Boy string inverters because we could locate one small inverter on each of our four trailers, making it easier to connect the electrical system since the house must be transportable. Sunny Boy inverters are among the most efficient inverters available.
SOLAR COOLING
The singularly greatest innovation in our house is the complex system that uses evacuated tubes to create hot water.
The entire south face of our house is covered in fifteen Sunda Seido 2-8 Evacuated Tube arrays. Seido 2 tubes were chosen for their superior thermal performance, flow-through design and availability in the United States. Though the arrays will be mounted vertically, the tubes will be rotated individually so that the metal collection plate is 54 degrees above horizontal. Upon return from the competition, when the house is in its permanent location, the modules will be elevated to the latitude in Cincinnati to maximize year-round efficiency.
The hot water produced by the evacuated tubes, which reaches 190 degrees F, is stored in custom-built tanks in the house. One of those tanks holds the hottest water directly from the tubes, while another slightly cooler tank receives water from the first as the temperature declines. As needed, water is pumped back through the tubes to maintain a high temperature. The tanks serve as thermal storage batteries, and their capacity is greater when hotter water is stored in them.
In order to cool the house, the hot water is pumped through our Rotartica absorption chiller, a device that functions much like a typical air-conditioning unit, but runs much more efficiently. Absorption chilling uses a thermally activated vapor compression cycle, with Lithium Bromide, a salt, as its absorbent and water as its refrigerant. Both of these are environmentally friendly substances.
Absorption chillers are currently used primarily in industrial settings, but the device we are using is appropriately sized for residential use and is somewhat scalable based on the cooling demand of the house. While the cost is still quite high, we expect that with greater exposure and higher demand the price will reduce significantly in the future. In both heating and cooling situations, water is then cycled through a fan coil unit with a water-to-air heat exchanger.
In colder months, warm water will also flow through a radiant floor, gradually heating the house comfortably from the ground up. The radiant floor provides an even distribution of heat and fairly constant temperatures with respect to time. The radiant floor will minimize heating demand from the fan coil unit, which makes for a more comfortable living environment without the unsettling effect of air blowing around.
