Engineering Features - Photovoltaics
The roof is covered with 630 ft2 (7.2 kW), of Astropower single crystalline PV modules. Astropower is one of the few photovoltaic companies that is not owned by an oil company or other large corporate conglomerate. They are also unique in that they use reject silicon wafers from the semiconductor industry for their feedstock. The 65-module array will produce about 30kWh of energy on a sunny day. The plenum between the PV modules and the roof will be vented with a PV driven fan to increase the performance of the PVs by keeping them cooler. The hot air vented out can also be used to increase the efficiency of the clothes dryer, among other things. Because four modules (144 cells) in series are required to charge the 48-volt battery system, the main array must be a multiple of 4 modules. Therefore we have an extra module which cannot be used in the main array. This will provide more than enough power to directly drive fans for venting the PV array plenum. Simulations predict that with forced air cooling of the PV array, daily energy production will be increased roughly 5% over an unvented roof integrated array, with 10% higher peak power.
The slope of the PV array is slightly less than optimal, but on an annual basis, it will provide 90% of the energy of the optimal array, and the 5:12 and 3.5:12 slopes used are much more compatible with standard residential construction today.
Solar input from all sub arrays is controlled with maximum power point tracking charge controllers. During the hot summer weather, these will not have a significant advantage over the more common voltage following controllers. However, when the battery voltage is low, especially during the winter months, they will increase the actual power generation of the array by up to 20%.
The batteries and the control and conversion equipment are contained in the tech pod. Each module of the house will be pre-wired for 120 or 120/240vac, with junction boxes used to connect each module to the main AC breaker panel after assembly. Although we are paying a slight efficiency penalty in using only AC power throughout the house, this vastly increases the amount of equipment available to us, and allows the same design to be used for grid tie versions of the house, or even super efficient houses without PV systems. The only use of DC will be for lighting in the tech pod and battery room in case of catastrophic inverter failure (unlikely since we have four redundant inverters).
The integrated DC-AC power system from Outback Power systems is used to decrease wiring time, complexity, and size of the power system. This integrated system includes four of their new FX-2000 sinewave inverters and two 60-amp power point tracking PV controllers, in addition to all required breakers and disconnects. These inverters use a PWM synthesized sinewave which has less than 1% harmonic distortion, but unlike most sinewave inverters it uses a 60 Hz step up transformer, which gives much better surge capacity and lower idle power consumption. These inverters are conservatively rated at 8kW, which is available at 240vac, or on either leg of 120vac with Outback's unique phase balancing transformer. They can provide the full rated capacity at 40íC, unlike most inverters which are rated at 25íC. Outback also hermetically seals its inverters, which means that they are immune to dust, insects, leaves and other airborne debris which can lead to early failure of other inverters.
A microprocessor controller monitors the activity of all the components, as well as staging inverters for maximum efficiency. Computer data collection is also easily supported by this system.
The batteries are flooded lead acid deep cycle L-16 batteries from Deka. These are standard batteries used in solar electric systems and are rated at 6 volts, 350AH per battery. We are using four parallel strings of 8 batteries each. A 48-volt nominal system voltage was chosen to reduce current and wire sizes while maintaining a wide selection of equipment. The batteries are in their own room in the first floor of the tech pod. Since the tech pod will be shipped lying down, the batteries will not be installed until we get to Washington D.C. They will pre-assembled and wired into four 12 volt 1400AH modules which a forklift can place on the shelves in the tech pod. This reduces the number of connections made on site from 64 to only 8. This also allows cross tying of the parallel strings every 6 volts, which will extend battery performance of a bank with many parallel strings.
The high-powered array and the full UPS system that we are using for the competition would only add about $399/month to the mortgage, and monthly utility costs will be completely eliminated. A less ambitious grid tied PV array using laminated amorphous PV roofing would only add about $119/month to the mortgage, while reducing the monthly utility costs by at least 2/3rds from an "average" house. (note: A 30 year mortgage at 7% is assumed)
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