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Fig. 17. Illuminance distribution of the final placement of the openings (64.7% glass transmittance and 27.8% furniture reflectance).
Table 1
Geometric parameters for three design stages
5. Electric system
Unlike conventional houses, whose main or only power source is the utility grid, ElementHouse and the other nineteen homes competing in the Solar Decathlon did not have a utility grid to which they could connect and were restricted to using solely solar power. In other words, all the twenty houses had to rely solely upon stand-alone PV electrical systems.
Fig. 19 shows the topology of the stand-alone PV electrical system used by ElementHouse. The topology is governed by the number of PV panels, the ratings of the PV panel, and the ratings of the other electrical components. To obtain the maximum power out of these PV panels, the team carefully chose one of the most effi-cient multi-crystalline PV panels on the market. The final roof configuration in Section 3 can accommodate forty such PV panels. Each of the forty panels has an efficiency of about 15% and is rated at 185 watts (W), totalling to a system of 7400 W.
The forty PV panels had to be pided into four separate arrays of ten PV panelsdfive parallel strings of two serial panelsdto comply with the input voltage and current ratings of the controller unit.Note that the PV panels, which were shown to be shadowed in the morning in Section 3, were grouped in one array so that shadowing never affected the other arrays. The controller unit is a combination of a maximum power point tracking (MPPT) device and a charge controller. The MPPT device maintains the operation of each PV array at its maximum power point (MPP) [6], while the charge controller ensures the proper charging of the battery stack.
For the same energy storage capacity, deep-cycle lead-acid batteries were found to be the least expensive, lightest, and least voluminous. Consequently, the battery stack comprised sixteen such batteries, each having a capacity of 215 ampere-hour (Ah) at 12 volts (V) for a 20-h discharge rate. This is equivalent to an energy storage capacity of 41 kilowatt-hours (kWh)denough to power ElementHouse for more than a day in the absence of solar energy. The life, performance, and safety of the batteries were increased by using an innovative battery equalizer that uniformly distributes the charge among the batteries. The batteries were connected in a four-by-four configuration to provide 860 Ah at 48 V to interface with the 48-V direct-current (dc) bus at the system voltage.
This system voltage depends on the dc input voltage rating of the inverters that convert dc power to alternating-current (ac) power required by the ac loadsdmost appliances and lightings throughout the house. Note that the controller unit also steps down the higher dc voltage from the PV arrays to the 48-V dc system voltage. Two inverters are used in a master-slave configuration (as shown in Fig. 19), in which they are synchronized to convert the 48-V dc system voltage into two 120-V ac legs that are 180 out of phase to be able to power both 120-V and 240-V ac loads. Each inverter has a continuous power rating of 3600 W and can handle surges up to 6000 W. Load management is critical to ensure that the overall continuous power usage does not exceed 7200 W.
The entire electrical system of the house complies with the 2005 National Electrical Code (NEC) [7] and the suggested practices for PV power systems given in [8] were carefully followed. The interior wiring was done in a conventional mannerdTHHN cables through electrical nonmetallic tubing (ENT) within the walls and through electrical metallic tubing (EMT) outside of the walls. Special connectors were used to connect the interior and exterior wirings from one house module to the other, making the assembly of ElementHouse fast and easy. There was also provision for a backup generator for cases of consecutive cloudy or rainy days, when energy from the PV array(s) and battery stack might not suffice to run the house loads. Due to the limited selection of household dc loads on the market and in order to eliminate extra dc wiring and electrical conduits throughout the house, no dc loads were included.