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					                                         Solar Decathlon 2002
                                         University of Delaware
                                            October 3, 2002

                                         Topic: Solar Electricity
                                            Newsletter entry




              Photo Credit: The AstroPower Company                    Photo Credit: Trace Engineering

The overall strategy for the UD solar house is to configure an efficient PV power system and minimize every aspect
of the electric loads posted by a normal household. An energy budget has been established, which represents a
reasonable loading on the PV system. Unfortunately, it will not be possible to guarantee that the house’s systems
will handle worst case scenarios, as designing a system to handle this would be prohibitively expensive, and would
not necessarily fit in the footprint that was allotted for construction. . Budget constraints also limited the UD team
from incorporating a larger array.

It was decided that the system should be ready to tie into the utility grid under "net metering" (marketing excess
power to the utility) protocols. The design of the system ended up being based heavily on what materials could be
obtained through material donations. In the end, the AstroPower Company offered to donate a complete power
system based on their SunUPS series of grid tie systems. The SunUPS is intended to operate as a grid tie system
with battery backup. Trace Engineering and the M DAVIS COMPANY donated additional monitoring, control, and
distribution hardware.

Our PV array used on the UD Solar house consists of 40 AstroPower AP-120 panels generating 120 watts peak each
at 12 volts. This particular panel was chosen due to its higher efficiency per unit area, and the generosity of the
AstroPower Company. The panel uses single crystal silicon cells manufactured from waste silicon from the semi
conductor industry. The life span of the array is expected to be at least 20 years, and probably much more. Peak
power output of the array is 4800 watts. The array was sized to handle all of the loads of the house for nearly all of
the year.

The battery bank for the UD house was sized to power the house for approximately three days with no power input.
This size was a compromise between budget constraints, size limitations, and maximum capacity. The choice of
battery was based on the standard battery used in the Sun UPS power system from AstroPower. The standard
SunUPS uses Concorde Sun XTender sealed lead acid Absorbed Gas Mat (AGM) Batteries. These batteries are
essentially maintenance free. Another advantage to the Concorde batteries is their low Hydrogen off gassing
potential. According to the manufacturer, no hydrogen gas will be produced under normal circumstances. Even
under the most severe conditions (Overcharging with high battery temperature) hydrogen levels would remain
below explosive levels. In other words, the batteries are essentially explosion proof. A passive ventilation system
was added for an extra margin of safety.
Power Conditioning, Monitoring and Control:

At the heart of the UD power system is a complete Trace Engineering Power Module assembly. This unit
contains all disconnects, battery charge controller, GFCI protection, and inverter hardware. A single
inverter was chosen over twin stacked inverters to keep the system simple and save money. The inverter
selected was the Trace SW5548. This inverter can produce 5500 watts at 120 volts AC continuously, and
was designed to work with a 48 volt power system. With the addition of a Trace Grid Tie unit, the inverter
would be UL Rated for utility inter tie operation. (The Grid Tie unit was not installed or needed in
Washington DC.) To power 220 volt appliances, a Trace T240 Auto Transformer (Donated by Trace
Engineering) was used. Although twin inverters could have been used, using the transformer saved several
thousand dollars. Battery charging is handled by Trace C40 three stage charge controllers. These
controllers use a three stage charging cycle along with battery temperature monitoring to efficiently and
safely charge the batteries. The inverter also has provisions for up to 60 amps of charging capacity from an
AC power source (the Utility Grid or generator.) To power 220 volt appliances, a Trace T240 Auto
Transformer (Donated by Trace Engineering) was used. Although twin inverters could have been used,
using the transformer saved several thousand dollars.

It is important to have sufficient monitoring facilities to identify exactly what the PV system is doing, and
how much power is available. A Trace TM500 cumulative amp hour meter was selected to provide
continuous monitoring of battery state of charge, and general system performance. The meter allows
monitoring of battery percent charge, battery voltage, amperage into or out of the battery bank, and
cumulative amp hours into or out of the batteries. This sort of meter is critical when it comes to answering
the question "How much power do I have left?" A Trace remote control unit was also selected to allow
easy control and monitoring of the inverter. A separate cumulative Watt Hour meter supplied by
AstroPower will allow for full monitoring of utility inter tie operation once the house returns to Delaware
and is tied into the utility grid.

Power Distribution:

The Power distribution system used in the UD Solar House is designed around the concept of a high draw
power panel for large loads, and a smaller UPS (Uninterruptible Power Supply) panel for critical loads. In
Washington DC, all panels will be supplied with power from the inverter. When grid tied, the UPS panel
will be tied to the inverter and the utility grid, while the high draw panel will be tied only to the utility grid.
This design is called for in the AstroPower SunUPS design theory, and will allow for more flexibility in the
future. The idea is to extend battery life by limiting the number of loads used during a grid outage while
still allowing for lots of high draw appliances. It must be emphasized that the system is sized to allow the
entire house to run off of PV. The addition of a high load power panel simply allows for the connection of
additional electrical equipment in the future

				
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