STEP Solar Thermal Electric Panel
Experimental Setup
The roof section tested consists of six STEP panels. The overall dimensions of the collection area are 16.67 feet tall by 8.91 feet wide for an area of 148.61 square feet. The angle measured from the ground is 61 degrees. This angle is optimal for November 21st and January 21st for the latitude in which Rolla, Missouri inhabits. The testing was delayed until late March so the collector was not at an optimal angle. The data in this report is not transposed for the optimal collector angle. The results presented here are for the collector angle of 61 degrees in late March. Figure 2 shows a picture of the test roof.
Figure 2: The Test Roof
A 40 gallon solar hot water tank held the fluid to be circulated through the collectors. The difference between a solar hot water tank and a conventional hot water tank is that the electric element of the conventional tank is replaced with a heat exchanger which entails coil of copper inside the tank. The fluid is half Dow Frost, which is a food-grade antifreeze with a rust inhibitor, and half distilled water in the amount of 30 total gallons. The fluid is pumped in the bottom right corner and flows out the top left corner. The equal geometric flow pattern lends itself to uniform flow. Pumping from the bottom up ensures that their will be a solid column of water in the thermal collection piping.
Figure 3: Fluid Flow System
The pump used for this test is a standard 1.5 horsepower water pump. The fluid is pumped out of the supply pipe in the bottom portion of the hot water heater. After flowing through a fluid flow meter the fluid is passed through a type-T thermocouple to measure the inlet temperature. When the fluid has passed through the collector area it passes through the outlet thermocouple before entering the hot water tank via the top hot water supply pipe. Figure 3 shows a graphical representation of the fluid flow system.
To determine the output of the PV panels an I-V, amperage vs. voltage, curve was obtain by charging a capacitor bank. Because of the high wattage, exceeding 130 Watts at times, the panels are tested one at a time. The panel voltage was obtained by measuring the voltage across the capacitor bank while panel amperage was obtained by measuring the voltage drop across a low resistance resistor in series to the flow of electricity. Knowing the amperage and voltage the maximum power can be obtained.
A pyranometer was used to obtain the available radiation numbers used in this report. The pyranometer was mounted on the roof surface to see the same angle as the STEP panels.
Figure 4: Capacitor Bank (left), Relay Board (right)
Figure 5: Laptop Setup
The equipment used to run the testing included two laptops, a data logger, a digital acquisition board and a relay board. One laptop was used in conjunction with the digital acquisition board which was used to run the relay board. The basic operation of the relay board is to turn on the discharging relay for ten seconds then pause for one second then turn one of the panel relays for ten seconds then pause for one second and this cycle repeats. The capacitor bank is discharged through a high watt resistor bank. Figure 4 shows the capacitor bank and the relay board. The second laptop is connected to the data logger. The data logger collects all of the temperature and voltage data from the components mentioned previously. Figure 5 shows the laptop setup. The laptop on the right is for the relay board setup. The capacitor bank is in the box behind the laptop. The laptop on the right is hooked into the data logger. The ambient temperature was taken via a digital temperature gage.