Solar Power System

In this chapter, we will describe exactly how we sized the solar electric power system for the Florida Solar Cracker House. The two main problems to deal with are defining your power requirements and assessing the solar radiation available at your site. Beyond that, you must decide on whether to track or not, and correct for thermal derating and power losses in batteries, inverters and wiring. All of this is described below. Keep in mind that a solar-powered home is basically a battery-powered home, with a solar battery charger. Before building our house, and before realizing that we could supply all of our water needs from rainwater and didn't need to have a well drilled, we had a well drilled and built a "watershed" to store water under pressure and house the power for the pump. The watershed, pictured below, contains a minature version of the power system for a solar house. The well is about 50 feet in back of the shed and is 120 feet deep (cased 100 feet), typical for a potable water well in this area. Inside the well is a Shurflo submersible diaphragm pump, about 20 feet below the surface of the ground. There are two 6 Volt, 230 Amp-hour batteries in the shed, wired in series to provide 12 Volts to the pump. At 12 Volts, the pump draws about 4 amps and provides a stream of water of about 1 gallon per minute. We flow the water into a WellMate pressure tank, having a capacity of 80 gallons and a total drawdown of 25 gallons. The tank is set to a pressure of 40 psig and is controlled by a pressure switch. The batteries are charged by a 53 Watt Siemens M-55 photovoltaic panel.

Defining your power requirements

Take a look at your last electric bill. Somewhere on it you should be able to find your average daily kW-hrs used that month. If you have a conventional, all electric home, it will probably be something like 50 kW-hrs or more per day. Building a solar electric system to feed an appetite like that is probably financially prohibitive for most of us (the solar modules alone would run about $90,000). So, the first line of defense is conservation. You will be very surprised how far conservation can take you.

Make a complete list of all of the appliances (everything that requires electric power) you really need. Go through room by room and think of everything you have plugged into the wall, and if it is a power hog, ask yourself if you can do without it or replace it with something more efficient. If you plan to use natural gas or propane, your job will be a lot easier, because things like a stove, a clothes dryer, and furnace, which take an enormous amount of power, can run on gas. In our case, we decided to try to do without all fossil fuels, so we had a harder time. Our stove became a wood cookstove and a solar oven, our heating system is also based on an efficient woodstove, and our clothes dryer became a clothesline. We live in a wooded area, and have an ample and renewable supply of oak firewood for our woodstoves.

For each of the electric appliances on your list, write down the rated wattage (this can usually be found on the appliance somewhere), and write down the estimated number of hours per day you usually use the appliance. Multiply these last two together to get the W-Hrs/day for that appliance. Large items that contribute 500 W-Hrs/day or more need to be considered very carefully, and probably new, efficient models need to be considered. Our list came to about 4 kW-Hrs/day for the winter and 4.5 kW-Hrs/day for the summer (see Table below), not including air conditioning. This is more than a factor of ten less than we currently use in our conventional, all electric home. As you will see in the next section, by sizing our PV system for the winter, we will have enough excess power in the summer (because of the additional solar radiation) to power our small air conditioner (up to 3 kW-Hrs/day).

Sizing Worksheet for Photovoltaic System

ApplianceRated WattsHrs/Day (S)W-Hrs/Day (S) Hrs/Day (W)W-Hrs/Day (W)
Refrig./Freezer, Sun Frost RF-19, 24VDC-- 984-744
Microwave15000.25 3750.25375
Bread Machine500 (mixing)0.30 1500.30150
750 (baking)00 0.20150
Washing Machine, Staber Industries-1/2 load 831/2 load83
Water Pump, Shurflo 93001001 1001100
Toaster Oven16000.25 4000.25400
Kitchen/Bath Lights (11 AC)186 10816288
Kitchen/Bath Fan4012 4804160
Toilet (DC fan)1024 24024240
Living/Bedroom Fan406 240280
Porch and Breezeway Fans (3)4010 40000
Livingroom/Bedroom Lights (11 AC)186 10816288
TV702 1403210
Radio/Sterio304 1206180
Computer/Monitor2502 5002500
Message Machine2.424 602460
TOTAL-- 4488-4008

In the table above, (S) stands for summer and (W) stands for winter. The Sun Frost Refrigerator/Freezer has a 19 Cu ft capacity, and is one of the most efficient refrigerators on the market (Sun Frost, Box 1101, Arcata, CA 95521 (707) 822-9095). Also, the Staber Industries, Inc. clothes washer is a very efficient (165 W-Hrs/load) tumbler, rather than agitator, and saves water and detergent as well. (Staber Industries. Inc. 4411 Marketing place, Groveport, Ohio 43125, (800) 848-6200). All of our household water will be collected from the roof into a 3000 gallon cistern, then pumped into a pressure tank for use in the house. The pump will be a Shurflo DC submersible, 9300 series, diaphragm pump. (Shurflo, 12650 Westminster Ave. Santa Ana, CA 92706-2100 (800) 854-3218). The lights are all compact fluorescent and the fans will probably be 24 VDC paddle fans.

Assessing your available solar radiation

The easiest way to find out how much solar radiation is available at your particular site is to use the Solar Pathfinder (Solar Pathfinder, 25720 465th Avenue, Hartford, SD 57033-6328, (605) 528-6473). This is an ingenious little device, about the size of an inverted salad bowl sitting on a small tripod. The price is roughly $200.00. It takes just a few minutes to set the gadget up, and with one measurement, you will know exactly what hours during which months a solar array at that site will be shaded by nearby objects (trees, hills, buildings, etc.). The first picture below shows my friend, Allison, using the pathfinder.

You can easily move the thing around to find the optimum spot, because sometimes it isn't obvious from just looking. We used it not only to analyze the site for our solar arrays, but also to find the best spot for our garden! It can also tell you whether an array mount that tracks the sun is worth your money. This next picture is a closeup view of the pathfinder.

The Solar Pathfinder comes with all of the information you need to measure and calculate how much power you can generate at your site, and figure out how many solar modules will be required. The easiest way to show how it's done is to go through our calculation step by step.

Available Solar Energy Worksheet

Month(A) Percent of possible solar radiation available at site(B) array tilt angle(C) Gainesville, Florida BTU's/sq.ft. per day(D) Tracking factor(E) Thermal derating factor(F) Equivalent hours at 1 kW/sq.met.(G) W-hrs per day output per 53 Watt moduleExcess W-hrs per day for 24 modules
Jan.0.8850 deg.15301.1514.88 hrs.259936
Feb.0.9450 17701.216.29 3332712
March0.9430 18401.250.96.13 3252520
April0.9630 19701.30.96.97 3692856
May0.910 21601.350.86.62 3512424
June0.9410 20061.40.86.65 3522448
July0.8910 19171.50.86.45 3422208
Aug.0.9610 18731.40.86.34 3362064
Sept.0.9630 17301.350.96.36 3372088
Oct.0.930 16401.250.95.23 2771368
Nov.0.9150 17301.215.95 3152280
Dec.0.8150 14401.1514.22 22496

In the table above, column A is the data obtained from the Solar Pathfinder. We found a site that was fairly clear of obstructions almost all year. The numbers are smaller in the winter months because the sun is lower in the sky, and distant trees shade the site during the early morning and late afternoon. Column B is the expected tilt angle of the solar array. We assume that we will manually adjust the array tilt angle four times a year between 50 degrees in the winter and 10 degrees in the summer. Our site in North Central Florida is at a latitude of about 30 degrees. Column C is taken from tables of climatic data, commonly called f-chart data, that are available for 250 cities in the United States. These data give values of incoming solar radiation averaged over many years, and include the attenuation of radiation due to clouds, rain, dust, etc. The town nearest our site for which data are available is Gainesville, Florida.

Column D gives a correction factor if a solar tracking mount is to be used. For our site, we have a very wide unobstructed angle to the sun, and in that case, a tracking mount will pay for itself. The correction factors listed are guesses based on data given by different manufacturers of solar trackers. The thermal derating factor (column E) takes into account the reduced efficiency of the PV modules when they are hot. Again, this is a guess based on data from the manufacturers. Column F is the product of columns A, C, D and E, multiplied by 0.00315, which is a unit conversion factor. It gives the equivalent number of hours of solar radiation at the standard level of 1 kW/sq. met., which is the radiation level assumed by most manufacturers of solar modules when they quote the output of their modules. For example, one module we are considering is the Siemens M55, rated at 53 Watts. The energy output of this module for our conditions is listed in column G.

Finally, we can estimate the number of modules we will need to supply our required power, and how much excess power we may expect during the summer. As a safety factor, we up our estimated power needs by 10%. Note that our leanest month is December, with 224 W-Hrs/day generated per 53 Watt module (column G). In December, we estimated we needed 4kW per day, and with the 10% increase, this becomes 4400 W-Hrs/day. Next, we multiply this minimum power required by 1.2 to account for losses in batteries and wiring. This comes to 5280 W-Hrs/day required in December. Divide this number by 224 W-Hrs/day per module in December to get the required 24 modules to get by in December. The last column in the table gives the excess power we should have for 24 modules, based on a use of 5280 W-Hrs/day in winter and 6000 W-Hrs/day in summer. Thus, we should be able to use about 2.5 kW-Hrs per day for our air conditioner in the hottest months of summer.

This picture shows our power situation as of Fall, 2003. We have three Zomeworks trackers, which hold the equivalent of 8 Siemens SM-55 PV's, 53 Watts each.

The next picture shows our Power distribution and monitoring setup.