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Issue #209 December 2007
INTELLIGENT ENERGY SOLUTIONS
Solar-Powering the Circuit Cellar
Part 1: Preparing the Site
by Steve Ciarcia
Start | Getting Started | So, How Do I Describe this Project? | Location, Location, Location | The Solar Panels Are the System | What's Next | Sources & PDF
GETTING STARTED
After walking around your house and determining that you can indeed see the sun, for most people, installing a PV system simply involves looking at the electric consumption, adding up the numbers, and justifying the cost benefits. As I mentioned before, I have determined to avoid ROI as a personal justification, but I did have to go through the exercise in order to size the system.
My “house” uses about 20,000 kWh of electricity per year. (The typical house uses about 8,000 kWh.) When I included this number in one of my editorials, I got a lot of e-mails asking whether I was smelting aluminum in my spare time. So, when I say “house,” I need to clarify it. My property consists of a central house with six additional outbuildings. These outbuildings are things like garages, a tool shed, a heated greenhouse, a potting shed, etc. I even call one of these buildings “the annex” because by itself this shop/guestroom is the size of a 2,200-square-foot house with an attached two-car garage. (It has its own four-zone oil heating system.) When I built it along with an adjacent garage, I attached them to a separate 100-A electrical service rather than expand the 200-A service in the main house. Originally, I thought I could install one PV system that would serve power to all of the buildings on the property, but the wiring distances along with the cost of electric service changes made it prohibitive. (Perhaps I will add a second PV system to those buildings later on.) The corrected power consumption for my real house (including the three outbuildings also powered from it) is 15,000 kWh/year.
At this point, a person interested in PV technology is supposed to create a spreadsheet itemizing the consumption for all of their appliances, TVs, light bulbs, etc. in their house. While this makes obvious sense for an off-grid system where you need to size battery capacity, the major variable contribution to my power consumption is air conditioning and lighting. The down side of having a large contemporary-style house with dozens of windows along with a solarium/greenhouse is that it requires tons of air conditioning. Interestingly, I admit that one good side benefit of this PV exercise was that it physically changed my energy consumption dynamics. Besides the significant lighting modifications I made while installing the PV system (to be described later on), I drastically reduced the amount of air conditioning I used last summer. I opened the windows. ;-)
A few years ago, I just flipped on the 12 tons (yes, 12) of air conditioning and kept the entire house like a meat locker from May through November. These days, when I am not satisfied with just opening the windows, I turn on only about 3 or 4 tons (1 ton is 12,000 BTUs) and I’ve moved the thermostats a lot higher. The irony is that I think I used to feel hotter going from 68° to 85°F outside than I do today going from 78° to 85°F outside (I know, it’s all about humidity). The good news is that comparing the month of July 2006 to July 2007, I can already see a 30% drop in electricity usage. I will endeavor to continue this trend.
When I originally sized my PV system, I used 15,000 kWh as the annual consumption. I’ll explain the numbers later, but I wanted a system capable of producing 80% of my electrical needs. It only gets better if I also reduce consumption after the fact.
Of course, sizing a PV system and paying for it are two different things. Fortunately, Connecticut is one of a number of states that has a very good PV subsidy program, the Connecticut Clean Energy Fund (CCEF). The only stipulation is that it has to be a grid-tied system professionally installed by a licensed and approved PV contractor. The amount of subsidy is based on the actual physical performance of the installed PV system after accounting for the calculated effects of shading, component efficiencies, and wiring losses rather than spec sheet maximums. With the typical commercial PV installation costing about $10 per watt, making the system performance as close to 100% as possible can easily result in offsetting almost 50% of the system cost. Not a bad deal and something to be seriously pursued.
I received quotes from four different PV system installers. I have to say that they were all well qualified and their quotes were fair. My final choice, however, was Sunlight Solar Energy of Milford, CT. I chose them because they seemed most agreeable about configuring a system that met the capacity I desired along with specifying electronic components, conductors, and solar panels that unquestionably met the engineered performance necessary to maximize the Connecticut rebate and pass muster with technically savvy Circuit Cellar readers.
Another requirement of the CCEF rebate program is that the system configuration must be utility-interactive net-metering—basically, a grid-tied PV system. Unlike charge-controlled stand-alone PV systems that store PV power in batteries, generally called an off-grid system, a grid-tied configuration uses the utility company as a big energy capacitor (see Figure 1). During the day, when the sun is strongest, PV power is used directly by the house and no consumption is added to the electric meter. Any excess PV power is fed back into the utility power lines, adding energy to the entire grid. The kilowatt-hour energy fed into the grid this way is registered as a separate meter reading and counted as a credit.
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| Figure 1—This is a block diagram of a grid-tied PV system. |
In Connecticut at least, the two readings offset each other on a one-for-one basis. For every kilowatt-hour I pump into the grid, I can receive the same kilowatt-hour back credited at the same cost. While I’m trying to ignore the ROI economics of this installation, it’s hard to ignore the $0.20 per kWh in Connecticut. (According to Paul Davidson in the August 10, 2007, issue of USA Today, Connecticut and Massachusetts have the highest electric rate in the continental U.S., and Connecticut’s has increased more than 50% in the last five years.)
After going through all the numbers with Sunlight Solar Energy, we configured an optimal grid-tied system for my location. The irony of having all the buildings I’ve described is that I don’t have a single adequate true-south-facing roof (the least expensive way to mount PV panels). The best I could do was a SSW-facing roof (215°) over the solarium that could hold about 4.2 kW. The only way to add another 6.5 kW was to mount additional panels on two 11¢ high steel poles (16 on each pole) set in concrete.
The final system configuration is 10,760 W and designed to produce 82% of my electrical consumption (based on 15,000 kWh/year). The system’s technical specifications are listed in the System Specs sidebar.
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