Photovoltaic Solar Energy

The Strawbale House is grid connected to the utility. Although we have completely eliminated our dependence on fossil fuel for heating and cooling we do use grid electricity for lighting and appliances. Electricity usage for our family of three is approximately 350 KWH per month well less than half the residential average for Wisconsin of 800 KWH per month. We plan on using PV in the future to lessen our grid demand.

You can see small scale PV at The Strawbale House. The active solar thermal heating and DHW system run off PV. The system uses a 40 watt PV panel directly wired to a 12V DC pump. The pump drives the whole system and works extremely well. When sunlight hits the PV panel the system starts to run, when there is no sunshine the system is idle. We also use two roof mounted solar powered vent fans to ventilate the attic. They use a temperature control switch or “snap” that tells them when to vent hot attic air.

The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell. Solar cells are made of the same kinds of semiconductor materials, such as silicon, used in the micro-electronics industry. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current — that is, electricity. This electricity can then be used to power a load, such as a light or a tool.

A number of solar cells electrically connected to each other and mounted in a support structure or frame is called a photovoltaic module. Modules are designed to supply electricity at a certain voltage, such as a common 12 volts system. The current produced is directly dependent on how much light strikes the module.

Multiple modules can be wired together to form an array. In general, the larger the area of a module or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct-current (DC) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.