This is the first of two articles that will focus on solar pumping. We start with a review of solar energy, followed by a description of the various methods of converting solar energy, and will conclude with a list of resources, should you wish to pursue the subject further.
Solar energy is one of the free benefits that come with residence on planet Earth. There is enough solar energy falling on the surface of the earth to meet humankind’s energy needs forever. You and I have enough solar energy falling on our rooftops to heat and cool our homes, and to provide for most of our electricity needs. The problem is in economically converting this energy into a form we can use – heat or electricity.
The technology now exists to convert solar energy into a form of energy we can use around our homes, but without government subsidies, it is not cost-effective because the cost of conventional energy – i.e. natural gas, heating oil, propane and electricity – is so low, the payback on the solar equipment is too long to be generally acceptable. With increasing conventional energy costs and the currently available government subsidies, solar becomes more financially feasible.
One notable exception is solar swimming pool heating, which is economically viable right now in most sunny areas. And, if you live in a relatively sunny locale, you can put a solar hot water heater on your home for a few thousand dollars, which will pay for itself in less than 5 years if your conventional energy source is electricity or propane, and in less than 7 years if you heat water with natural gas. Unfortunately, most of us won’t spend the money with that long of a payback.
Back in the early 1980s, the federal government offered tax credit incentives to encourage us to install solar systems on our homes, and many Americans took advantage of this program and already are enjoying the benefits of solar. Today, many states offer incentives to encourage the installation of solar equipment, and through the end of this year, homeowners can get a 30-percent federal tax credit on qualifying solar products. But until the cost of conventional fuels reaches a point where the cost to purchase and maintain a solar system is lower than the cost to purchase the energy it replaces, solar energy equipment will be relegated to the hinterlands and/or to the rooftops of visionaries.
How much solar energy is available? There are approximately 300 BTUs per square foot per hour of solar energy falling on the surface of the earth in the United States on a sunny day. BTU stands for British thermal unit, and one BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree Fahrenheit. To heat the water for residential use, approximately 80 square feet of solar panel area would serve the needs of a family of four.
Solar Thermal PanelsConverting solar energy to heat energy is fairly simple and efficient. All you need is a black surface to collect the solar energy and convert it to heat energy, and some way of carrying away the heat energy. The most common method is to attach copper pipes to a copper sheet, paint or coat the sheet black, mount it in an insulated glass covered box, run water through the pipes, and, presto, you have a solar water-heating collector. Such a device will convert solar energy at a rate of 80-percent to 90-percent efficiency, depending on the types of black coating, glass and enclosure used.
Converting solar energy to electricity is not as simple or efficient. One method is to use a solar thermal panel as described above, add mirrors to concentrate the energy so temperatures above the boiling point of water can be achieved to run a steam generator, just as you would to generate electricity with gas, coal or nuclear energy. This technique is used in a number of solar electric-generating plants in the southwestern United States and around the world, but is impractical for small-scale residential solar/electric conversion.
Photovoltaic PanelsThe more common method of converting solar energy to electricity – and how most solar pumps are powered – is to use a photovoltaic (PV) panel, which provides direct conversion of solar energy to electricity. PV panels consist of a number of PV cells, usually silicone, which are wired together, and housed in a weather-proof panel. PV panels are relatively expensive and not that efficient – less than 20-percent efficient. But the cost is coming down each year, and the efficiency is improving, so we are heading in the right direction.
PV panels vary in size from a few square inches to many square feet. The exact size needed for pumping applications takes a number of factors into consideration, and will be discussed in detail next month.
Also next month, we will look at the various types of solar pumps available, describe the advantages and disadvantages of each, and list sources of supply for equipment and sources for information. ’Til then .... ND