Web Site of Robert John Morton
Sustainable Energy: Solar Energy
Photovoltaic cells transform light into electricity directly. They use light (not heat) in the range 0.6 to 0.3 microns wavelength. This is the same octave of the electromagnetic spectrum as that used by the human eye. So if the day looks bright, you will know that your photovoltaic array is producing plenty of electricity.
Data on the amount of solar flux in your locality is available from a data source provided by Sandia. The same site also provides data on scaling photovoltaic systems to suit the size of your house.
Photovoltaic cells are not very efficient. The default value in the first field of the following table is the percentage of incident solar energy which is converted to electrical energy by a typical photovoltaic cell. The default value in the lower field is the typical percentage of incident solar energy which can be retrieved from a photovoltaic array via a pumped water cooling system acting as a heat co-generator.
The amount of electrical energy you get from a photovoltaic array is proportional to its active area. Individual photovoltaic cells are often circular. Circles do not tessellate. When you place them in contact with each other on a flat plane, some space is left between them. The total area of your array is therefore always more than its active area. How much more depends on the pattern in which you arrange the circular cells. With the cells arranged in a square pattern, the total array area is 1.273239544735 times its active area. With the cells arranged in a hexagonal pattern, its total area is only 1.102657790844 times its active area. The hexagonal arrangement leaves you with a zigzag edge at each side, but it is the tightest way of packing the cells on a plane.
Change the default value for the active area of your photovoltaic array in the following table. The approximate total array area will then be posted automatically to the lower field. It assumes that the cells are arranged in the more efficient hexagonal pattern.
The default insolation figures si in the following table are adapted from Monthly Maps of Average Daily Solar Radiation from the US National Renewable Energy Laboratories. si is the solar energy in kilowatt hours per square metre per day which is absorbed by a plane collector facing due south and tilted at your latitude + 15° from the north pointing horizontal. This angle maximises its winter yield. The figures are for the extreme north west of Washington State. This is far from where I live but I could not find any data for the United Kingdom.
The electricity and heating demands ed and hd are my own domestic data. The hd figure includes energy for hot water.
se = total incident solar energy on the PV array in Mj per day.
eo = total electrical output of the photovoltaic array in Mj per day.
ep = fraction of ed being provided by the photovoltaic array.
ho = domestic heating from the PV cooling system in Mj per day.
hp = fraction of hd being provided by the array cooling system.
The final 'batt' column gives the number of hours back-up your battery will provide. If you have changed the figures in the Battery section of this project then your new figures will be the ones used to compute the figures in the 'batt' column of the above table.
You can find insolation figures for specific areas of the United States from the Solstice Web Site. They also provide lots of links to web sites dealing with photovoltaic and solar heating technology.
© January 2001 - Robert John Morton