SOLAR RESOURCE IN UK AND EIRE
You may be suprised to learn just how much solar energy is available in the UK and Eire. The map shows the light energy (in kWhr) falling upon a square metre of horizontal ground each year.
A tilted surface can collect more energy than a flat surface, depending upon which direction it is facing. A surface tilted towards the South at 35 degrees will intercept around 11% more light energy than a horizontal surface.
(See also The Effect of Tilt and Orientation)
A home of 80 sq. m, built with modern levels of insulation will have an annual space heating demand of 2,000 kWhr, and a domestic hot water demand of 2,000 kWhr, for a total demand of 4,000 kWhr.
A single roof pitch of the same house will intercept 54,000 kWhr of light energy each year.
Viridian Solar hot water systems represent an extremely cost-effective way to harvest this abundant free energy.
The Seasons and Solar Energy
One consideration for the implementation of solar hot water systems is the seasonal nature of the solar energy. Changes in the weather and to the hours of darkness throughout the year act to limit the availability of solar energy in winter compared to summer.
The graph shows the average daily light energy for each month of a typical year. The total light energy is made up of two component parts - direct irradiation (straight from the sun and shown in orange) and diffuse irradiation (light that has reflected off clouds, the sky and the surroundings, shown in blue).
It can be seen that the daily average in the summer months is six to seven times higher than in the winter months.
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Domestic hot water demand is relatively constant throughout the year, so the designer of a solar hot water system faces a choice - should the system be specified to meet hot water demand in winter, spring or summer?
A system that is sized to meet a household's requirements in winter will be six to seven times larger than it needs to be in summer. The solar panels are sitting idle for large parts of the year. A system sized to match demand in summer will use more auxilliary heating in winter.
The compromise that is usually chosen, maximises the ratio of useful energy collected to the capital investment. Normally this works out to be a system that just meets demand in the summer, and provides around 50-60% of the hot water throughout the year. |
Next: The Effect of Tilt Angle and Orientation
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