ROBERT J.R. GRAHAM: Use Rain Power and Solar Power Together!

by: Marcus Bicknell

The rainwater harvesting industry is constantly looking for different ways of making rainwater harvesting more economical and practical. Rainwater harvesting means you can use between 30% and 50% less on mains water consumption, and since we are all now under strong government pressure to do this. The Code for Sustainable Homes recommends rainwater harvesting for its higher target levels (maximum of 80 litres per day per person) and new Building Reg Part G 17K coming in April 2010 will, mean that, for the first time, water consumption limits will be a legal requirement in new buildings.

Rainwater harvesting does, however, in most circumstances use an electric pump to take the stored rainwater to where it is needed, and this pump needs power. Steps have already been taken to reduce this pump energy use. There are now specially-designed gravity feed systems whereby a rainwater header tank is only refilled when empty rather than every time water is drawn. This avoids the pump hunting, reduces pump activity considerably and, therefore, reduces energy use. The next logical step has been to use solar energy to power the pump.

A solar panel the size of a small car windscreen can power a gravity-feed rainwater management system. The solar panel, in combination with a submersible 12 volt pump in the main storage tank and a battery ensure rainwater can be pumped to the rainwater header tank without any use of mains electricity. The control mechanism of the system and the pump are both powered by the battery that is kept topped up by solar power in daylight hours. Even on a cloudy day, tests show that the most up-to-date solar panels can give six times the charge necessary to run a rainwater harvesting system in a typical family home. In the unlikely event of the panel not providing enough charge to the battery, mains electricity is used as backup. Furthermore, unlike normal chargers, the standby mode of the mains charger takes no current at all, not even a trickle.

So in such a system, the powerful mains-electric pump is replaced by a small 12 volt pump which pumps to 8-10 metres high, enough to feed a header tank on a two story house. One or more relay pumps, with supplemental solar panels, can be added for taller buildings. It should be noted that this system does not provide enough pressure for garden sprinkling but gravity fed irrigation is fine.

For quite a while, solar pump kits have been available allowing rainwater to be moved without the need for mains electricity and without having to turn on and off manually. Water can be moved, for example, from a low level collecting tank in the garden (or even at the end of a polytunnel or greenhouse) to a storage tank at a higher level. The applications are numerous in allotments, equestrian properties, market gardens and for water features. These kits can supply a drip feed watering system through a header tank (so that water is available for gravity feed at any time, even through a time switch). In more sophisticated kits, when the float switch detects water, it starts pumping, then when the header tank is full, it turns the pump off.

Using solar energy to power the pump is a big step forward in making rainwater harvesting an attractive sustainable proposition by saving water while not using extra power to do so.

Source