Why not use a solar panel to run the pump?

We have been asked many times whether it is a good idea to use a photo-voltaic solar panel to run the pump in a solar hot water system.

Lets ask ourselves a few questions:

If there is a perfectly good source of mains power, why not use it?

Mains driven pumps typically will be rated at less than 50W, less than a lightbulb. The pump is only powered when there is solar gain. The power used is not negligible but it is small.

How long will the PV panel take to pay for itself?

As with all PV systems, a small solar panel to drive the pump will take a long time to offset its cost. I'd take a guess and say 30 years.

Is the level of control provided by a PV driven system as good as using a temperature controller?

Unless a proper differential temperature controller is used, there is no measurement of the hot water cylinder temperature. This means that theoretically, the pump could run when the cylinder is hotter than the solar panel. This would, of course, waste perfectly good heat.

Considering the above, we generally recommend a mains driven pump. We can and do supply PV driven pumps with PV solar panels to suit if customers want it.

Hope that helps.

Solaris Joins Ethical Junction

Today we have had our application to join the "Ethical Junction" approved. This means that they have vetted us an found us to be ethical in our business practices and sales approach. Why not put us to the test and get an instant quote or arrange a survey?

Thermal Stores vs Dual Coil

Thermal stores are cylinders in which hot water is delivered via a heat exchange coil mounted in the top part of the cylinder. When used with solar, there would be an additional coil mounted at the bottom of the cylinder and the solar circuit would be indirect and filled with antifreeze. The water in the cylinder itself is not at mains water pressure and would have a header tank either integral to the unit or mounted in the loft.
If you consider what happens when you draw hot water form a tap, the top DHW coil has cold water introduced to it which absorbs heat as it flows. The water surrounding the coil will necessarily cool and tend then to fall to the bottom part of the cylinder. As this occurs, the water will be stirred resulting in less stratification of the cylinder. I.E. The water in the cylinder will be of a uniform temperature.

Standard "Dual Coil" cylinder - well stratified cylinderThermal Store - reduced stratification

There some advantages to the thermal store design despite its shortcomings. One of which is that it offers mains pressure hot water without the inherent safety issues associated with un-vented cylinders and as such the requirement for specialist training and certification can be avoided. In addition it is theoretically possible for the solar input to make a contribution to space heating. This carries a huge note of caution and will be dealt with by a separate blog post. If you have any questions feel free to contact us.