THE LIMITATIONS ON SOLAR SPACE HEATING

As we have seen, solar energy is seasonal. Space heating demand is also seasonal. The challenge for using solar to provide space heating is that their seasonality is out of phase.

The graph below shows the timing and relative sizes of energy demand for space heating and domestic hot water for a well-insulated home of average size.

Typical heating demand through the year for a residential building

Overlaying a line of annual solar yield on the same plot illustrates the challenge for solar space heating. The line illustrates energy output from a typical solar installation sized for domestic hot water. As can be seen, there is a potential excess of energy relative to hot water demand in the summer, but a shortfall in the heating season due to lower light levels as a result of winter weather and shorter days.

Overlay showing seasonal bias of solar energy availability

To make a meaningful contribution to space heating, the area of solar panel needs to be increased. The diagram below illustrates the effect of three times more solar panel area compared to a panel sized for domestic hot water only.

The effect of increasing solar panel area

A system with larger solar panel area can produce a large excess of energy in the summer months compared to the hot water demand. The amount of auxiliary heating for domestic hot water has been reduced in the winter, and there is sufficient excess of solar energy to shorten the heating season and make a contribution to space heating in spring and autumn. However the overall effect on the space heating demand is somewhat negligible. The case for the additional complexity of the system (see previous section) and upfront capital costs are generally less persuasive than for a system that heats domestic hot water alone.