Ground heat or sun heat for your house?

Two sources, two timetables

Ground heat is available at roughly the same level day and night, summer and winter. Its limit is the rate at which the earth can deliver it to your pipes. Solar heat comes in strong bursts: abundant in July, scarce in December, and absent at night. A house needs heat when the sun delivers least. That seasonal mismatch is the whole problem solar heating has to solve, and the idea is laid out in solar space heating for homes.

What each approach needs

• Ground-source heat pump: a buried loop, a compressor and electricity. Output follows the heat pump size. See how ground source heat pumps work.
• Solar thermal heating: roof or ground collectors, a storage tank or larger store, pumps, and a backup heat source. Output follows the weather.

In running terms, a ground-source unit buys electricity and multiplies it. Solar thermal buys almost nothing once built, but delivers little when the days are short. A solar photovoltaic array feeding a heat pump is a third path, which separates electricity generation from heat delivery.

Can they work together?

Yes, and the interesting designs do exactly that. Solar heat collected in summer can be pushed into the ground, which warms the soil or rock around the pipes. That heat can then be drawn out later. This is seasonal storage, introduced in seasonal thermal energy storage.

At Drake Landing, 800 flat-plate collectors on garage roofs fed 144 boreholes, and 52 homes drew heat from a district loop. By 2012 monitoring reported a solar share of 97 percent of space heating, and 100 percent in the 2015 to 2016 heating season. Read the figures on the performance results page. It took several years for the ground to warm up, which is covered in why seasonal storage needs time to ramp up.

Which is simpler for one house?

For a single detached home, a ground-source heat pump is usually the simpler thing to buy and live with, because it behaves like a conventional appliance. Seasonal solar storage at household scale is difficult: a large insulated store or borehole field costs a lot per home, and economies of scale improve with the number of homes sharing it. That scaling point is made in community solar heating vs rooftop solar.

How to decide

Start with the house. Cut heat loss first, since both systems perform better in a tight, well-insulated shell. Then compare lifetime cost: the loop and the heat pump on one side, collectors and storage on the other. Check what each qualifies for in current rebates, a task described in geothermal rebates and financing. Where a community can share the capital cost and the land, seasonal solar storage becomes a serious contender.

A note on electricity and photovoltaic panels

People sometimes treat solar heating and solar electricity as the same thing. They are not. Photovoltaic panels make electricity, which can run a heat pump, charge a battery or feed the grid. Solar thermal collectors make hot fluid directly. For a house, the cleanest pairing today is often photovoltaic panels plus a heat pump, because electricity is flexible and can be exchanged with the grid, while stored heat is not easy to move around. The difference is set out in solar thermal vs solar PV.

The thermal route earns its place when there is a large, steady heat demand and a way to store heat cheaply. Drake Landing used flat-plate collectors, described on the solar collection page, because heat for 52 homes could be gathered in summer and held in the ground until winter.

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