[Footprint] Solar Thermal Energy Systems

Let's take the stance that we don't have enough land to use biomass energy but circumstances are such that solar energy can be used for heat.

Again, we are looking for a simple sustainable system AND we are looking to provide heat over a wide duration of time - not just when the sun is shining. We need:

1. A way of capturing the heat in sunshine.
2. A way of storing the heat.
3. A way of moving heat from the capture area to the store.
   

Enclosures with transparent covers (like a greenhouse) has always been a good way to capture heat from the sun. As for storing heat we need to heat something up and then later, when we need the heat, we need to allow the heat to come out of it.

'Passive solar gain' into buildings through windows is useful it can be a very inefficient and expensive form of heating due to the large amount of glass, the need to variously shade and unshade the glass, the need to capture the heat within floors and walls and the loss of heat through the large expanses of glass. So I'm tempted to think that windows are for lighting and not for heating!

A great way of storing heat is in water. However, it can become a little expensive to have an insulated watertight pond. Another option is stone or masonry - often this is cheap and just needs to be piled up and insulated. For both of these options we need to move heat from the point of capture to the store. A fluid is what is needed - something that can flow from the capture point to the store. For a water store we can use water to move the heat, for a stone store we can use air.

The simplest option that I can see is as follows:

1. Capture: On the ground mount glass on bricks. The bricks form the edge and the glass is mounted over them to trap a volume of air within the brick area. Now, we want the volume of air to get hot, but we want the heat to only be in the air and not in the brick or the ground where it will be lost to the surroundings if the sun goes in. One thing I have seen is to use long black ceramic fibres just randomly thrown into the container - like a ball of wool after a child has been playing with it and you have had to gather it up. These black fibres rapidly get hot by absorbing heat from the sun and in turn heat up the air in the container. They do not store much heat in themselves and so little heat is radiated back through the glass when the sun goes in. As hot air rises it is best that these glass covered enclosures are on a slope with ambient (unwarmed) air coming in at the bottom and leaving at the top toward the...
2. Store: ...which is uphill from the capture point. The hot air passes into the stone heat store, heating up any stone or masonry before it is exhausted from the coolest part of the store which is most distant from where the hot air came in.
3. If heat is needed in the building at the time you have hot air coming in then it can be directed straight to the building, perhaps via the heat store (via the heat store exhaust?).
4. Note that the building is best positioned above the heat store, so that air can be used, without need for a fan, to bring the heat up. However, it may be important to insulate between the heat store and the house to avoid heating of the house during hot weather. This depends on your exact layout and your climate.
   

If the capture point can't be at the lowest point, with the heat store above it and the building above that then you may need to use fans to move the hot air around the system. In the interests of simplicity it can be useful to avoid fans.

If you have an solar air heating system you may wish to use the air to heat water or to heat water directly from the sun to provide your hot/warm water needs.

As an aside I'm tempted to think that it would be useful to have three water supplies - cold water (~8C), warm water(~45C) and hot water(~70C) (which is much more difficult to achieve) - there is little point wasting precious hot water when all you need is warm water.