Off the keyboard of Geoffrey Chia

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Published on the Doomstead Diner on April 1, 2016

Discuss this article at the Doomsteading Table inside the Diner

The ongoing Tiny House Design Chronicles: Maximising solar energy collection

Further thoughts about using the Header Tank as a source of thermal mass:

Here is a brief commentary expanding on the previously mentioned idea of using the water in the indoor header tank as thermal mass and how it can be optimally configured to collect solar heat. Further considerations:

As the header tank is located high within the house, any air that this tank heats up will stay high and will not heat the lower level, unless active mixing of the indoor air is enforced. This is simply done by turning on the ceiling fan. Proper flow of air passing through the house is best achieved not by the fan, but by opening a low window (eg in the kitchen), to let cool air in; and by opening a high window (eg in the loft above the head of the bed), to let hot air out. On a hot summer day these windows (and others) will be kept wide open, but on a cold winter night all windows are likely to remain closed and the hot air in the loft can be allowed to accumulate for comfortable sleep. It is essential that the wood stove extracts oxygen from outside air and not from the air within the house. For an optimal pressure head, the header tank should be positioned as high up as possible. The highest internal location for the tank will be up flush against the ceiling. In this configuration, the obvious location for the (double glazed) window to receive the sun's rays will be on the roof directly over the header tank*. Whereas at first glance this may seem like a good idea, it is actually not (see Figure 1 ). A roof window/skylight will optimally receive heat from an (almost) overhead sun, which will occur in summer. However the skylight will be inefficient at receiving the oblique rays from a winter sun. Accordingly this configuration may tend to overheat the house in summer and be poor at gathering solar heat in the winter. Hence my suggestion for optimal collection of solar heat for the header tank is shown in Figure 2 where an optimally angled reflective sill reflects light into the side window (North** facing) against which the tank is positioned.

Materials for the reflective sill: the most efficient reflective surface is of course a mirror, however glass can shatter with hail impact and mirrors are heavy, with sharp edges. A better and cheaper option may be mylar film glued onto (waterproofed) marine plywood.

*The considerations described in Figure 1 relate to a roof with a flat north-south axis or a skillion roof sloping down towards the sun-facing aspect and do not apply to a skillion roof sloping away from the sun. In the latter case the configuration in Figure 2 is the only sensible arrangement.

**South facing if you are in the Northern hemisphere

Maximising other solar energy collection systems:

The logical extrapolation of the idea of a reflective sill described above, can be extended to a reflective surface placed in front of your solar PV panels or solar evacuated tube array. This is relevant for steeply angled arrays optimised for the winter sun in high latitude locations (see Figures 3, 3a, 4 and 5 ) but does not apply to flush roof mounted arrays.

It is possible to estimate the extra solar energy collected by calculation, however the best method is to simply try these out using various different angles and measure the extra power output (either displayed on your battery charging monitor or measured with an ammeter).

Super duper ultimate solar energy collection:

Those with OCD may adopt the "frilled lizard" approach, emulating the reflective panels surrounding the front of solar ovens, to try to harvest even the weakest oblique rays of the rising and setting sun. For my part I think the configurations in Figures 2, 3 and 4 may be worth adopting but those in Figures 3a or 5 are impractical.

Conclusion:

Simply placing a (more or less) horizontal reflective surface in front of your header tank, solar PV panels or solar evacuated tube array can significantly augment your solar energy collection. This may not be relevant for warm, high insolation locations such as Queensland, Australia or Southern California or Arizona in the USA. However for those living at high latitudes, it can be important for harvesting the oblique rays of the sun during short winter days. Such a strategy may accrue (wild guess) perhaps 20% extra energy, using the dirt cheap accessory of mylar film on plywood. It will certainly be much simpler and cheaper than purchasing a 20% larger solar PV or solar evacuated tube array.

As mentioned in previous articles, "stick on" mylar film and plywood can be used to construct solar ovens, sea-water distillation boxes and "greenhouse" composting chambers (to accelerate the composting of toilet waste). Hence off-grid wannabes should think about putting aside a store of mylar film and marine ply for future use.