This continues the series of articles by which we hope to empower readers to exit the "killing fields of the future" (the cities), to help you achieve and maintain a comfortable offgrid lifestyle for at least a couple of decades after TSHTF (assuming you have purchased sufficient spare parts for maintenance and/or are creative in your repairs). For example, solar evacuated tubes invariably break, therefore it will be prudent to purchase many spares, in addition to those in your working solar thermal array. (Click all pics in this article for a larger view.)
In this article I outline my plumbing preferences for my tiny house which, to say the least, is a little unconventional compared with "standard" arrangements. The plumbing here is specifically configured for my tiny house design which I have described in a previous article: https://www.resilience.org/resource-detail/2544932-building-a-tiny-house
Preferences for the cold water system:
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A steel rainwater storage tank within the lounge (under the seats) doubles up as a thermal mass tank which picks up passive solar heat in the daytime. This minimises the need to operate the wood stove at night. The water in this tank (eg 500 litres) may not last long depending on the rate of use, hence my preferred configuration is a permanent connection to an additional external rainwater tank with capacity of perhaps 1000 to 2000 litres.
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In off-grid configuration, the 150 litre header tank is all-important feature and serves three main purposes: first and most important is that if a tap is accidentally left open, the greatest amount of water that can be lost is only 150 litres (even if a hot water tap is left open, the last 50 litres of water in the cylinder will not be lost because at the end there will be no pressure head left to empty the cylinder). If however the system is directly connected to a community shared large (eg 40,000 litres) water tank uphill, it will be possible to lose many thousands of litres. The second reason for a header tank is that the ritual of filling of this tank each morning, either by electric or manual pumping, will reinforce the value of fresh water and encourage daily limitation of water consumption (of course this is not true rationing because the header tank can always be refilled at any time, but being creatures of habit we will probably just fill it once a day or even on alternate days, thus limiting water consumption to <150 litres per day per tiny house). Thirdly, a header tank eliminates the need for a frequently operating electric water pump (triggered by the pressure drop detected by an electronic sensor whenever a tap is opened). It eliminates another layer of electronic complexity (even though a high volume electric pump is part of my configuration, it does not require any electronic sensor and also has a manual backup). Another purpose of this header tank is additional thermal mass.
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This system includes the option of direct connection to town (reticulated water) supply at normal mains pressure. This high pressure port will also be suitable for permanent direct connection to a larger water tank situated uphill, although as stated before this is to be discouraged.
The diagrams are self explanatory
Preferences for the hot water system:
Contemporary conventional solar / hybrid hot water systems are highly complex and depend on sophisticated electronics. I initially describe my general preferences, then outline the workings of proven "standard" setups, then go through a process of deconstruction and simplification to pare things down to the bare bones system I personally prefer.
My general preferences:
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I prefer solar heating of water with an evacuated tube system (the "heat pipe" type evacuated tubes, NOT the hollow core type) with no integrated gas or electrical backup. Evacuated tubes are more efficient in temperate climates in winter compared with flat panel arrays*. Best orientation is facing the equator (ie facing North if in the Southern hemisphere) and permanently angled around 15 degrees higher than your latitude eg if you are 40 degrees South, it should be angled around 55 degrees from horizontal, which is optimal for winter. Suboptimal angling for the summer sun is in fact desirable, to avoid overheating in summer.
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If there are several overcast days, the wood stove (or LPG stove) can be fired up and hot water obtained from the backboiler tank or by heating a kettle. Adding the hot water to cold water in a bucket will create a comfortably tepid wash mixture. For me the expense and complexity to plumb a system which connects pipes from the woodstove backboiler (eg from the Salamander Hobbit system) to the hot water storage tank is not worthwhile.
Conventional systems:
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Contemporary conventional domestic solar hot water systems use a microprocessor controller with electronic sensors. The "Heliatos" system http://www.heliatos.com/ obviates the need for microprocessor control of the pump. I have no pecuniary interest in Heliatos but mention them repeatedly because their components and configurations enable simplification of conventional complex solar systems (and easy retrofitting of non-solar to solar systems) while still working well, and I have had productive dealings with them previously. The key components are the "bottom feed connector" and a simple 12V DC electric pump + 10W photovoltaic panel. The standard Heliatos configuration assumes the solarthermal array is on the roof, ie above the level of the hot water cylinder, and the cylinder incorporates backup gas/electric heating. Typical cylinders operate at around mains water pressure. Whenever hot water is taken from the top of the cylinder, cold water under mains pressure replaces it at the base to keep the cylinder full, to enable ongoing sourcing of hot water from the top. The entire water mass in the cylinder is always kept hot because backup heating kicks in as needed, as determined by temperature sensors. Please note: all pipes containing hot water must obviously be heavily insulated, this is not shown in the diagrams for simplicity.
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My modifications:
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My modifications involve use of evacuated tubes rather than the Heliatos flat panels and placing the tube array on the ground rather than on the roof for ease of cleaning and maintenance (also easy to cover with a tarpaulin to shut down the system if it overheats or to protect against a hailstorm). Tubes are thus located at a lower level than the hot water cylinder. I also choose not to have backup gas/electric heating. The mode of operation is described on the diagram. Thermosiphoning during the day should be enabled, thus eliminating the need for an electric pump and PV panel.
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My aim is to reduce complexity (resulting in only minor inconvenience) and thus ensure long term robust performance. This configuration is pretty much guaranteed to work, because there are already well proven "stand alone" outdoor evacuated tube systems which utilise passive convection currents, with the tank situated above the tubes. Such standalone outdoor systems are suitable for warmer climates such as Queensland but not ideal for cold climates such as New Zealand, where it is best to locate the hot water cylinder in a warmer indoor environment for greatest efficiency. I sought the opinion of the Heliatos consultant, Dr Abtahi (Phd) about my split system preference, who emailed me back that what I propose is not only workable, it is actually not uncommon. Thus I cannot claim any originality here and can be quite confident of its feasibility. His main caveats were that the pipes must be properly insulated and the array should be tilted so that the hotter end of the manifold sits higher, to kick start thermosiphoning in the morning.
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It is always important to seek the advice of your local plumber, which I am also doing. We can expect problems to arise if the sizes of the tank and solar array are mismatched between each other and also with regard to the climate. For example of the tank is too big, solar array is too small and winter sun is too feeble, you can expect persistent poor heating performance. Conversely if the tank is too small, solar array is too big and summer sun is too strong, the system can boil away the water in the tank and cause the tubes to overheat. The good thing about "heat pipe" evacuated tubes is that one or more tubes can be removed from the array and the system will continue to function perfectly (obviously with less heating power). So you can reduce the array size in summer and increase its size in winter very easily. Alternatively simply cover one or more tubes if the day is too sunny.
Frost
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If frost is a likely problem, a glycol solution must be run through the manifold and this circuit must be kept separate from the domestic water. Heliatos have an external heat exchanger which connects to the bottom feed connector, hence if retrofitting, there is no need to purchase a hot water cylinder with internal heat exchange tubes. The Heliatos external heat exchange system requires two pumps and a 20W solar PV panel in the usual "high panel" configuration (compared with the standard Heliatos arrangement which uses one pump and a 10W solar PV panel).
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If establishing your system de novo, obtaining a cylinder with internal heat exchange tubes will be preferable and more efficient. As the internal heat exchange tubes will be much wider than the tiny tubes of the Heliatos external heat exchanger (thus posing less resistance to flow), there should be no need for any electrical pumps at all in the "low panel" configuration. This arrangement may turn out to be the simplest yet most robust configuration, which can suit all climates (even with freezing winters), as seen in the final diagram. Hence this is my preferred configuration. As in all things the proof of the pudding is in the eating and the end user must try their own system out for themselves and tweak things if necessary to make it work. There will be different specifications of different components purchased by different users in different climates, hence no two systems are likely to be identical and some customisation may be necessary.
Exclusive use of rainwater will avoid the problem of lime deposits from hard water.
CONCLUSION: This article outlines a variety of options. Different configurations will suit different people depending on whether they want roof mounted or ground mounted panels and what level of complexity they are happy with. Conventional systems are convenient (hot water is available at all times with backup heating which however requires complex electronics) but also have more potential points for failure. I do not mind some inconvenience (no hot water in tank after several heavily overcast days) but prefer an easily maintained, simple and robust system with greater longevity. Just remember to buy good quality components from the outset and obtain plenty of spare parts (eg extra evacuated tubes, magnesium anodes etc) and you should be able to enjoy using the same system for at least the next twenty years.
G. Chia Feb 2016
*Footnote:
Boat based solar thermal arrays must by necessity be mounted flush on deck, which when stationary will be horizontal (or near horizontal), but due to boat movement will be constantly varying in angle. Evacuated tube systems are not feasible for boats because:
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Irrespective of latitude, the tubes need to be angled at least 20 degrees from horizontal to allow convectional forces to operate within the tubes
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Even though designed to cope with small hailstones, tubes are easily shattered (whereas a flat panel with polycarbonate cover will not break if a heavy shackle drops on it)
A boat based, horizontally mounted flat panel system will therefore require water to be circulated by electric pump: there is no option for passive thermosiphoning.
