Ask most people about “going solar” and they think automatically about huge, expensive silicon panels with all the inverters and all. I’ve been there and done that too. My 2KW roof installation happily generates electrical power whenever the sun shines – and this being Los Angeles, it shines just about all the time. Great. But probably not a good investment. Even with all the rebates and tax incentives, I doubt it will ever really pay for itself – not until we’re facing 20 or 30 cent per KWHr. Still, I put the system in to “do my bit” and fuel the industry etc. Installation prices are falling quite considerably since I put my system in. I’m still not a big fan though.
More recently though, facing an excess of free time, I decided to get creative and install my own solar domestic hot water system. So far, the results are brilliant – I get all the hot water I can use for the cost of about 1 penny per day – that for the small electric pump which drives the water through the collectors. For the life of me, I can’t see why the building regulations don’t make such systems compulsory in all new constructions. For one or two thousand dollars (depending on your creativity) you too can live off the fat o’ the land. Here’s my story…
My quest started by finding a couple of previously-loved collector panels on Craigslist. Back when Jimmy Carter was president, tax incentives created a large industry making these things – and they were built to last. Even the White House installed a system. I’m told that the first thing Raegan did on taking office was to repeal the incentives and then remove the panels. Without the tax incentives, the burgeoning solar industry died in childhood and early adopters found no-one available to maintain the systems in place. This was bad news for the country, but happily for me – it means there are a lot of components available for next to nothing. I picked up a couple of 4′ x 8′ solar collectors for little more than the scrap metal value – and these things were build to last.
The next step was to research on the internet how best to use these panels. It turns out there are three main systems, each suitable for a different climate. The simplest is a thermo-siphon design, where a large storage tank is installed above the collector panels. When the sun shines, hot water from the panels rises into the tank without pumps or anything. This design only works where there is zero chance of frost though. Even here in Sunny California, we get one or two nights a year below freezing, so this design is out.
The most popular system pumps anti-freeze in a closed circuit, transferring heat to the storage tank via a heat exchanger. This system is more complex and hence costly to install. One problem faced by this design is in dealing with overheating – once the storage tank reaches the upper set point, something needs to be done to prevent the antifreeze mixture from over heating (and in the process becoming corrosive and reducing the freeze protection). Many of improperly maintained installations failed with corrosion and burst pipes and/or leaks caused by not changing the anti-freeze early enough. For much of the country though, this is the only workable system.
In temperate climates though, such as here in Southern California, there is an ingenious design refereed to as a drain-back system which is almost maintenance free and more or less fool-proof. In a drain back system, plain water is used in the closed loop, avoiding breakdown of the antifreeze due to overheating. A simple electronic pump control made just for this purpose sensors the temperature of the water in the collectors and in the storage tank and whenever there seems to be a useful differential, turns on a small circulatory pump driving water through the collectors and heat exchanges etc – just like in the anti-freeze system (Using plain water as the heat exchange medium turns out to be more efficient in both the heat capacity and in lower viscosity for the pump – a double win). When the controller senses no need for heat (either due to the storage tank upper temperature limit being reached or when the sun no-longer shines enough) the pump turns off (saving electricity). Rather than keeping the collector panels full of water though, which could freeze and burst, this design incorporates a small reservoir tank mounted inside the building’s warmed space. When the pump turns off, all the water in the collectors flows back into the reservoir – the drain-back tank – and is protected from freezing. Care must be taking in installation to ensure that all the pipes slope continuously back to the drain-back tank for this purpose. Installations tend to look a bit odd as a result – a small price to pay, I think.
Here is my simplistic diagram of the arrangement showing the closed circuit heating loop. The drain back tank is shown here with an open top while in practice the solar loop is sealed to prevent evaporation.
When there is no useful temperature difference between the collectors and the tank – or when the hot water tank reaches the top set point, the controller (not shown) shuts off the circulating pump and water from the collectors quickly drains into the drainback tank avoiding any possibility of freezing or overheating.
The most expensive part of this – not counting my labor – was in the brand new 80 gallon solar hot water tank. My old gas powered tank was rusting through and I was unable to find something better to replace it. The model I choose incorporated a heat exchanger in the bottom part of the tank and an electrical heater element in the upper part. Once or twice a year we go for three or four days without significant sunshine. I though I might need the electrical backup for these days, but so far we have been able to brave it out during these periods with shorter and shorter showers etc. so the electrical elements remain un-connected.
Next in expense was the dedicated drainback tank. I’m not too happy about this as the drain back tank is really low tech. I think I spent something like $400 in the end which seems criminal. I tried a number of alternatives first, but finally accepted the inevitable.
The differential temperature controller came next at something like $120 or so. I picked a model with regular household plug sockets making it easy to trouble shoot – I can plug the pump directly into the wall power if needed.
The last part was the water pump – there were a bewildering set of choices here and a wide range of prices. Nearly everything I read online said that I needed a bronze or stainless steel pump and fittings, costing 2 or 3 times as much as regular cast iron pumps. The idea being that using water as the solar fluid would cause the iron pump to rust. One link pointed out though that this was not a problem with a drain back system so long as the solar circuit is closed. Rust is caused by dissolved oxygen in the water which being sealed is quickly depleted. I shopped around until I found a three speed cast iron pump on special – I think the whole thing cost no more than $50 and it works like a charm. The drainback tank includes a parallel site-glass which I look at from time to time to confirm that the water levels remain constant (no leaks) and rust free.
During the design, I was concerned about the energy consumed by the electrical pump. In the end, I run my pump on the lowest possible speed which according to my kill-a-watt unit uses 50 Watts. Note that the pump only runs while the water is being actively heated, so most of the time it is off. In practice, I find the pump runs for about 3 hours a day. 3 hours at 50 Watts makes .15 KWh per day which at our utility rate of $0.12 per KWh becomes about 2 cents per day. Not bad for some simple plumbing.
One last thing before wrapping up – many of the things we take for granted with domestic hot water are no longer true once we go solar. Advice like switching to a washing powder that works in cold water is no longer necessary. Similarly, lowering the hot water temperature setting does nothing to save energy. Taking long hot baths still uses water – but that’s another thing. In practice, I set the tank cut-off temperature well above the normal 45C (115F) – more like 70C (160F). To prevent scalding though, I installed a tempering valve which constantly adds cold water to the mix. The effect is like having a tank much larger than the actual 80 gallons.
So that’s how it works. I highly recommend something similar to anyone living in a high solar region. There is nothing like the feeling of smugness that comes from a piping hot shower of liquid sunshine.