Sing hey! For the bath at close of day
that washes the weary mud away!
A loon is he that will not sing:
O! Water Hot is a noble thing!
- J.R.R. Tolkien, The Lord of the Rings
A hot bath is one of the essential elements of the good life, and one which has been lacking from mine ever since I moved to Spain, where showers seem to be the rule. So September 6th, when we enjoyed our first baths with hot water from our very own solar panels, was a day to remember.
To be clear, we are talking here about solar THERMAL panels: the kind that make hot water directly from the sun – not solar PHOTOVOLTAIC (PV) panels, which make electricity, at a much lower efficiency and far higher economic and ecological cost.
In general, solar PV is appropriate in places with plenty of uninterrupted sun, no connection to the electrical grid, and/or generous government subsidies. These conditions do not apply to Abrazo House, so we have yet to go down the PV route. Besides, we don’t have the spare cash. Some friends who live off-grid not far from us invested €10,000 in a solar PV system (including panels, mountings, batteries, inverters, etc.); the retail price for our solar THERMAL system (panels, storage tank and all the associated valves, etc.,) was €2200, plus €400 or so for installation.
Also, from what I hear, the life expectancy of a solar PV system is rather unpredictable – especially the batteries, which are the Achilles’ heel of any off-grid electrical installation. Whereas with a simple solar thermal system like ours there’s really very little that can go wrong (crosses fingers). There’s nothing electrical involved: no pump, because the water circulates by convection, provided the tank is located above the solar panels (this is called a “thermo-siphon” system); and no thermostat, because when the sun stops shining, the water stops circulating automatically. It’s that simple. (Well, almost, because there are some safety valves and the like which have to be installed, but they all came with the panels. And provided that you install everything the right way round – which our plumber managed to do on the second attempt – it should work fine.)
As a matter of fact, there were a couple of vital elements missing from the kit, which we had to supply ourselves. One was a thermostatic mixer valve (mechanical, not electronic), which set us back a further €60, but which is essential to bring the temperature of the hot water down to a safe level; on a sunny day (especially if you don’t take enough baths) the water in the tank can easily reach 80°C. The other is a thermometer, so that we can tell when the water is hot enough for that long-awaited bath. For this we used an electronic oven thermometer (€8) with a probe which we stuck to the pipe between the tank and the thermostatic valve, and covered with lagging.
You can, of course, make solar thermal as complicated as you like, with electrical backup, thermostats, pumps, etc., for dubious gains in efficiency; but why would you bother? When you have a simple system that works, the way to improve it is not to make it more complicated, but to optimise the system while keeping it simple.
In our case, we did tweak it a bit. The system we bought was designed to be installed on a rooftop, but I didn’t like that idea for lots of reasons (e.g. structural, ease of cleaning, and well, if you’ve got it, why not flaunt it?).
Instead we decided to located the panels at ground level, at the sunniest corner of the house. This meant that instead of mounting the tank on the frame with the panels, we were able to put it inside the house (which anybody will tell you is the most sensible place for your hot water tank), on the second floor, and located so as to minimize the distance from panels to tank.
The panels themselves were designed to be mounted at a 45° angle to the horizontal, but we tilted them nearer to 60°, the better to catch the scarce winter sun; reasoning that in summer we expect to have more hot water than we can use, whereas in the winter we will be glad for every scrap of heat.
Thus far we had a system (see diagram) that worked perfectly, with only one tiny drawback: no sun, no hot baths. This wouldn’t be much of a problem in a lot of places, but here in Green Spain, like in any maritime climate, we have a lot of cloudy days in the winter.
You can buy solar panels that make hot water even on overcast days; I once visited a factory where they are made (in the north of Ireland, appropriately enough). But ours aren’t that kind, so we needed a backup source of heat.
The only type of fuel around here that is both free and ecologically sustainable is wood. The wood stove in Snail Cabin has been keeping us cozy for several winters already. It and the views are what we have instead of TV. So a wood stove seemed like an obvious choice for the main house, but this time we wanted one that would not only keep us warm, but provide us with hot water as well.
We managed to find a stove that includes both secondary combustion (which is supposed to make it burn cleaner and more efficiently) and a back boiler – a water tank fixed to the rear of the stove. The stove wasn’t cheap – €1200 – but it was easy to install, anyway, once we managed to make a hole to take the chimney out through the cob wall. Then we got the plumber in again to hook the back boiler up to the hot water system. The plan was that the stove would simply act like a second solar panel, on another branch of the same circuit, with the water again circulating by convection from the back boiler up to the tank and back again.
But once we fired up the stove, we realised that there was nothing stopping the water from flowing the wrong way around the circuit which was now formed by the solar panel and the wood stove. And since water always takes the path of least resistance, most of the hot water decided to go and heat up the solar panel instead of the tank. Not only that, but when the sun came out the same thing happened in reverse – instead of heating up the tank, the sun was heating the wood stove!
The solution was to install two anti-return valves (one in each loop of the circuit) in order to stop the water flowing the wrong way, and also a cut-off tap in the solar panel circuit which you can close when the stove is in use (and which you should remember to open again afterwards).
If you forget to open the tap, and then the sun comes out, there should not be a safety problem, because the solar panel is still connected with the safety valves. But, obviously, the solar panel will heat up and the heat will not reach the tank, but will re-radiate back into the outside.
For safety reasons we decided not to install a cut-off tap in the wood-stove part of the circuit; I have heard stories about what can happen when someone lights the stove without opening the tap first. (Since we do have safety valves installed, the result should not be a life-threatening explosion; but still, you don’t want steam or very hot water at high pressure exiting through the safety valve or bursting the pipes.) But because the heat of the stove is much more intense than that of sunshine, I would rather not risk someone forgetting to open the valve before lighting the stove.
As I write this we haven’t tested the new system yet, so I will post an update soon to let you know if it works…
25 February 2013
We’ve now been using the redesigned system (including the anti-return vales) for a month and a bit. So how well does it work?
Pretty well. On cold, cloudy days we have the stove going pretty much the whole time that we are in the house (which varies from a couple of hours to the whole day). If we have the stove going for at least half of the day then there’s typically enough hot water by the evening for the four of us to have showers (though I don’t usually bother, I wait till there’s enough for a bath!) But because our tank is 300 litres, there’s never enough heat from the stove to heat up the whole of the tank — only the top layer of the tank heats up. This isn’t a big problem but it does mean that by the morning, the water has usually cooled down again (apparently by mixing with the cold water) even if we don’t have showers. Perhaps it would be more ideal to have two separate storage tanks — a large one for the solar panels and a smaller one for the stove. But we’re pretty happy with the system as it is.
We’ve found the cut-off tap in the solar-panel part of the circuit to be unnecessary; in fact we’ve never closed it. The two anti-return vales are sufficient to prevent the hot water from the stove reaching the solar panels. For safety, we will probably remove the handle of the valve to prevent anyone closing and forgetting to open it!