Solar panel performance 07

Rather impressed to hear the solar panel working today, New Year’s Eve, on a grey, overcast day with no hint of sun.  It ran for less than an hour so if I hadn’t heard it, it wouldn’t have been recorded in the stats.

Since it started working in September, its mini-computer calculates that it has generated 385kW of heat for the hot water tank.

Using Nottingham Energy Partnership‘s energy ready reckoner, that’s about  £10.72 worth of gas.  We will have to see how well it performs when confronted with an actual summer.

Solar panel update

After a few false starts and complications (nothing is ever simple with this house…) the panel has now been running for a few days.

Nothing is quite perfect – the panel is in shade in the afternoon because of a tree, and doesn’t get the earliest sun anyway. The last few days have been very mixed in terms of weather. The end of September is clearly not the best time to get sun.

And yet every day so far, the panel has got solar gain. Not much, admittedly: four and five degrees a day – but that’s still taking a bit of gas out of the equation. Hopefully we’ll get more in strong sun.

Before I went away from the weekend, I toggled something in the controller that now makes it display how many watts of heat it’s gaining, and keep a running total. During Saturday and Sunday, it thinks it got 19kWh of heat out of the sun. I think that’s roughly equivalent to 80p worth of gas.

I paid extra to have a top range controller I can use across my computer network. So I can have a program on my computer that tells me what the panel is doing – here’s it’s current readout:


On the right-hand side S1 (sensor 1) is the temperature at the panel (which is actually double the current air temperature outside). S2 is – as the diagram shows – at the bottom of the tank; S3 at the top. It’s at 62deg at the moment because after sunset, the gas comes on to get it to the right temp for showering in the morning. On the left side, the black spot represents a green LED that comes on when the pump is turning, and the pump triangle in the diagram also turns.

It does all this both on the program on my PC and on the digital read-out of the controller. If you’re the sort of person who checks what the temperature is several times a day, or who taps a barometer when passing, a little readout like this is grist to the mill. I find myself making detours to the airing cupboard to see how hot my water is now.

The reason it needs these temperature readings is that the controller turns the pump on whenever there’s an appreciable difference between the temperature at the panel and the temperature at the hot water tank.

Although the controller is pretty cool, there doesn’t seem to be a way of getting at the data without the fancy graphics. I was hoping there was going to be a platform independent doodad that just let me read the temperatures and keep a record, and make calculations separately. In my mind, I had Automator on the Mac keeping records at quarterly intervals, and creating a web page to say how much solar gain I was getting at any given point. Maybe it could even be linked in to Skype so that my solar panel could send a text message when it turns on…

Solar panel progress – in pictures

So, work has begun on the solar panel.

This is Tuesday:

Roof access

The company turned up, and spent an hour looking at the roof. Access is really tricky. There’s a car-port preventing them putting a ladder straight up to the south-facing roof. Eventually, they decide the easiest way, short of entirely removing the car port then replacing it, is to go up into the attic, out through the sky-light and clamber over the roof to the panel.

Solar panel - incomplete

By the end of the second day, they have fitted the top part and some of the tubes. Because they only have access to the top of the panel, and not the bottom from a ladder of platform, it’s slow progress.

Evacuated solar tubes

Here’s what the tubes look like close up.

Evacuated solar tubes

They’re longer than I am tall – but once they’re up on the roof, they don’t look nearly so big.

Solar panel

The panel was completed by the end of Thursday – but so far it’s not connected to anything inside! Work continues next week – replacing the hot water tank, moving the header tank, plumbing in a new shower, connecting the controls, lagging the pipes.

Here’s a picture of the new tank – with a wheelie bin in the background for size comparison pictures.

hot water tank

And here’s a picture taken through the hole in the top where the immersion heater goes. I love this picture. You can clearly see both coils – a larger one around the middle which will be connected to the ordinary boiler, and a smaller one with fins around the bottom, which will be connected to the solar panel.

inside hot water tank

Shower technology

We’ve bitten the bullet and decided to go ahead with the solar panel. It should be up by the end of next week. I’ve also requested a special controller that lets me keep a log of how well the panel is working. We’re not really going to find out over the autumn and winter, although it will make some difference to our gas usage.

In going ahead, I’ve learned that showers are much more complicated than they look.

Currently, we have a simple electric shower. Cold water at mains pressure goes into a little white box and comes out hot.

But with a solar panel heating our water it makes sense to have a shower fed by the hot water tank. This means more options than I’d ever thought about before.

We could have a pressurised hot water system, with the hot water tank at the same pressure as the cold mains. This would mean that the hot water tank could heat the water over 100 deg C – getting the maximum possible out of the solar panel. But the setup costs are much more expensive, so this gets ruled out.

Which leaves us with a gravity fed hot water tank, with a header tank relocated to the attic to give some pressure, but not as much as mains pressure. You can’t mix mains pressure cold water with a lower pressure hot water system, so some sort of clever has to go on in the bathroom.

You could have a separate cold water feed in the bathroom, also fed from a header tank. But this means an extra run of piping.

You could put a pump on the hot water system to bring up the hot water system to the same pressure as the cold water system. I was a bit worried about this option because it seems to me that it might be a way of using the water up quicker, and emptying all the hot out of the tank.

The last – and simplest – solution is to put a pressure limiting valve on the cold water feed to take it down to the pressure of the hot water. I think this is what we’ll be doing.

Solar panel

Today we had a visit from the Notts Energy Partnership to consider whether solar hot water might work for us. They run a project called Sungain, which is about bringing solar hot water to as many homes as possible – and in the process drive capacity amongst local firms for putting solar panels on local roofs.

Just for good measure it’s the first properly sunny day for ages, so we could see clearly that the roof over our car port gets its fair dose of the rays – at least until 2.30pm when the sun goes behind the tree. Our house is oriented with the corners on the compass points, which means we don’t have a south facing roof, but a south-east facing one. This is less good, but not impossible.

It’s not a trivial job to fit one in this house. We already have a hot water cylinder based system, but at present our hot water is only used for hand washing and the little hand washing up we do. There’s neither the pressure or capacity in the current old cylinder to run showers, which would be the main point of embarking on something like this.

So to get solar hot water, we would need to

  • fit evacuated solar tubes on the roof
  • replace the hot water cylinder with a larger one
  • relocate the hot water header tank to the highest possible point in the attic
  • pipe the whole lot together
  • replace bath taps with a mixer shower combo

It is quite a lot to do, and won’t be cheap. But in the context of some of the other things we are spending on the house it is an affordable cost. It’s not a great deal more than the cost of new windows.

There are still some things to weigh up.

  • Pay-back time. This should give us cheap or free water heating for much of the year. Even over the winter, on bright days, the kit should mean the boiler has less work to do to get the tank of water up to temperature. But calculating payback will be complicated. It will mostly be electricity we save (no need for electric showers).
  • Changes in the bathroom. Neither of us has particularly fond memories of the times we spent in homes with tank-fed showers instead of on-demand heating from an electric shower or system with a boiler that heats as you use. Don’t want to run out of hot water, or get scalded when loo flushes or washing machine starts. This is mitigated by the fact there’s only two of us, and we usually shower at different times of day
  • Fitting a mixer tap might mean replacing the bath
  • There’s a chance a yob with a rock could damage the solar tubes
  • Fitting complicated things – or even just “different” features – might make it harder to sell
  • Still more work to do on the house and plumbing!

On the plus side

  • Low cost hot water, even when fuel prices increase
  • A bigger airing cupboard
  • Feeling better about banging on about sustainability issues in planning meetings
  • Solar panel will improve house performance on energy performance certificate element of home information pack
  • Watching the control gizmo, which monitors the temperature of the panel and the temperature of the hot water tank, and switches on the coolant pump whenever the panel is warmer than the tank, will appeal to my temperature nerd elements 🙂

EDIT: Good grief, a bath/shower mixer tap costs over £100, according to B&Q website!