Solar water heating efficiency

corktree · Post by **corktree** » 11 Jul 2009, 16:13

I've asked this on another forum before but I still haven't been convinced of the answer:

I had a flat solar panels water heating system installed early last year but I have a question about the running of it.
The circulation pump for the fluid between the panels and the tank has 3 settings. These are 35watt, 46watt and 65watt.
Is there a setting that is most efficient or should the wattage be changed based on weather conditions outside?

I've tried observing the running time with different settings but as weather conditions are not consistent it tells me nothing.
I know it's small wattage but the pump seems to run for several hours a day so it could cost between 100 and 200 kwh in a year. I know that's less than £30 but it all adds up and I'm more concerned with the longer term impact.

My "back of an envelope" estimation of the logic would be: When there is direct sunshine a low wattage will be most efficient but in poor sunshine a higher wattage would be more appropriate to "harvest" as much of the sun's energy as quickly as possible.

Am I right or is there a single setting that is more efficient regardless of weather conditions?

Bandidoz · Post by **Bandidoz** » 11 Jul 2009, 17:30

I would guess the power applied to the pump would determine how quickly the fluid flows in the system, as well as how much pressure is generated in the heat exchanger. If the pump is OFF, no cold water would be getting into the panels to be heated.

I'd say that if you're getting enough hot water from the system on the low power setting, then use that. Otherwise use the higher setting.

corktree · Post by **corktree** » 11 Jul 2009, 17:46

Thanks Bandidoz, I guess the problem is that sometimes I'm getting enogh hot water and other times I'm not (weather depending). I'm guessing what would be best is some intelligent pump that would increase and decrease flow when necessary? But the issue is I don't have an intelligent pump so I need to decide on a setting, or should I switch it based on weather.

woodburner · Post by **woodburner** » 11 Jul 2009, 23:07

Those power levels sound suspiciously like a normal domestic CH circulator. That is much more than is needed to shift the heat from a panel, and it's consumption makes a bit of a joke of the savings of solar heating.

Post by **kenneal - lagger** » 12 Jul 2009, 04:55

The Solar HW tutors at CAT say that, for maximum efficiency, there should be a 5 deg C temperature difference across the panel from input to exit of the water. Adjust the wattage to achieve that.

We have a Solartwin panel which uses a 5W PV panel to drive the pump directly (not through a battery buffer) and we have anything up to 20deg C temperature gradient across the panel. But then the CAT people hate Solartwin with a vengeance.

Compared with the 5W of Solartwin, even 35W seems excessive.

caspian · Post by **caspian** » 12 Jul 2009, 10:43

woodburner wrote:Those power levels sound suspiciously like a normal domestic CH circulator. That is much more than is needed to shift the heat from a panel, and it's consumption makes a bit of a joke of the savings of solar heating.

Why? I don't understand your reasoning. 60 W is the power consumption of a smallish incandescent lightbulb, and the pump is only on for a short amount of time (on ours it's probably only on for about 1 or 2 hours a day). The heat is provided gratis.

PS_RalphW · Post by **PS_RalphW** » 12 Jul 2009, 11:59

And remember the 60W would be converted to heat, heating the water mostly. So the energy is not entirely lost.

contadino · Post by **contadino** » 12 Jul 2009, 12:36

My pump is a 12vdc jobby that runs from a small PV panel. It is designed to run at different speeds based on the amount of current coming from the PV panel. IMO, all solar circulation pumps should work like this, but then, IMO all solar circulation pumps should be 12v.

I suspect that even with the 35w setting, your water is being circulated much too quickly. Do you know the volume of your primary circuit, and the pump throughput on the 35w setting? Do you have a controller? Even a simple differential temperature controller will make a massive difference.

corktree · Post by **corktree** » 13 Jul 2009, 17:35

Thanks everyone. I don't know enough about this to answer some of your questions like volume of the primary circuit, and the pump throughput.
The pump (yes it looks exactly like an ordinary domestic CH pump) has just 3 settings.
Based on your responses I'll be setting it on the 35w setting for now but think that is still way too fast.
The controller is connected to the pump by an electric cable but it seems to only have an on/off effect on the pump.

contadino · Post by **contadino** » 13 Jul 2009, 17:58

If you have a controller, the pump speed issue shouldn't be a problem. If I were you, I'd find out how the controller is setup to see if that's where the problem lies. Maybe if the sensors are incorrectly placed it would have a negative effect. They should (AFAIK) be on the exit of the panel as close to the panel as possible, and 2/3rds of the way up your tank (depending on what sort of tank you have - how many circuits, etc..)

emordnilap · Post by **emordnilap** » 15 Jul 2009, 11:15

kenneal wrote:We have a Solartwin panel which uses a 5W PV panel to drive the pump directly (not through a battery buffer) and we have anything up to 20deg C temperature gradient across the panel. But then the CAT people hate Solartwin with a vengeance.

Compared with the 5W of Solartwin, even 35W seems excessive.

I suspect they hate the early models; the 'solar controller' has perfected the SolarTwin system and it, along with the pump, is run by the pv panel.

I really like the system and it is saving us using a considerable amount of electricity, cheap though that is at the moment.