Solar Power-is it worth it?

thomas.dahl

This is a very interesting thread and I like the arguments in the earlier stages.. good stuff.

I am considering upgrading our 33 year old oil fired heating system (24KW - 6000L consumption per year) with:

1. A modern condensing oil fired boiler
2. Some solar panels
3. Wood Stove in the our sitting room for comfort, convected heat as well as a back-boiler (https://www.brunner.de) for:
2. A 1000L thermal storage unit (hot water system with a coil for DHW) which will be connected to all of the above.

This thread talks about how poor solar panels have proven in DHW systems (10 year pay back). When used in the above scenario it may make a little more sense.

This whole system will probably cost around 25k pounds..

I expect to burn about 9 cubic meters of wood (3 tons) per year. This should keep about 50% of the oil consumption at bay.. There are people out there who have managed to get away with using practically only wood, but they burn some 6 tons of wood per year and need to fill their fireplace twice a day with 10Kg of wood.

Am I mad.. or does this make sense?

Thomas

Cardew

thomas.dahl wrote: »

This is a very interesting thread and I like the arguments in the earlier stages.. good stuff.

I am considering upgrading our 33 year old oil fired heating system (24KW - 6000L consumption per year) with:

1. A modern condensing oil fired boiler
2. Some solar panels
3. Wood Stove in the our sitting room for comfort, convected heat as well as a back-boiler (www.brunner.de) for:
2. A 1000L thermal storage unit (hot water system with a coil for DHW) which will be connected to all of the above.

This thread talks about how poor solar panels have proven in DHW systems (10 year pay back). When used in the above scenario it may make a little more sense.

This whole system will probably cost around 25k pounds..

I expect to burn about 9 cubic meters of wood (3 tons) per year. This should keep about 50% of the oil consumption at bay.. There are people out there who have managed to get away with using practically only wood, but they burn some 6 tons of wood per year and need to fill their fireplace twice a day with 10Kg of wood.

Am I mad.. or does this make sense?

Thomas

Solar domestic hot water payback in 10 years - how?

In calculating 'payback' times or return on investment(ROI) the loss of interest or cost of borrowing must be taken into account.

For instance to borrow £25k at 7% will cost £1,750 a year in interest.

A solar Hot water system at, say, £3k will cost over £200 p.a. and even with oil you won't save £200.

I would have thought for £10 -15k you will get a good Ground Source Heat Pump system that will be much cheaper to run than your proposed system

thomas.dahl

Cardew: I think that you are quite right that current Solar DHW is a long term proposition. Even the most serious forecasts suggest that the payback is no less than 8 years and maybe as long as 15. I think the decision to install a solar system is based on a feeling that you are getting "something for free", even if it costs you heavily to get it. Of course, to further justify it to oneself we can point at the environmental benefits and the "likelihood" that energy costs are likely to rise. In other words you can start to feel that you are "doing your bit for the environment".

Here in Germany we can get subsidized bank loans of 3.5% which obviously makes the financing of such a project a little more attractive. These loans run over 20-30 years. In addition "energy efficient" houses are now seeing a higher re-sale value so the loans may well pay for themselves in the final analysis.

One technical aspect of all this which I do not fully have a handle on is:

The whole concept of a Solar Collector is to provide as large a temperature delta between the water coming off the roof and the water currently in the hot water storage tank. This is the only way to "add" heat to the storage tank efficiently and quickly. As the tank temperature rises the temperature from the roof needs to keep up and remain higher. One nice trick here is to vary the speed of the circulating pump to give the roof time to heat the water as much as possible before drawing it off the roof.

As the summer comes to an end the roof temperature starts to drop and there is no net gain to be made between the roof and the storage tank the whole point of drawing heat off the roof comes to an end. This is when the oil/gas heating needs to kick in to meet the demand for hot water.

Soo.. to my question:

Heat-Pumps. You mention that you feel that a heat-pump may be a better proposal. My concern is that I need to dig-up half an acre of lawn, or drill a well in my sloping garden that is massively deep to get to any water with a stable temperature and which actually contains some heat that I can be extract. Air-to-Air systems like those so common in Sweden become very inefficient in winter (when you need them most) and you are practically using electricity with no pump efficiency when it is below 5 Celsius (and I have been there when it is -27!).

The Solar Collector is still a little warm, even in mid-winter, during the day. It simply does not rise enough to provide the temperature needed to be useful. So why not circulate this "lower" temperature water through a storage tank (500L maybe) and then use a heat-pump to extract and boost this to a temperature that can be used to heat-up the larger storage tank (1000L). Would this not extend the seasonal usefulness of the Solar Collectors? Are not heat-pumps more efficient when the "step-up" that they need to work against is as low as possible?

Obviously at night, when very cold or on rainy days, you would not expect much of a boost out of such a system, however I am only trying to make the current rather inefficient Solar Collector technology more useful - for longer. I do not expect to replace the primary heating source such as gas/oil.

Does anyone have any thoughts or experience with this?

Thomas

paceinternet

Thomas, my thoughts are that your 500L tank would still need to provide a "volume" of warm water to satisfy the heat pump, or you would need a control system to provide the heat pump with an alternative supply when the temperature was insufficient. Maybe an immersion heater to keep it simple?

Air source heat pump supplier data continues to evolve and some do show a COP of around 2 at sub zero temperatures, and some even show they are operable down to -25 deg C.

So, the advice seems to be, do not expect to have the full benefits at your lowest temperatures, as they may be for only a few days or hours a year. But create supporting systems for those situations, and use the higher COPs of 3-5 for maybe 90 + % of the year.

If you are doubtful about the performance capabilities, I can post some manufacturer links for you.

thomas.dahl

PaceInternet: Thank you for your comments. Very interesting.

I think I would probably stick the immersion heater straight into the hot-water storage tank (1000L tank) rather than let the heat-pump repeat the exercise...

If you have some link I would be very grateful as I want to learn more about all this.

What is COP? Is this the net gain? My understanding so far is that the net gain using water based systems could be around factor 4. ie. 1Kw in and 4Kw out.

If indeed the gain is only 2 at low temperatures and 4 at modest temperatures, any "boost" in source water temperature would move the gain from 2 towards 4 which should be a massive cost saving.. Using a solar assisted buffer tank may well be useful.

Of course I am now going to struggle with having two types of heat-pumps in my equation. One for the water-to-water and one air-to-water. Very different animals.

Still, worth considering the physics further to see what conclusion we may reach.

Thomas

paceinternet

I'll post the links later, but a quick clarification comment:
Remember that the heat pump will lower the temperature of it's source, probably proportionately to the amount of heat it is extracting. So if your solar water heating only produced 100 litres at 20 deg C and the heat pump tried to heat 200 litres at 50 deg C, the supply tank would freeze. You would need some temperature sensors and controls to switch it off at limits to prevent it trying to freeze. Effectively you have a closed circuit with the 500 l water tank if the solar is insufficiently active.

Conceptually you are right though, and another way would be to solar heat the air going into the heat pump which would then not be constrained by supply.

Also be aware that heat pumps have an upper limit for the supply air or ground source. Something quite practical, maybe around 45 deg C, but the links will show you.

thomas.dahl

PaceInternet: Good points. There would have to be some good controls for the system to prevent freezing up the "source" tank. I think these types of heat-pumps should have a built in feature to sense and control this condition anyway..

A neighbor of ours has a buried swimming pool that has not been used for 15 years and is empty (he keeps old furniture in it). I keep telling him to fill it and buy a heat-pump. He could theoretically add a battery of solar panels to help keep its temperature up and hence the efficiency of the heat-pump as high as possible.

My idea is to simulate this..

Thomas

paceinternet

See pages 11, 12, 13:
http://www.mitsubishi-heating.com/pdf/Ecodan_Brochure.pdf

easytreasure

You could consider a DIY setup https://www.solarfriend.co.uk if you are handy with tools

thomas.dahl

Thank you guys, but you do not really address my question about buffering water in a seperate tank and using a heat-pump to boost the heat stored there into my main tank.

Anyone know anything about this?

Thomas