Solar Domestic Hot Water - calculations

muymalestado

We've put panels on the roof and have been delighted and amused at the sun's heat being captured into our hot cylinder - watching the temperature numbers is great fun.

I installed a datalogger and get about 8,400 spreadsheet rows each month full of 5-minute readings.

Now; how to do the calculations. I have some set up but am not convinced of my prowess here.

Eg, one outcome I would like is savings in gas not required.

Does anyone listening wish to enter a dialog on just exactly how to best to calculate what is going on ? I realise this would be utterly boring for most readers.

Samples readily provided on request.

Ken68

Don't know May...for comparison wouldn't the weather have to be the same. If you save, e.g., £60 per heating season, doesn't follow the same next year.
I have a similar problem calculating the savings from some DIY exterior window shutters.

Cardew

If you provide the data, we can work out the savings.

Typically a good well designed system will save you £50 to £70 a year.

muymalestado

Thanks for replies:

Ken68 - weather is all, some days go by with zero operation.

Cardew - I have an outside hope of better than £50 savings a year as that rate would mean a payback of several generations.

As a fr'instance I get 3 temps recorded and the flow rate of the heated fluid, thus I should be able to calculate mass of water/fluid passing through and the heat it loses to the cylinder as joules of heat captured. Then "knowing" joules of heat in a litre of gas should give the amount of gas not required. Gas price per litre should give savings each 5 minutes. Add that up and gloat!

Simple? I just don't trust it, as a small alteration to the calculations gives very big change to end result - thus my wish to be right.

I hope that a robust set of calculations could be used by anyone who can capture similar data.

Cardew

There are so many variables in any set of data that it will be impossible to measure accurately, and you will have to make assumptions of efficiencies and heat losses. However it all boils down to calculating the output of your panels in kWhs.

If you do a search on this site you will find masses of posts on solar energy savings.

The average family with Gas CH spends between £100 and £150 a year on Domestic Hot Water and you will be very fortunate to save 50% of that.

If you want to see some unbiased tests carried out by the DTI read:

http://www.berr.gov.uk/files/file16826.pdf

They tested 8 systems and found that the average output was in the region of 1,200kWh per year(in Southern England) which if you have gas CH will represent a saving of £30 a year.

So let us assume that somehow you can double that saving(and ignore any costs for running the system) you might get your savings to £60 a year.

Also take into account that you have to pay for the electricity to run the pump; also there will be breakdowns/maintenance in the future. Another factor is the requirement to get up on the roof from time to time and clean off the grime and bird droppings from the panel(s).

In economic terms solar heating does not make any sense; the loss of interest on the capital expenditure alone will be more than the savings.

Obviously some people want solar to 'save the world' and that is of course their prerogative, but at present prices it cannot be justified on economic grounds

muymalestado

I do so envy, Cardew, those who have even your max £150 spend on gas. Our monthly DD is £26; because the gas is delivered by our Dutch friends via road tanker. Higher cost makes even a small saving more worthwhile.

Adding in a simple desire to spend on green-like projects I feel we will get value in the long term.

What I am aiming for is accuracy on my calculations, not justification for the spend.

Prior to purchase I consulted the document you refer to and our local solar panel maker turns out to be a reasonable choice in the tests (AES). Subsequently speaking with the owner, I have to say that they have improved, as you would expect, since 2001. AND, the pump we installed modulates by an optimising algorithm.

One key feature I would flag up is attention to insulating the pipe-run as small losses 365 days a year will mount up.

All that said - I wonder whether ground source heat may not be a better option if our weather turns cloudier. As we have too little soil it is a non-starter here.

Cardew

muymalestado wrote: »

I do so envy, Cardew, those who have even your max £150 spend on gas. Our monthly DD is £26; because the gas is delivered by our Dutch friends via road tanker. Higher cost makes even a small saving more worthwhile.

.

Not certain what you mean by that statement???

Solar heating only heats Domestic Hot Water i.e. the water from taps. NOT heating the house. The £100 to £150 is what is spent on hot water - not the total gas bill.

So if the solar systems they tested save approx £30 a year, how much do you think they have improved? 10%? 20%? 50%?

What is your estimate of annual savings?

Would you care to say how much your system cost to install?

As I have said before, people can buy what they want, for whatever reasons they want - that is obviously their choice.

However surely nobody can pretend that solar heating makes economic sense? and this is a Money Saving website.

penrhyn

This must be an entrant for techno babble phrase of the week!:

"the pump we installed modulates by an optimising algorithm"

One trusts the pump is also powered by an extra terrestrial source of nuclear fusion.

.........I'll get me coat:rotfl:

greyteam1959

penrhyn wrote: »

This must be an entrant for techno babble phrase of the week!:

"the pump we installed modulates by an optimising algorithm"

One trusts the pump is also powered by an extra terrestrial source of nuclear fusion.

.........I'll get me coat:rotfl:

Thats just what I was thinking !!!!!!!!!!!!!!!!
:eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek:

Ken68

A manual set-up DOES give hot water (tho sometimes only warm) in the late spring/summer/early autumn. Capital cost about a fiver.
My neighbour uncovers both our systems in the early morning, and because he gets in late, I draw the 10 litres into a large industrial type insulated flask.
Did I mention is was FREE.

muymalestado

Cardew, gas delivered by road tanker of course is CALOR. Its good stuff but paying for it makes you weep. And, I cannot separate hot water from the house heating. But our wee intiatives over the past few years have halved our delivery litreage which is a start and is contributed to in some small way by the solar panels.

I don't want to turn this into a technobabble thread or site, thus the desire to be able to state confidently what performance we see - ie, are my calculations sense or nonsense. And, was this a good money-saving ploy (at this house) ?

As for the optimising algorithm it just means the solar system controller is smart enough to monitor several temperatures and the flow rate and to adjust that flow rate to consume the least amount of electricity at the pump whilst taking as much heat out of the collector as possible at that moment.

Cost; is definitely not free. And whether SDHW is worthwhile must depend on capital cost as well as daily benefits. After the grant we spent just above £2K.

We have friends with 'free' systems which freeze in winter and boil in summer
and consume time in repairs. Who is to say that route is better?

One point of displaying my situation here is so others can form some opinion on whether, or how, to invest in the solar gambit.