How to calculate solar fraction of a solar thermal system

[najeeb]

If we have following data:
1. Solar collector type & size.
2. Required hot water temperature.
3. Application area example House with 2 floors.
4 .Number of people.
5.Climate data (Air, temperature,relative humidity,solar radiation etc)

How can we calculate solar fraction for a solar hot water system from the above mentioned parameters.

I found many online software but I am not able to find those data or reference which can guide and provide formulas to do these calculation manually.


Anyone who can help in this regard, the assistance in this regards will be highly appreciated.

[Mike90250]

I'm not too much into the solar thermal end. You need to know the collector efficiency, the BTU of solar in your area per sq meter, the insulation efficiency of your pipe and storage tank, and the input temperature of the water. Too much for me, so I'd say stick with the generic calculator.

[russ]

Hi najeeb - Welcome to Solar Panel Talk!

If you have

1) the collector area and efficiency
2) The watts/m2/day for your area - insolation

You know the kW possible to collect. 50% average efficiency won't be all that far off.

The rest - 2 floors & number of occupants is only the total volume required.

Russ

[najeeb]

I asked these questions several times, but no one knows and ever provided the real basis and formulas/equations used for engineering and designing a Solar Thermal Hot Water System.

[russ]

If you want someone to hand you 'how to design a solar thermal system' with all engineering details I expect you will have to pay for it.

There is zero difference between the US and elsewhere in this regards.

Russ

[Mike90250]

http://en.wikipedia.org/wiki/Solar_water_heating

or check out www.solarroofs.com. They may have a page on design.

[Naptown]

If we have following data:
1. Solar collector type & size.
2. Required hot water temperature.
3. Application area example House with 2 floors.
4 .Number of people.
5.Climate data (Air, temperature,relative humidity,solar radiation etc)

How can we calculate solar fraction for a solar hot water system from the above mentioned parameters.

I found many online software but I am not able to find those data or reference which can guide and provide formulas to do these calculation manually.


Anyone who can help in this regard, the assistance in this regards will be highly appreciated.

You need a few more things added to the list above
A location to determine insolation (zip code if in US)
inlet water temperature
Azimuth and tilt of collectors
Is the system direct or indirect. Water or glycol transfer fluid if indirect.
Efficiency of heat exchanger
Size of tank and will it be a single or two tank set up.

Biggest question is why do you want to do it manually?

[Solar33]

Solar thermal systems behaviour is not easy to predict. The most spread methodolgy for solar fraction estimation is the FCHART curves, however if you are willing to correctly size and optimise a solar thermal system you should give a try to a dynamic simulation package like TRANSOL: http://aiguasol.coop/en/transol-sola...ergy-software/

[MikeSolar]

A pretty good, and free, xls based program called RetScreen is what you should try to use. Goggle it.

Aside for that, the efficiency of the panels is actually less important than the surface area, pumping and the heat exchanger. A lot of people say "our panels efficiency is 95%". Pure BS. There are 2 main factors to determine efficiency of the absorber, one is how much of the total energy available to it, will it absorb. The second and more important in a northern climate, how much will it lose back to the environment through the insulation, back out through the glass. Typical REAL energy delivered to your tap is about 50-55% of what falls on the collector.

That 95% panel, if it has a selective surface such as most European panels and the Gobi will absorb 94-96% of the energy but will emit only 4-6% back out. After that there are losses in piping, pumping, control strategy and heat exchange.

Many people, such as those who make panels for southern climates state that a painted surface is just as good and longer lasting in the long run. They can absorb 95% but might re-emit up to 20%. The upshot of this is that the painted surface will not get as hot over the long term and cook the glycol as quickly. Therefore it will be under less stress and enjoy a long life. Remember that we are talking about a difference in stagnation (temps with no fluid flow and peak solar insolation) temps from 200C for a selective surface and 160C for a painted surface. Collectors don't operate in this temp range (40-80C is typical) so it doesn't affect your performance much.

That said, a well designed system for DHW will have a fraction of 50-55%, 65-80% if you are a conserving household, but at high fractions like that glycol will need to be changed more often.

[Naptown]

Actually solar fraction is more a function of what the system will produce based on the daily load.
say a system will produce 50 gallons a day of hot water
If you only use 50 gallons a day the fraction would be 100%
If you use 100 gallons a day the fraction would be 50%
This is based on a clear sunny day The annual fraction will always be lower due to cloudy days, rain etc.

[MikeSolar]

Actually solar fraction is more a function of what the system will produce based on the daily load.
say a system will produce 50 gallons a day of hot water
If you only use 50 gallons a day the fraction would be 100%
If you use 100 gallons a day the fraction would be 50%
This is based on a clear sunny day The annual fraction will always be lower due to cloudy days, rain etc.

Haha, I think I said that, in a round about way. It is pretty hard to get to 100% fraction on an annual basis (maybe in the desert), certainly not where I am.