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Old 12-03-10, 03:49 PM   #6
osolemio
Hong Kong
 
Join Date: May 2010
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Quote:
Originally Posted by strider3700 View Post
Although I agree with most of your post I believe this is wrong.




if your water started below 110F then any heating the panels managed to do is energy produced. Yes you will still require an additional source of heating to get the desired temperature but that source will not need as much energy.

In my case my desired final temperature is 105-110 F My starting temperature is 45-50 F. Any rise that the solar panels accomplish is energy saved that the electric hotwater heater doesn't need to achieve the final temperature.


I agree about the larger tank sizes but there is some sort of ratio between collector size/efficiency and tank size. If you make your tank the size of a swimming pool, 50 sqft of collector isn't going to be able to heat the tank to a useful temperature. if your tank is the size of a garbage can the panels will quickly heat it beyond the desired temperature so you will stagnate the panels instead. My tank is roughly 160 gallons. This should be a fine size for DHW heating but is probably too small to be really useful for space heating. I'm guessing I'd draw the available energy out rather quickly running it through a couple of radiators.
In the example you give, your need is lower. When I say a need of a higher temperature, then I mean that all buffers and other demands are already at a higher temperature than the solar panel.

If the coldest point in your buffer, hot water tank or equivalent is warmer than the hottest point in your solar panels, then you cannot transfer any energy. Not unless you amplify the low-temp heat using a heat pump or similar.

It is quite common for solar panels to be around 100 F in winter, unless they are with mirrors, vacuum tubes or similar. If your radiators require 150 F to keep your house heated, it is not much good if you have 100 F. On the other hand, if you have underfloor heating, you are more likely to require a lower temperature.

What I am saying is, that from the very same panels, the amount of realistically extracted heat depends on what temperature you require, and how hot the panels get. Whereas a direct electric heater does not care much about the existing energy or temperature, it increases the temperature from what ever it is, adding the energy you put into it.

I finally realized this when I was discussing whether a solar panel was working or not. A user had the liquid changed, and complained that after the change, the solar panel was cooler than it was before. He was not happy that changing the liquid had lowered the temperature. Until I explained him that this is actually the purpose!

We do not install solar panels to make them as hot as possible. We install them to extract as much heat as possible. All other things being equal, the cooler you keep your panels, the more energy you have extracted.

Think about that ...
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Space heating/cooling and water heating by solar, Annual Geo Solar, drainwater heat recovery, Solar PV (to grid), rainwater recovery and more ...
Installing all this in a house from 1980, Copenhagen, Denmark. Living in Hong Kong. Main goal: Developing "Diffuse Light Concentration" technology for solar thermal.
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