Temperature & Heat In Solar Hot Water Systems


IMG_4736I was talking with my friend Gordon the other day, a man who installed four solar hot water collectors at his house to provide heat for these three loads: radiant floor, domestic hot water and hot tub heating. He had asked me for some advice on the design layout and was now just going through his first winter. He was excited to report to me that he was seeing temperatures of 180F degrees in his solar thermal heat storage tank!

Very excited…

Do you see any problem with this?

After congratulating him, I began to think about it and told him that his temperatures were too high! What? He looked puzzled so I began to gently explain that with such high temperatures, his solar collector panels were running at quite low efficiency. Why?

Because they are losing too much heat out the glass and the collector box before the solar heated water gets delivered into the house. I would guess his solar collection efficiency at 180F would be around 25% or less than optimum on these cold days. Hotter solar collectors lose heat fast to the cold winter air surrounding them.

He explained to me that he had a 250 gallon tank of water where the solar heat was delivered to, the thermal storage, from which the solar home heat was delivered to the radiant floor, the domestic hot water loads in the home, and then to the hot tub if there was any heat left.

Basically, in a nutshell, I quietly suggested to him that he could double his solar collection efficiency by doubling his solar thermal storage capacity! I did not do an exact calculation to verify this to the dime, but the general idea is totally valid…in the right direction, and simply stated so that he (and you) can grasp the point easily and picturesquely.

panels-in-salida-400x300The effect of doubling the solar thermal storage tank capacity is to lower the operating temperature of the system. Gordon thinks that 180F hot water in his tank is great! But actually running the system at lower temperature would actually increase the amount of “heat” collected. This is because the solar collection will be more efficient, delivering more heat to the house while losing less out the collector boxes to the cold winter day.

The lesson here is that “temperature” is not “heat.” Heat, as defined here, is a quantity of energy that is the product of two things: temperature, and the amount (volume or mass) of water or other storage media.

The closing argument here is that none of the heating loads in his home require high temperatures. The radiant floor, the showers, and the hot tub operate just fine at temperatures in the range of 110F to 115F.

The other lesson here is that with solar heating, the design objective is to match the source temperature to the heating load requirements. In this case, it means not collecting solar heat at temperatures higher than required by the heating loads. A design strategy based on this knowledge results in higher solar collection efficiency in the panels.

There are also problems with drawing solar heat from the same tank for loads that require different operating temps. In this case, the radiant floor can operate from 80F – 100F just fine, whereas the domestic hot water load requires temperatures of 115F and higher. But more on this fine point of switching loads at another time. Meanwhile, here’s an energy tip.

IMG_4246Did you know: Enough sun falls on the Earth in one hour to supply the planet’s energy needs for an entire year? In spite of this abundance…what we should be doing with energy is to limit entropy in our world. Essentially this means matching energy sources to tasks for which they are best suited. It is the ultimate action for energy conservation, and solar energy is going to play a significant role.

Related Articles