Solar heating: How to calculate energy

[osolemio]

I thought about this recently. We compare direct electric heaters, oil and gas furnaces, with heat pumps and solar heating. And we just talk about kWh or other similar energy units.

But it is really not that simple, is it? Heat pumps have their limitations, and varying efficiency depending highly on factors like input and output temperatures.

As for solar heating, one cannot just measure the temperature rise and multiply with the flow. It does matter what the temperatures actually are, and what the need is. If you have a need of 140F and your solar panel peaks at 110F, the energy produced - in reality - is zero.

Also, the hotter the solar heating panels become, the more energy is lost from heat loss to ambient air, around the solar panel. In the extreme case where you do not pump any liquid at all, whether it is summer or winter, the temperature will simply stagnate at whatever it takes to equal solar heat produced against heat loss. One should really think about this, it is really vital to a solar heating system where space heating is required (and not just hot water).

My way of mitigating this is to expand the heat storage and heating units to optimize them for as low a temperature as possible. This allows me to keep the solar panels colder. Or in other words, I can suck out much more energy from them, especially when the supply is lowest, and the demand is highest - during winter!

Underfloor heating is a must for this to work, and only part of my house is not converted to underfloor heating. The rest has such nice wooden floors and it hurts my heart (and wallet) to think of tearing all that up to install underfloor heating. But I really want to. Both ground floor and 1st floor has concrete just under the wooden floors, perfect to absorb a lot of heat from a good sunny winter day, and keep it for as long as possible, into the darker and colder days (and nights!).

I also have large water tank storage, almost 1000 USG apart from the 300 USG buffer. And I have had inserted tubes under the existing house, so that I can long term store excess heat energy. This is especially from late summer and fall, but also during winter, to be sure I can cool the solar panels as much as possible (as described above).

I hope to have the system up and running by next year, and cannot wait to see it work and document with raw figures, what it is that I am babbling about!


For now, just have a think about the fact that you cannot just talk solar panel energy as "how much energy does it produce" without looking at what your need is.