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.

[Daox]

I completely agree with what you've said. Since I recently got some free solar hot water panels I've been looking into the same things. I'd love to get a huge tank of water. Can you tell us more about these tanks?

[strider3700]

Although I agree with most of your post I believe this is wrong.

Originally Posted by osolemio

If you have a need of 140F and your solar panel peaks at 110F, the energy produced - in reality - is zero.

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.

[Daox]

The other issue with large tanks is the heat loss from them. You would have to have a lot of insulation to negate the heat loss.

[osolemio]

These are just the reasons why a large mass is required, and a low temperature heating system.

The smaller the heating area, the larger temperature required to heat up a given space.

The higher the temperature of a heat storage, the more heat loss.

A huge mass heated to a lower temperature has only a slight loss, compared to a small mass heated to a higher temperature.

Imagine 1000s of cubic feet of clay or earth, below your existing (or new) house. This has a certain temperature, typically quite constant year round, in the order of 40 to 55 F. Older houses might not be that well insulated downward, modern houses should be well insulated in all directions, also below the floor.

All summer, there is lots of excess solar heat, but no-where to use it. Even if you heated several thousand of gallons of water to boiling, it would not last very long into the fall. And it would not take long time until it would be "fully charged".

But try to heat up the earth below your house, down to 10 or 15 feet, using pipes "shot" into the clay/earth/sand below it. Dig down and insulate on the sides, sloping downwards somewhat. You now have yourself a massive heat storage. It will take all summer to heat it. But imagine, your solar panels all summer heat a little hot water, and for the rest, they just dissipate excess heat back into the air. Unless you have that earth under your house, to cool it off. THAT is where you LONG TERM STORE heat, from all of summer, long into the fall and even winter.

The clay will not conduct the heat so readily - which is fine! Because s l o w l y is the keyword here. Forget huge water tanks. You just need a "small" one of a a few thousand gallons, to keep heat for some days, overnight and until you can have it absorbed into the ground.

This technique is already known, but in a more simple setup, if you search the net for "annual geo solar" or similar. It is simply has such a huge potential for using solar heating to a further extent, a whole new level. But to work fully, you should really have underfloor heating as much as possible, or even partial wall heating as well. Heat up as much of the house to 70-80 F in the coldest of winter, and you won't need the low-area, high temp radiators at all.

When I finish this project, and prove that it works, I really need to make a graphic video to show it with pictures, rather than words.

[osolemio]

Originally Posted by strider3700

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 ...

[Daox]

I have heard of what you talk about, annual solar heating, or seasonal solar heating I've heard it called. Its a great idea that I have never seen implemented. I bet its also quite a lot of work to do.

Do you have any documentation of your project thus far? We'd love to see it.

[osolemio]

Originally Posted by Daox

I have heard of what you talk about, annual solar heating, or seasonal solar heating I've heard it called. Its a great idea that I have never seen implemented. I bet its also quite a lot of work to do.

Do you have any documentation of your project thus far? We'd love to see it.

Not yet, I am afraid. I am still waiting for "the engine for the car", the solar panels. They are in themselves quite spectacular, check out the "hybrid" model on solarus.se. I should have had them this spring (2010) but not likely to get them before spring 2011 ... waiting, waiting, waiting ...

I will document eventually, when I have some data. The annual geo solar would obviously take longer time to document, as it is a seasonal process.

[strider3700]

Originally Posted by osolemio

Think about that ...

OK I can see your point now that you've explained it.

We have different goals with our systems. You want to extract every possible BTU and have it available for later usage. I simply want 100% of the cold water entering my hotwater tank to have been heated to 110F or hotter.

So for me having more energy then I can use is just fine and I will "waste" it by stagnating the panels. You on the other hand will always be wanting more.

[osolemio]

Originally Posted by strider3700

OK I can see your point now that you've explained it.

We have different goals with our systems. You want to extract every possible BTU and have it available for later usage. I simply want 100% of the cold water entering my hotwater tank to have been heated to 110F or hotter.

So for me having more energy then I can use is just fine and I will "waste" it by stagnating the panels. You on the other hand will always be wanting more.

The reason I need this is because I need space heating also. Don't know about your needs in this regard? But one of the significant limitations of solar heating in places with significant season changes are that you obviously have a lot of excess heat available in summer, when you don't need it. And in winter, when you really need a lot, there is close to nothing.

The second significant limitation of solar heating is unpredictability. Sure, the sun will shine, but when, and how much? It is pretty much the same problem as electric cars are facing.

Storage.

We need more capacity, to cover one or more cloudy days, from sunny ones, or in this case, to even cover the winter, as much of it, from long term summer heat stored.

This is really practical if 1) your heat storage is high-mass, low temp and 2) your heating system is OK with a lower temperature (large heating surface, like underfloor heating, even combined with wall heating, or other means of heating as much of the house as possible.)

This is some of the main ideas in my project, but as mentioned before and other places, there's even more to it.