Solar heating: How to calculate energy
 

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.

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?

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.

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.

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.

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 ...

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.

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.

I think this thread is missing some math! The title is how to calculate energy.. but I see no calculations.. At least, perhaps, some links to sites with calculations?

So many variables: latitude/longitude, local weather patterns, shading in the immediate vicinity, panel orientation, size, and type.

Type of fluid circulated, flow rate, any heat exchangers and their type/efficiency, storage volume(s) present, their dimensions/insulation and surrounding environment - or ultimate rate of heat loss, the desired output temperature/flow rate or heat rate..

There are many possible configurations. Seems the best we could do is to coagulate reference data and formulae for any of these possible components and configurations. I'd assume such a collection already exists somewhere on the web..

I am interested in seeing solar water(or other liquid) heating being made more versatile and applicable. Let's simplify the concepts and the systems to reduce the barriers to entry for parties interested in integrating such systems into hot water/space heating.

The panels and attached hardware up to a point constitute a system acting as a source of heat. This system is dependent on certain exterior factors to produce this heat, and it provides that heat to an attached exterior system. The rate of heat production is dependent not only on the exterior factors (location/weather/temperature) but also dependent on the internal state of the system.

That is, as the internal system heats up, it's efficiency of heat absorption is reduced. However, we would typically need a high temperature to transfer that heat into our destination, like a hot water storage tank.

A simple system could heat up a certain amount above our destination temperature, then we can induce circulation and allow heat transfer until we hit another lower temperature threshold. And allow this cycle to repeat. It's simple, but it's not efficient. We could optimize the efficiency if we minimize the temperature inside our heat source system!

A heat pump would be perfect for this, it could use refrigerant to efficiently transfer heat from our solar source to whatever our destination is. A path for hardcore do-it-yourselfers to follow to design and build their own heat pump to meet their needs would be sweet.

Although this is all speculation on my part. I'm unsure how much efficiency of transfer is affected with a rising solar panel temperature. Data would be necessary to determine if it's economically justifiable..

Also, integrating a heat pump would allow the storage of heat at a very low temperature to minimize heat loss and allow greater durations of storage...

I was not intending to arrive at a precise formula of how to calculate produced or anticipated energy. This was mainly about the fact that it is not straightforward to compare different heat sources like electric, combustion and solar.

As I mentioned earlier, the main problem with solar is to

1) Get a temperature which is high enough to meet demand (or lower the required temperature of the demand, to the solar temperature available)

2) Store solar heat either seasonal, weekly or daily

During all my research, thinking and experience so far, I have come to the conclusion that one of the vital points of solar heating (space heating in particular) is to maximize the difference between temperature available and temperature required.

To mitigate this challenge, it is vital to make a heat system with as large a surface as possible. Imagine if you could heat all floors in the house, and even all walls. In this case, even by the worst of winters, you just need to heat these floors and walls to 80 F or less. And the heat would be much more comfortable than a house with alternating cold windows, hot radiators and the resultant drafts and difference in radiated heat.

It is like a business and the bottom line. You can improve by increasing the sales, lowering the cost or a combination of the two.

To "increase the sales" is using a heat pump to transform the temperature available to a higher level, but there is a cost. This "cost" could be seen as "advertisement" in the business parallel.

To "reduce the cost" is in this case to lower the required temperature of the heating system. Although not exactly the same, the principle is the same. Because we use the simple method of combining two masses of heat (solar heating liquid, mixed with heating system liquid, in a heat exchanger). The temperatures of these two will merge, for the purpose of heating the house. But without a heat pump, there can be no heat transfer unless the produced temperature is higher than the required one.

To have a lower required temperature of a space heating system with a large surface not only increases the efficiency and extent to which you can extract heat from the solar panel. It also means that storing heat can be done at a lower temperature. The lower the difference between the heat storage, and the surrounding temperature, the less the loss.

In my system, the seasonal heat storage is high volume, low temperature, placed below the house. Insulated on the sides, the top is the house, and the bottom will eventually saturate over the years until it stabilizes somewhat. (providing ground water is not too close to the ground).

Whatever heat loss is upwards, goes into the floor of the ground floor. And whatever heat loss there are to the other sides, is not really that great a loss. Remember, this heat is excess summer heat, which is close to free and could not otherwise have been used. The energy it takes to pump it into, and out of the ground, is so minor it is easily made up by photo-voltaics.

As the final backup, one can add on a heat pump to make hot water for taps and shower out of the heat storage temparature, and to get the last bit out of the heat storage in late winter and early spring. Since the heat increase here is quite minor, this heat pump would run at a quite high CoP, especially compared to a heat pump trying to make hot water out of freezing air!

As for the calculation of how much heat is actually produced it is quite easy. All it requires is one flow meter, two temperature probes and a meter. The meter needs to be calibrated for the liquid used, like in the case where glycol or other anti-freeze liquid is used. If the meter has a logging function, one can compare the heat produces against other factors, like outside air temperature, available sun, time of year and so on.

I will take measurements in my system at the relevant places, and log these data for later analysis, to prove how much heat it is possible for me to extract and store from this system. And I will hold future energy bills toward historic ones, and compare energy production and usage from before and after the installation of this system. As electricity and heating rates increase over the year, I will keep producing my own heat, and the same close-to-nothing rate. All I need to pay is interest on loans, and minor occasional maintenance.

Finally, I will sum up all the expenses, and all the savings, and I will include all the added benefits as "savings". It would not be fair to expect the energy bill savings to entirely sponsor all the improvements done to the house while doing this project.

Missing posting?
 

Michaelgale40 posted this text below, according to an email notification I got - has it been deleted?

Delays, delays
 

I am still here, and I am still working on my project.

The solar panels are just about ready from the factory (finally) but as winter is approaching, I will have to wait for the spring. To remove the tiles from the roof, I need warmer weather, as the tile glue is solid below 60F or so. On a normal sunny day, the tiles can be removed easily without too much effort.

I am working on the garden (hard surface, the garden is so small, there is no lawn!). The seasonal insulation in the ground is all done, just a few cosmetic touches to go, yet the control system inside the house is not assembled (no point yet, as there are no solar panels yet)


Rainwater system is up and running, and has been for a while. The tank is 7500 liters, which is roughly 2000 USG. At the moment it supplies water for toilet flushing, washing machine, cleaning purposes and so on.


I am in the process of investigating the possibilities of making a legal installation where rain water is also used for hot water and most of the remaining cold water in the house. We would retain a number of dedicated taps (cold water only) for drinking and cooking.

The filters in question would be three stage: particle, active carbon and finally UVA filter. Probably in dual, with monitoring, to ensure quality and supply (auto shut down if a UVA lamp fails, and then the other system keeps working).

Financing is a problem too, paying as I go out of the paycheck - with what remains after the worlds highest marginal tax rate has been applied. Delaying the solar panel installation until spring gives me time to breathe (make more money) for the final but crucial investments.

I look forward to be able to present data here, to document what I meant when I started this thread.

It will be accompanied with graphic presentations to illustrate the function of the system - any suggestions to what programs (Mac!) to use? I prefer something which is not too complicated, and if it can make web enabled presentations, that would be preferred (no flash, please, or other proprietary or complicated formats. Html5 format preferred!)

Yeah, he was a spammer.

Great info, I will be looking forward to this thread as things move along. Please post pics.

Heating with the sun
 

osolemio

We have been pursuing solar hot water heating for a couple years now. It has been quiet successful. There are 160 sq ft of flat plate collectors mounted to the wall of the house and have been in operation for two years. With good sun on them we heat for a 24 hr period via in floor heat. Back-up heating is via ground source heat pump. This weekend we have installed the last panel for the house. It is an additional 88 sq.ft. that can be adjusted for the seasons.
Your system sound great. Keep forging ahead. Solar heating works well. Who ever thought you can heat a house with the sun.? Here in Canada there is a comunity with an installation such as yours. The comunity uses bore holes in the earth to store massive amounts of heat. Check out Drake Landing (dlsc.ca).

Randen

I currently have a ground source heat pump in my house and am looking for ways to incorporate solar heat. I prefer a large tank and low temperature. I have considered a large water tank and floor heating. Another option I have not seen suggested is to use a large tank to pre-heat water going into my heat pump. I have an open loop system that uses water at about 53 degrees. If I can heat it to 70, or even 60 degrees, it will increase the efficiency of my heat pump. It would be even more effective on closed loop systems because the ground temperature decrease as heat is removed during the winter.

You could do that. But, it would be more efficient if you could simply use the hot water directly for heat. Running it through a heat pump would just be an unnecessary step. However, the heat pump would be useful to pull heat out of the water after the water tank's temperature has dropped below a usable level, say between 75F and 53F.

Spaceheating with solar hot water
 

The results we have had with the solar hot water heating are nothing short of spectacular. When we had first began to think of the possibility of heating with the solar we had a look around to see if anyone else had some success. My research lead me to Gary Reysa of Build It Solar and his solar garden shed. He had some very good results with his system. The reduction of propane for his space heating had been reduced markedly. Gary was installing tubing on top of the floor and covering, I think with ceramic tiles. The system I'm familiar with is quik trak panels. We were very lucky to already have a heated concrete slab which is one of the key elements of the lower temperature heating and to act as a heat battery. If one was to convert all their living space to the heated floor tiles and circulate the solar heated fluid through it would make the house very comfortable. The addition of a solar insulated tank 500-1000 gals would help to carry the heat through the evening. I had seen a suggestion of 10-30% of the floor area would be the recommended amount of flat plate solar panel area. If the panels are mounted vertically on a south facing wall they collect the most amount of heat from the lower angle of the suns exposure in the winter and don't collect snow as well in the summer they don't over-heat.

Gary and my system both use the suns heat directly for the space heating. The solar heat is a priority. By this I mean if the solar heat is avalible the other method of heating is suspended until there is no longer any solar input. This is quite simple when the circulation pump for the solar is turned on a relay interrups the backup heat source.

My result last year after installing solar hot water for both domestic and space heating reduced my electric bill by $1200.00 During the months from about March to October the electric water heat element was used only about three days. The amount of space heating is hard to quantify as I don't meter the run time but the end result is very good.

Randen

I definitely agree with you thanks for sharing useful information

Hi, Better you can have some pool water heaters instead of looking for heaters to tank of water. That will be cool idea to get plenty of hot water. There are many companies who are selling many types of solar panels and equipments. They provide you many solar panels that can be installed on roof of the houses, and also portables that we can carry anywhere, and pool heaters as well. Try them out!:):):)