Solar Thermal Collectors 101

Solar thermal collectors are used to collect heat for space heating, water heating, pool heating, ...

They are very simple, efficient, low maintenance, and make good DIY projects.

I have tried to start from the basics, but have also tried to take the discussion far enough to allow you to make intelligent design decisions and sizing calculations. By the time you finish this, you should be able to evaluate all the goofy ideas that come up on the Internet :)

How A Solar Thermal Collector Works

Lets first consider a typical solar collector for heating water for showers or space heating.

Let's build up the collector element by element to see what each part does, and why it's needed.

The key element is an absorber -- sunlight shines on the absorber and is (surprise!) absorbed. So, we now have a hot absorber.

[absorber picture with sun]

We need some way to collect the heat from the absorber and get it to where we want it. For this kind of collector, the usual approach is to circulate water through the absorber to pick up the heat. This is often achieved by passing water through a grid of pipes that are thermally bonded to the absorber surface. The absorber heat is transferred through the pipe walls and into the water flowing through the pipe. So, this is basically a functioning solar collector, but not a very good one. With the absorber surface exposed to outside air, most of the absorber heat is just lost to the surrounding air instead of being harvested to heat your space.

To overcome this problem of heat loss to the outside air, the absorber is normally enclosed in a flat box to reduce heat loss. Obviously, the front of the box (the sun side) must be transparent, or no sun would reach it. The sides and back of the box are normally made from insulating materials to reduce heat loss.

Adding glazing on the collector is a tradeoff: on the plus side, it greatly reduces heat loss from the absorber to the outside air, but, on the negative side, it normally absorbs about 10 to 15% of the sun's energy, and this absorbed energy never gets to the absorber plate. As you will see below, some types of collector applications don't use glazing at all.

So, now we have a complete water heating collector consisting of: 1) an absorber plate, 2) tubes coupled to the absorber to collect the heat, 3) glazing on the sun side to prevent heat loss, and 4) insulation on the back and sides also to prevent heat loss.

[ complete collector -- cut away?]

Collector Heat Flows

Let's take a look at the incoming solar energy for the type of collector described above and see exactly where it goes.

[ diagram showing heat in and heat loss arrows]

In full sun, about 93 watts of solar energy shine on each square foot of the front surface of the collector.

Of the 93 watts, about 10% gets absorbed by the glazing and never gets to the absorber -- so you start with a maximum possible efficiency of around 90% for glazed collectors.

So, about 84 watts per square foot makes it through the glazing and reaches the absorber. Absorbers (even simple ones) are good at absorbing the solar energy, and typically 95% of the incoming 84 watts is successfully absorbed -- or, about 80 watts per square foot of collector.

If we could get all of that heat that the absorber has sucked up to the room or storage tank we are trying to heat, the collector would be (80/93) or about 85% efficient. But, this is normally not the case due to heat losses from the absorber to the outside.

Since the air outside the collector is normally a lot cooler than the absorber, heat flows from the absorber to the outside. The glazing and the insulated back and sides are there to minimize this heat loss, but the loss is typically still substantial.

Just as an example, let's assume that the absorber plate is running at 150F, and that the outside air is at 50F. This means there is a 100F temperature difference trying to move heat from the absorber to the outside.

The back and side insulation is typically about R6. For these conditions, the heat loss out the back is about 5 watts per sq foot. So, the loss out the back and sides is fairly small, and can be reduced further if desired by just using more insulation.

[link to the calculation -- (1ft^2)(100F)/R6 = 17 BTU/hr = 5 watts

The loss from the absorber out the glazed side is larger and more complex. The absorber looses heat in a combination of ways: 1) the absorber (like any body) radiates heat out through the glass, 2) the absorber conducts heat to the air around it, and 3) convective air currents between the absorber and the glass carry heated air away from the absorber that gets replaced by cooler air. If we lump all these heat losses together into a single R value, the R value might be around R1. This is kind of a gross approximation, but it lets us compute a loss that is close enough to get a rough idea. So, using the 150F absorber and 50F outside air, and an effective R1 between the absorber the outside air, we get a loss of 29 watts per square foot of collector.

[ link to calc -- (1 ft^2)(100F)/R1 = 100 BTU/hr = 29 watts ]

The heat that gets through the glazing, and is absorbed by the absorber, and is then not loss to the outside is:

solar input                                                           93 watts
absorbed in glazing (10%)                             -9 watts       Leaving 84 watts
not absorbed by absorber (5 %)               -4 watts                Leaving 80 watts
lost out the back                                              -5 watts                Leaving 75 watts
lost out the glazing                                          -29 watts             Leaving 46 watts

So, of the 93 watts coming into the front surface of the collector about 46 watts gets delivered to the house -- or about 50%. While things change with collector design, and with absorber and outdoor temperatures, 50% is a fairly typical real world efficiency for real world collectors for space heating and domestic water heating.

[diagram that shows incoming solar, part absorbed by glazing, part lost out glazing from absorber, part lost out back and sides, AND part that goes to storage -- make it clear that is a sum = 1 game, and anything that goes out 1,2, or 3 does not go to storage. Then question becomes how do you minimize 1,2, and 3 -- discuss each in turn]

Other Types of Collectors

The solar thermal collector design has been tailored to a number of applications.

Pool Heating

In pool heating, the pool water is normally not very hot, and the air temperature is usually fairly warm. This allows several changes from the design discussed above -- these changes reduce cost and actually improve efficiency.

Since the air temperature is about the same as the absorber temperature, there is no need to enclose the pool heating collector in an insulated glazed box, because there is little to no heat loss from the pool water to the air -- there may even be a heat gain if the air is warmer than the pool water. This saves the cost of the box and glazing, and since you don't have the transmission losses of the glazing, an unglazed pool collector is actually typically more efficient than a glazed collector.

Since the absorbers on pool heating collectors operate at much lower temperatures, they can be molded from rubber or plastic as a single unit that contains both the absorber and flow passages -- this makes them cheaper.

[picture of molded pool collector]

The low cost of the pool heating collectors coupled with their high efficiency make pool heating the most cost effective of common solar applications. Other factors that reduce the cost of solar pool heating systems is that the existing pool pump can used for circulation, and the pool itself is the heat storage "tank".

Can't resist my favorite quote about solar pool heating collectors:

"Three gallons of oil refined and burned provides 400,000BTU ... Once. Three gallons of oil, made into a 4 by 12 foot solar collector, can provide over 10 million BTUs per year ... year after year"

Tom Lane

The same type of collector technology could be applied to any situation in which water need only be heated to a low temperature and the air temperatures are moderate.

Air Heating Collector

Solar air heating collectors heat air, which is then normally distributed directly to the space to be heated.

The basic physics for the air heating collector is the same as the water heating collector described above except that instead of circulating water through the absorber to pick up heat, air is circulated along or through the absorber to pick up the heat.

Air heating collectors have the advantage that the heated air can often be ducted right into the space to be heated. In addition, they don't have to include freeze protection as water heating collectors do, and when they leak, they don't make puddles -- they just lose some efficiency. All of this means that air heating collector systems can be very simple and cheap -- every home should have a few :)

The real challenge in air heating collectors is to get good heat transfer from the absorber to the flowing air. This is difficult because air is a poor conductor of heat, has a low density, and does not have a high specific heat. All of this means that a lot of air has to be brought into close contact with the absorber to efficiently remove the heat. As an example, a 40 square foot collector with the absorber

[add the equation to calculate required airflow ]

Various designs are used to achieve the efficient transfer of heat --here are some:

Forced air, backflow collector

The "backflow" collector uses an air passage behind a solid absorber sheet. Air flows through this passage and picks up heat from the absorber. The air passage is shallow so that the flow velocity is high and the airflow is turbulent -- this makes for good heat transfer, but also requires a relatively large fan with a high pressure drop capability. The larger fan uses relatively more electricity.

Other design requirements for this collector include having a long enough flow path to allow the air to heat up enough to be useful -- in some cases this will require an airflow path from entry to exit with turns in it to extend the path length.

[ link to more detailed design rules]

Thermosyphon Collector

The "thermosyphon" collector is at the other extreme from the backflow collector. Air flows through the thermosyphon collector using only the buoyancy of the heated air to propel the air flow -- there are no fans. These collectors normally use porous flow through absorbers, meaning that the air must flow through the absorber to get from the inlet to the outlet -- this helps to insure good heat transfer. They also tend to use absorbers with a lot of surface area such as meshes or screens, also to increase heat transfer. Since these collectors rely only on the buoyancy of the heated air to provide flow through the collector, they must be designed to have as little flow resistance as possible. This takes the form of a relatively large cross sectional area, low resistance flow through absorbers, and large vents that are well distributed across the top and bottom of the collector.

While it might seem that thermosyphon collectors would not be able to achieve the same efficiency levels as forced air collectors, if they are carefully designed, the efficiency can be similar, and they have the advantage of not needing fans, controllers, or electricity.

[link to design rules]

Others

There are a number of collector designs that lie between the two described above. For example: fan forced collectors that use flow through absorbers. With the addition of a fan, an absorber with more surface area (and more flow resistance) as well as fewer and smaller vents can be used.

Some collectors use an arrangement of slats that the air flows through -- think of partly open Venetian blinds.

Evacuated Tube Collectors

The evacuated tube collectors are another form of water heating collector.

There are endless debates about the relative efficiency (both thermal and cost) of flat plate collectors and evacuated tubes. Without going into a lot of detail on this, it depends on your climate and how hot you want to heat the water. Evacuated tubes tend to do better in applications where the water needs to be heated to high temperatures and when the climate includes a lot of cloudy weather. For the usual range of solar water and space heating applications the two tend to be pretty close to each other, with perhaps the flat plate collectors winning by a small margin. If you are considing both types, I think that one of the best ways to get a handle on the relative performance in your area is to run Andy Schroder's simulation program. Here is an example with some comments. Remember that its really the heat produced per dollar you spend that counts, so you need to factor the cost difference into the performance difference.

There are also some differences in the physical characteristics of flat plates and evacuated tubes that can be important. This includes things such as snow removal -- these factors are covered to some degree here...

Another factor for Do-It-Yourselfers is that its easy to make good flat plate collectors that cost a small fraction of what commercial flat plate collectors cost, but making evacuated tubes that perform well and have a good life is very difficult.

Efficient Collectors

So, if the main idea is to reduce heat loss, how do you do this?

- Maximize the amount of solar energy that reaches the absorber in the first place by using glazing that transmits solar light well. Any energy that gets absorbed by the glazing is not going to heat your house. Good single glazing transmits about 90% of solar light.

- Use an absorber material that absorbs solar energy well. This turns out to be pretty easy, as ordinary black paint absorbs about 95% of solar energy.

- Keep the absorber as cool as possible. This is very important. The rate at which the absorber loses heat out the glazing depends strongly on how much hotter the absorber is than the outside ambient air. The reason that collectors are more efficient in hot weather than cool weather is that there is less temperature difference between the absorber and the ambient air, and therefore less heat loss, but there is not a lot you can do about outside air temperature.
But, you do have some control over the absorber temperature. By circulating more fluid through the collector, you can cool the absorber so that it loses less heat to the outside. One of the main reasons for inefficient collectors is not enough fluid flowing through the collector. Basically, if there is not enough fluid flowing through the collector, the absorber just keeps heating up until it is able to get rid of its heat by losses through the glazing. It is also very important to have the fluid flow over the full surface of the absorber, so the absorber is cooled uniformly without hot spots.

This can be a little counter intuitive in that one would think that a collector that is running really hot inside is doing well, but its not -- its just losing a lot of heat to the outdoors.

- Reduce heat lost from the back and sides of the collector by insulating them. This is an easy step to reduce heat loss.

Measuring Performance

what -- covers efficiency?

Other Types of Thermal Collectors

Cover unglazed pool, air heating, evaulated tube

For low temperature applications, like pool heating, the absorber and tubes can be a single molded piece of plastic.

Design Variations

Cover double glazing, low iron glass, low e coatings,

How Do Solar Thermal Collectors Compare to Solar Electric Collectors?

Solar electric collectors convert sunlight directly into electricity. The electricity produced can be used to directly run some things, but more commonly, an inverter is used to convert the direct current output of the PV panels into household current.

Solar electric panels cost about $3 a watt these days with the power measured under STC (Standard Test Conditions) -- the STC system being a relatively optimistic way to rate PV panel output.

By comparison, commercial solar thermal panels cost about 50 cents a watt (about 1/6th of solar electrics). This is based on a cost of $25 per sqft of collector, and 60% efficiency. 60% is (perhaps) a bit optimistic, but probably less so than the STC system used to rate PV panels.

[based on 1000 watt/sm = 93 watt/sf at 60% efic = 56 watts/sf

($25/sf ) / (56 watt/sf) = 45 cents/sf ]

Solar thermal panels are relatively easy to make, so if you are into DIY, it is possible to make a good quality solar thermal collector for about $6 per square foot. This brings the price per watt of output down to 11 cents per watt -- about 1/30th of PV panels.

Have a table that summarized the key ratings:

efficiency

cost per watt

DIY (yes or no)

DIY cost per watt

Solar electric panels have the advantage that they produce electricity, which can be easily converted to other forms of energy. But, for heating applications (the largest portion of most peoples energy bills), it makes more sense to use solar thermal panels because they are much more efficient, take up less space, and cost less.

Another advanta

Collector Myths

Collector Design Misconceptions:

These are my 2 cents on some of the things that people tend to get wrong in collector design, and some of the things that are most important in a successful collector. If you don't agree, I'd like to hear about it.

That Efficiency is the be all/end all of solar collectors:

Efficiency in solar collectors is important, and should not be ignored -- you should always try and verify that your collector is performing in the same efficiency range as proven collectors of the same type.

But, that said, most people are more interested in how much economic return they get on the money they invest in a collector. So, the real figure of merit for a collector should be something like :

Economic Return = (dollars saved per year on energy) / (cost of the collector).

Most decently designed collectors do not vary widely in efficiency, but collectors do vary widely in cost, so maximizing economic return is usually more about building collectors for less money than it is about squeezing the last bit of efficiency out.

Collector AREA is king:

The most important collector design parameter is COLLECTOR AREA. You can work very hard, and spend a lot of money improving the efficiency by 10%, but its not going to get you anything remotely close to the effect of doubling or tripling the collector area. This is one of the beauties of DIY collectors -- you can build a DIY collector for $5 per sqft that performs just about as well as a commercial collector that costs $25+ per sqft -- so use that advantage to double or triple or quadruple the area! If you have the space -- build big. A corollary to this is that its not a lot more work to build a 4 ft by 10 ft than it is to build a 2ft by 3ft collector -- build the larger one.

High Output Temperatures Don't Mean High Efficiency:

Sometimes people think that the hotter the outlet temperature on a collector the more efficient it is, and the more heat it is producing. This is usually NOT true. In almost all cases collectors will be more efficient operating at relatively high flow rates, and at relatively low collector temperature rise. The reason for this is that the higher the collector temperature, the more heat will be lost out the glazing -- heat lost out the glazing reduces the heat output of the collector. So, generally you want to try to set the flow rate such that the temperature rise is the lowest that will work for what you want the hot air for. For thermosyphon collectors, you want the path through the collector and absorber to be as free flowing as possible -- this will give you higher flow velocities and lower temperature rises, and a more efficient collector. For thermosyphon collectors, this means choosing an absorber material that has both good absorption AND low flow resistance.

If this does not make sense to you, just remember that its the product of the temperature rise times the flow velocity that determines the heat output. If you make a change that increases the temperature rise by 10%, but in doing so you decrease the flow velocity by 20%, you have lowered the heat output of the collector.

You can also directly see the effect of high collector temperatures by examining the efficiency plot above. When the temperature rise through the flat plate collector is 50F, the efficiency is nearly 60%, but when the temperature rise is increased to 100F, the efficiency drops to about 40%.

Adding Thermal Mass to a Collector Improves Performance:

I don't know where this comes from, but a lot of people seem to feel that adding mass to a collector will improve its performance. It is better to keep the mass down, so that the collector comes up to temperature faster when the sun comes up or comes back out from behind a cloud. You want an absorber that absorbs well, and you want sufficient flow through the collector to pick up the heat from the absorber without undue rise in the collector temperature, but thermal mass is not required for this. If your system needs thermal mass for storage, the worst place to put it would be in the collector, which has very high losses to the sky once the sun goes down.

Old

A sheet of transparent glazing is placed over the absorber. The only reason the glazing is there is that without it, the absorber would lose most of its heat to the outside. The glazing insulates the absorber and reduces heat loss, but there this is a tradeoff, as the glazing is not perfectly transparent and absorbs some of the valuable solar light before it can get to the absorber -- so, glazing is a sort of necessary evil on collectors.

On the back and sides, the absorber is surrounded by an enclosure or box. The box is normally insulated to reduce heat loss to the outside.

Fluid is circulated through the collector to pick up as much of the absorbed heat as possible. For water heating collectors, the most common arrangement is to have the fluid flow through a set of pipes spaced a few inches apart. The absorbed heat is transferred through the absorber to the pipes and carried off by the fluid. In air heating collectors, the air is circulated over the absorber, or, for porous absorbs, right through the absorber.

The basic process is that about 80 to 90% of the sun passes through the collector glazing, and is absorbed by the absorber. The heat absorbed by the absorber goes one of two places: 1) it gets transferred to the space being heated or to storage, or 2) it gets lost out the glazing or the back of the collector as heat loss. The more heat that goes down path 1, the more efficient the collector, so a lot of good collector design is about minimizing heat losses to the outside.