Solar Thermal 101: Basics of Solar Thermal

Solar thermal has been around for decades, but when people use the word “solar” when talking about renewable energy they always think of the younger brother photovoltaic (PV), which creates electricity from solar, while solar thermal uses the heat from the sun to do meaningful work. In this article, we are going to discuss the basic concepts and design to solar thermal for residential applications and touch on some economics of solar thermal. Lets start at the beginning.

What is solar thermal?

Solar thermal is literally heat from the sun, this is done via infrared radiation from the sun in the form of light, to heat up materials through absorption. With definition we can determine that we need two things to make efficient solar thermal systems: 1) full access to the sun and 2) a material that does a good job of absorbing all the spectrum of light.

Traditionally we do this by taking a material and painting it with flat black paint, since the flat/matte paint will reduce reflection of the light and the black will absorb quite a bit of the light, obviously it will not absorb all of it. This will make this material hot, but it may not be “useful work” yet. To make it useful we need to put it into a system that works for us. The most common system is to heat up water (actually a home via passive, but we’ll get there, I need to explain a concept first.)

To heat up water we could put a tank of water in the sun and wait and it will get warmer, much like your garden hose does when it sits in the sun, but as the day expires it will also lose much of its energy that it gained through the cooling factors such as ambient air temperature, wind and actually a little evaporation. So how do we prevent these things from happening? One way is to enclose this tank of water and then insulate it. This is great for keeping the energy in the tank, but now we broke our first law — full access to the sun.

At this point we could do a few things to make the system more efficient, we could take that tank and place it in a box with a window and increase the heat by lowering the cooling factors and reflection factors, thereby increasing the solar gain of the system OR we could effect change from within the water tank itself. Since many people don’t want their hot water tank sitting outside (except in the case of hotter climates with less corrosion issues due to rainy weather), we are going to place this tank inside and run a loop of water to heat the tank from the inside of vessel.

Solar thermal design

Implementing Active Solar Heating

To be able to heat up the water from the inside out we need to create what is known as a heat exchanger, and aptly named it exchanges the heat from a source to another source, in this case from a heat source to our water in the tank. Now this heat exchanger could be made of many materials, but we find that metal works great compared to that of plastic. Metal is a great conductor of heat and it also removes potential issues of oils leaking into your drinking water over time, which could lead to cancers. The metals we use are aluminum, copper & stainless steel because they do not corrode as easily in water and are known to be safe for potable water systems.

A series of pipes can be wrapped around the tank or coiled inside of the tank to create a simple heat exchanger that is quite efficient. You could use a manufactured one like a flat plate heat exchanger or pipe heat exchanger that has uses fins like a radiator to go in the tank. Whichever way that you employ the transfer of heat you will still need a heat source. This is where our solar collectors come into play.

Solar Thermal Collectors

There are many types of solar thermal collectors, but for water heating we generally use two main types, flat plate collectors and evacuated tube collectors. Let’s discuss the difference between these two.

Flat plate collectors

A flat plate collector is essentially is a window with pipes that run through it. Just like passive solar, we can utilize windows to allow light to go in and glaze the internal side of the glass to reduce the reflection out of the glass, thereby increasing the solar gain of the system. This is known as the solar gain coefficient. A single pane of glass will have a higher amount of light that will pass through the glass, but a lower resistance to light leaving it compared to a double pane window. So to employ a good balance we use a type of glass that has micro prisms in it, while keeping it only one pane thick. This reduces the reflection from the inside to the outside, splits the light wave to get good coverage over the entire area of the pipes inside. Why is that important?

The collector is one continuous pipe if used in serpentine style or a series of pipes where you have two manifolds with several smaller pipes attached to it, in either case these pipes try to maximize the square area of the collector. If we had a lot of money, we would just take two flat sheets of metal and make it into a pocket for the water to flow through, but we find that even this needs to have channels to allow the water to heat up, so there isn’t a huge gain compared to using off the shelf pipes if we use one creative technique.

We take a thin sheet of metal and solder it to the pipes in the system to maximize this square area. This way when the light comes in we don’t need to have pipes everywhere, we can spread them out far enough to allow good thermal transfer and close enough to absorb it into the fluid that is running through the pipes. Without this sheet metal (flashing) on top of the pipes all of the light would need to physically hit each individual pipe leaving 75-80% not hitting anything. So how do we get it to absorb the sun then, metal is reflective. We paint all of the flashing flat black. There ya go.

Flat Plate Collector internals

OK, so we have light that comes in through a window, which is in a box, where it hits flat black paint that is on thin sheet metal (flashing) that then transfers the heat energy to the copper pipes that are underneath the flashing, and finally the copper pipe transfers it’s heat to the water flowing through the pipe.

So what about the cooling issues we talked about before, won’t it just get cooled down? Yes it will. So we use insulation in the box to keep it hot. We use rigid foam insulation on the perimeter of the box and the “floor” of the collector, as it has the highest R-value per inch and then we use bat insulation on the top of the “floor” to keep our pipes nice and cozy and reduce convection/transmission losses through our glass.

Evacuated Tube Collectors

Evacuated tubes are misunderstood and thought of an exotic thermal product, while pretty ingenious, it doesn’t mean they are hard to understand. Take all of what you learned about the flat plate collector and think about solar thermal as a thermos. A thermos can keep a beverage hot or cold for hours during the day utilizing a vacuum, so evacuated tubes do the same thing. Since light cannot pass through metal it must use glass instead.

We take two flat piece of glass, one that is shorter than the other, heat them up and blow the glass to fuse them together on the edge, then pull a vacuum on it. This process makes it evacuated glass. If we cap the end of the glass it will heat up, so much that we actually have seen people turn this into a solar barbecue, were talking about some serious heat here. So how do we utilize it?

gosun solar bbq
Same way we always do, we take a pipe, wrap flashing around it and paint it matte black and stick it inside the tube. Now the suns radiation will go through the glazed glass and hit the flat/matte black paint which will absorb the heat,  the heat will be trapped by the vacuum making the least path of resistance to go into the metal flashing, which will then transfer to a pipe which has a sealed fluid in it causing it to flash to the top of the pipe, which then transfers it’s heat into a manifold which is caring water out of the collector. The more evacuated tubes you have the more energy you will make because you are taking up more square area. To utilize more of the evacuated tubes surface area, we build a solar concentrator behind it to increase the solar gain of the collector.

inside of an evacuated tube

Pros/Cons of Flat Plate Collectors vs Evacuated Tubes













Evacuated tubes are pretty great generally, they are efficient making upwards of 220F water in hot climates and 190F in colder climates due to the vacuum seal around the glass, but with all that efficiency it runs into one issue in snow. The snow doesn’t melt around the tubes, and after a while it can bury them making them receive less and less light making them unusable in snow conditions. The old faithful flat plate collectors shine in many areas because they are just so darn simple. They are easy to repair, no special tooling is needed to build or repair these, they don’t shatter very easy and in snow the heat that usually radiates through glass now heats up the snow making a slick water film and the snow slides right off (in a fixed tilt). This is a huge benefit for snowy climates. I am a big fan of keeping it simple and prefer the flat plate over the evacuated tube unless I need the extra heating such as in cooling applications or space heating.

Tying It All Together

We have talked about several parts of this system, so let’s talk about the whole thing to show the ease of this system. You choose a solar collector for your location, solder 3/4″ – 1″ copper pipes from your collector to your heat exchanger which is in your hot water heater.  Put a pump on the cooling side faced toward the collector and that’s it pretty much it, essentially a pump runs through the collector and brings that hot water back to your storage tank. There are a few other design consideration like tilt, and component sizes and parts, but we will talk about this later in another article on how to design this system.

Super Basic Design Specs

So we need to calculate how much energy is needed for your solar water heater. Unfortunately, we can’t look at your utility bill unless it is just providing the energy for the water heater so we need to use some rough math to make it work. First we need to calculate the required energy per 20 gallons since most tanks are sized in 20 gallon chunks.

Let’s start with your equation from basic thermodynamics and we find that:

Q = m(h2 – h1), where Q (kj) is the total enthalpy needed, m (kg) is the mass of the system and h is the internal energy of the system in kj/kg. You can use this equation in imperial measurements also if you are working with gallons like we are and utilize it in lbs.

To do this though we would need to have the internal energy values in BTU as this measurement is in lbs, (A BTU is the value to raise 1 lb of water 1 F)

So where would we look for such a value? I have great news. Every solar panel sold has an Solar Rating & Certification Corporation Certificate (SRCC). Take a look at the common flat plate collector the SunEarth EP-32 SRCC. As you can see on the left hand side they have BTU’s/panel/day and they have the values in kWh/panel/day. This is good news for us as our equation is in KJ/Kg which also equates out to kW/Kg too and our irradiance factor of how much light is hitting the earth is also in kWh/m2/day, so we will just use this figure so we can limit the amount of unit conversions.

Let’s take Portland, OR as an example for the location. In Portland OR, we have 4.04 kWh/m2/day, so where does this fall on our SRCC chart? It is right between Low and Medium density usage. So we need to interpolate this number to come up with the true factors in the Solar Hot Water/Space Heating row (D).

We have low at 3.1 kWh/m2/day and we have medium at 4.7 kWh/m2/day with their corresponding numbers of absorbed kWh/panel/day or .7 to 3.0 kWh/panel/day.

To interpolate it we take the range and multiply it times the delta of the internal energy + the internal energy of the starting point, essentially mx+b in algebra.

We find that each solar panel can absorb about 2.05 kWh. This is our base figure.

Each person in your home uses roughly 20 gallons of water per day, so a home of 4 people will use 80 gallons a day, another reason our tanks our scaled up by 20 gallons roughly.  We can use this to scale this using 20 gallons per person per panel per day, which will just come out to panels/day in Portland, OR.

Going back to our equation of Q = m(h2 – h1) we can see that h2 is temp of the water we want to heat to (120F) and h1 is the starting temperature of the water, while m is the 20 gallons in kg (75.6 kg).

While looking in a chart we know that water at 55F, is 12.78C and we find the following values. 62.08 KJ/Kg @ 15C and 42.09 KJ/Kg @ 10C and interpolate it to find that 53.76 Kj/kg is the absorbed heat energy in 55F of water.

We go through the same interpolating of water at 120F, which is 205.13 Kj/Kg.

We plug that into the equation above:

Q = 75.6Kg(205.13 KJ/Kg – 53.76 Kj/Kg) and get 1,1443.34 KJ to heat 20 gallons of water at 55F to 120F. We normalize this to kWh by dividing the KJ by 3600 to get to hours and get, 3.178 kWh per 20 gallons. Since we have 1 person use that amount per day, we can just call that one person/day. So 3.178 kwh/person/day.

We take 3.178 kWh/person/day / 2.05 kWh/panel/day and get 1.55 solar collectors/person to size your system on an average day in the year in Portland, OR.

So a family for 2 would use 3 EP-32 SunEarth panels to heat up their water heater, while a family of 4 would use 6. You could use less if you conserved water more, and I took the pain of doing all the calculations here so you can plug in your own water usage to come up with a customized value.

So how efficient is this?

Let’s plug the numbers into a chart from the EP-32 SRCC.

High Radiation
(6.3 kWh/m².day)
Medium Radiation
(4.7 kWh/m².day)
Portland, OR (4.04 kWh/m².day)Low Radiation
(3.1 kWh/m².day)

As you can see the better the sun you have the better these systems are at trapping the heat. You can also see why it is important to interpolate the data so you don’t just go with the lowest number and buy 2x more panels than you need.

How much will it cost?

This is a huge question with lots of moving numbers, but I will be insanely generic here and use some assumptions that make make an … you get the idea.

In general these solar collectors will cost $1,500 each new and your hot water heater with a built in heat exchanger will cost about $2000. You have the controller, temp sensors, piping, pump, expansion tank and gauges and you are looking at around $15,000.

QuantiyCost ($)Total Cost ($)
EP-32 Collectors (6)6$1,500$6,000
80 gallon Hot Water Tank w/ HX1$2,000$2,000
Grundfos UP15-42F1$105$105
Expansion tank, Fittings, and Copper Pipe$600$600
Mounting Racking and Hardware1$900$900
Misc – flashing, roofing, tools$250$250
Total Hard Cost$9,855
Total Labor Cost25200$5,000
Total System Cost$14,855

Generally the labor is going to be about 50% of the hard costs, but this could be higher or lower in your area.

Closing Statement

I hope that you found this article to be helpful and if you have reached this far, I am sure you have! These statements are just based off some basic guestimations and will likely be different from your specific case, so if you need any design help you can contact me and we can work out a better plan. In future articles I am going to give you more design information such as how to size your pump, how to do space heating, how to calculate the resistance and flow rate in your piping and general best practices to solar thermal.

I find that solar thermal is one of the technologies that gets left behind, but it is so good at what it does, that you really shouldn’t, especially when we convert the excess heat into air conditioning. Yes that is right, that is a thing and it is real. Stay tuned for more as we learn to live free each and every day.

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Rick Reynolds
2 years ago

Wow! Such an amazingly thorough explanation. We love our PV system and it’s great to know the details of the thermal system as another possible addition to our home. Thanks, Abe!

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