Understanding Solar Hot Water

  This article is about solar hot water which is using the sun's energy to heat cold water. We talk about a specific type of solar hot water technology called solar evacuated tube. It's kind of a newer technology in the United States, especially in Florida. You'll see this a lot more in northern states and a lot of other countries such as Germany, France, England. This is a really popular design that's used in the northern climates. One of the reasons is that with lower wind loads the wind can go in between the tubes and also that the snow can fall in between the panels increasing their efficiency.

  The panels that you see on this picture right now are not electricity producing panels. These tubes are made out of copper and there's usually a gas freon or just air on the inside of these tubes or sometimes even a liquid depending on the manufacturer and the type of evacuated tube or solar hot water panel. Sun will heat those tubes up. The black metal bar at the top heats up the water flows from left to right or right to left. Thus this is an extremely efficient system. It's actually more efficient number-wise than using solar electric panels to heat hot water.

Hot water and Electric Bill

   One of the things that shocks most people is that hot water is usually the first or the second largest part of the electric bill. Most people have electric water heaters standard in their house and it takes a lot of electricity to heat even one gallon of water. So when we do an analysis of a customer's electric bill whether they're residential or a commercial facility usually twenty to thirty three percent about one third of that bill is usually hot water. When we install a solar hot water system properly, we pull all the permits, do all the engineering, make sure that won't blow off the top of your house, make sure everything is legally done, we have the city or county or building inspector come take a look at it and once it's hooked up to your hot water tank over the next few years that system should save you twenty to thirty three percent of money. This is because the hot water from that solar system is extremely efficient and it can keep up with that electric backup or the previous electric hot water tank. We usually take someone's electric bill, look at the last point and multiply that by 30%. That usually tells us a very good estimate about how much a solar hot water system will save you.

  Now another question that we have is how does this work? Above there is a diagram that shows the solar collector with the hot water tank. This system is called an independent solar thermal system. What this means is that hot water basically comes in too cold into the solar hot water panel and then moving down the hot water goes into a heat exchanger inside the tank. So this is a special type of tank that will be needed to install and is included with this package. It basically heats up the already existing water inside our hot water tank. This picture also has the back up like a boiler or an instant hot or other electric or natural gas water heater. We can hook that up to the system for backup just in case it's cloudy for like a week straight or we don't get a lot of sun. Even still the solar collector should keep up with heating up your hot water tank.

  The solar collector also has a controller which controls the pump which can actually be solar powered as well. For most installs, we don't have the ability to put a PV panel there but in case of say residential customer that wants everything off-grid we can definitely put a PV panel that powers that pump. See the dotted line on the lower right of that controller that goes to the hot water tank. Say the hot water tank drops to below 80 degrees or 90 degrees Fahrenheit then the customer can actually set that on the controller and the pump will turn on and take the cold water up to that solar collector. The Sun will heat that up and it'll drain back down or move down with the flow and heat up that tank. The controller has a temperature differential device inside of it that determines when it turns on.

  When it's nighttime we definitely can do different types of systems depending on what type of tank, how many solar collectors you have, how many controllers we've done, installations where we've had. We want to install solar collectors perpendicular with the sun's rays as much as possible to minimise heat loss. In Florida we typically install anywhere from 20 to 30 degrees angle or we try to make it as flat as possible with the roof.

Solar Thermal Water Heating will substitute Electricity in Future

   As more and more people find out about the efficiency of solar thermal, the market is increasing every single year. This graph unfortunately only goes up to 2008 but this is the best data that we could find. We have different types of solar thermal water heating and it's usually divided into solar thermal for your home, solar thermal for your commercial and then solar thermal for power plant generation.

Investing in Future Energy Consumption

  The return on investment is the number one selling point of solar hot water panels. When you install a solar hot water system, the savings are immediate the day that turns on. For a typical price of a solar hot water system you're looking at anywhere from three thousand dollars all the way to higher-end seven thousand dollars.You also get a thirty percent federal tax credit in the United States and its territories. If you're a business customer you can depreciate that over a period of six to seven years so it'll fully pay for itself.

Evacuated Tube vs Flat Plate Models

  In the picture, there are two different pictures of a solar hot water panel. On the left hand side is a picture of a flat plate collector. It's one solid piece of glass. The picture on the right is the evacuated tube. So two very different technologies but they operate in almost the same way. They both use copper and they both have water flowing through them. The flat plate and the evacuated tube are roughly the same price. The evacuated tube is much easier to install whereas the flat plate is one solid collector which can weigh 200 to 300 pounds depending on the size. It takes a heavier lift and a crew of at least two to three to get a flat plate collector up there whereas the evacuated tube has individual tubes. You just have to screw those in so it's a lot less roof penetration. For the flight plate collector you have to remove all those tiles whereas for evacuated the shingles will still exist underneath the panel. Their efficiency is roughly the same. In colder weather snow the evacuated one will actually perform a slightly better as the snow slips between the tubes and there's a lower chance of wind shear. For flat plates there’s a good chance that the snow will stay on the solar flat plate and not fall off until it heats up.

  The picture above shows you a cutaway of a flat plate collector. There’s the glass, the metal absorbing plate and the copper tubing inside. You have safety devices inside the circuit so that either the water vents out or the heat vents out so no one will be hurt. They also have the pump which shows the controller. We have to figure out where we're mounting that solar hot water panel, how long the copper run is going to be, if the solar thermal panel has a powerful enough motor to move that water and also that everything is heavily insulated because we want to minimise the heat loss through that copper piping. The more we minimise the copper run, the more efficient the panel is as more hot water will arrive and also will be a cheaper install.

   If we could replace the flat plate collector with the evacuated tube it would look exactly the same. Just the orientation of the copper piping on the top would be slightly different because there are two connections on an evacuated tube. Right at the top we have water flowing basically perpendicular with the tubes. There is also a heat dissipator device which acts like a radiator just in case the evacuated tube overheats.

Frequently asked questions about Evacuated Tube Collectors

●     Are these new?

No, the technology is about 20 years old. The tubes were developed in Germany by Daimler-Benz Aerospace in cooperation with a solar energy research company.

●     How many tubes do you need and how much space is required?

For DHW we typically use 4 tubes per person, which assumes 20 gal/person/day usage rate and raising the water temperature from say 55 F to 120 F at a minimum. A 2-person family would need a 1/5-8 tube collector and a 50 gal storage tank. A 4-person family needs a 1/5-16 tube collector and an 80 gal tank. The 16-tube collector is just over 6ft wide (75.6") and about 7 ft long (83")

●     How can you tell if you have lost the vacuum?

All tubes now being delivered have a silvery coating on the inside at the bottom of the tube. When vacuum is lost, this coating depletes and the tube becomes clear. Older style tubes did not have this feature and you had to rely on observing a build-up of condensation inside the tube as an indicator of vacuum loss.

●     Will the glass break in a hailstorm?

The tubes are manufactured and tested to withstand 35mm (1.38 inch) diameter hailstones. The glass is low-iron tempered glass that is 2.5mm thick.

●     Can I heat my house with these?

Yes, provided that the home is well insulated, thereby having a low heat loss to the environment. There are no rules of thumb for sizing a space heating array based on square footage of a house. You have to know the heat loss, design temperatures, weather data, and solar radiation available to estimate the size. It will be larger than the DHW component alone.

●     What is the life of a tube?

The statistical "mean life to failure" is 15 years.