The sun delivers energy to the Earth every day. Without it, there would be no life on this planet. All life-forms have developed ways to harness this energy to stay alive. Plants use it to grow through photosynthesis. Animals and humans use it to warm their bodies through heat. Our most useful and plentiful form of solar power actually comes from heat. Instead of converting heat to electricity and back to heat again, are there ways we can just collect the heat and use it to warm our homes and our water supplies?
The answer is yes! Solar heating is a real possibility for some home owners. I say some because, unlike fossil fuels, we can’t carry around sunlight like we can carry around pieces of wood or coal. In order to gain significant heat from the sun, we have to dance to its rhythm.
Solar Heat Basics
We first have to understand how the Sun’s energy interacts with the Earth. Due to the tilt of the Earth, measured at roughly 23.4 degrees, different parts of Earth get different amounts of energy. That’s why we have seasons.
Every child knows that the sun rises in the east and sets in the west. However, if you observe the sun’s arc over the seasons then you know that the sun appears to rise higher in the sky from the winter solstice to the summer solstice, and then goes lower in the sky from summer to winter. In the northern hemisphere, this arc leans toward the southern horizon, and vice versa in the southern hemisphere.
The closer that this arc of the sun gets toward the horizon, the less energy that reaches that part on the earth. On the equator, known for its extremely hot weather, the sun practically goes straight up overhead, with only a little deviation on either side of it to account for the tilt of the Earth. That part of the Earth faces the sun most of the time. As you travel toward the poles, the amount of sunlight per day gets less and less and results in colder weather.
If we want to heat with the sun, or prevent our homes from getting too hot from the sun, then there are two other factors we must consider. Different materials absorb and release the heat energy from the sun at different rates. We call this temperature when talking about the heat in the air around us. If you take off your shoes and socks and try walking on asphalt on a hot day, you know it can get hot enough to burn your feet pretty badly. If you do the same thing an hour or two after sunset, you’ll see that the surface will still be comfortably warm relative to the air. This is the concept of thermal mass.
The second concept is the greenhouse effect. When a material lets shorter wavelengths of heat energy through, but blocks longer wavelengths from escaping, heat builds up. Glass is one common material that does this, and it’s why greenhouses are made of it. However, this only happens when the sun is shining. Sleep near a window in a warm house on a cold night and you’ll quickly learn that glass can let a lot of heat escape!
The combination of the amount of energy coming in from the sun and the amount of energy your structure absorbs is called solar gain. The goal for any solar heating system is to generate enough solar gain for your needs without absorbing so much that you get overheated. Before you can even consider what kind of solar heating system to install, you need to know if your site gets enough solar energy to make them run. Follow the instructions at http://www.builditsolar.com/SiteSurvey/site_survey.htm for a very excellent and educational guide about how to find out how much sun your site gets.
Active vs Passive Solar Heating
There are two main ways to gather solar energy for heat, active and passive. Active solar heating uses an external system that runs on electricity to make the system work. The disadvantage of an active system is those electronic components. If those parts break down and can’t be replaced, then the efficiency of the system will drop, possibly to the point where it becomes useless.
Passive solar heating doesn’t use tracking mechanisms. Instead, through careful design, the structure naturally collects enough energy to maintain the house at a good temperature. It is an extraordinarily efficient way to heat a structure. The disadvantage of a passive solar structure is that not all sites can use this method and the building must be designed correctly. An incorrectly designed passive solar structure can end up being more like a cold cave than a warm house!
Active in detail
There are two basic active designs for solar energy collection, direct and indirect. In a direct system, a solar collector sits outside where the sun can reach it. The collector is an insulated box designed to concentrate the energy into water. This can be done by painting the box black, using glass to create a greenhouse effect, insulating it thoroughly, or all the above.
Water heated from the collector flows into a tank filled with cold water from the outside, much like a hot water heater. A thermostat measures the temperature in the tank and pumps water up to the collector if it is too cold. A separate line runs from the hot portion of the tank to the taps.
Direct systems are low maintenance, but they have a few problems. Since the system works directly with water, it’s quite possible for the system to freeze in winter and have a pipe burst. Also, they could overheat if the thermostat in the controller goes dead. Finally, scale accumulation can be a problem in the pipes over time. Direct systems are best in temperate climates.
In an indirect active heating system, the fluid in the collector can be water, air, or an antifreeze solution. The heated fluid goes into a heat exchanger much like the tank in the previous example. Cold water goes into the tank from the bottom and heats up from the hot fluid in the coiled pipe inside the heat exchanger. Hot water rises to the top and goes to your taps. A pump and a control box measure how hot the water and exchange fluid are and turns on the pumps if needed. Indirect systems are more expensive and have more points of failure, but they reduce the problems found in direct systems.
For home heating, the collection devices work the same way but the heated fluid is pumped through the home. In a radiant slab system, hot fluid is ran through channels in a heavy masonry floor. The mass of the stone soaks up the heat and releases it slowly. It can take a long while for these systems to absorb enough heat to feel warm, but once they are it’s easy to keep them there. However, installing one of these systems in an existing home can be quite costly.
Another type of system runs the hot water through a standard radiator or hot water baseboard system. However, the water temperature needed for these systems needs to be quite high. The circulating water may need to be heated a second time by a standard system. Alternatively, the radiators could be enlarged, or a special solar collector can be used called an evacuated tube collector.
The third type uses your existing heating system to help heat the air. The hot water pipe is coiled into your air return duct before it hits the furnace. This makes the furnace work less and drops your energy bills.
The advantages to active systems is that you aren’t constrained by how heat flows. The systems can be much more flexible and made to fit your existing home. However, they are reliant on industrial parts to keep functioning. Also, home heating with an active system normally only reaches efficiencies of 40 to 80 percent compared to your standard central heating unit.
These systems are also quite expensive. A two-collector system installed for a family of 4-5 can run $6000 minimum from a dealer. Installing these yourself is also not a good idea unless you have plumbing, electrical, and roofing experience, but if you do then go with evacuated tube collectors. They are light enough to carry up a ladder and much easier to repair by a homeowner if a hail storm hits.
For one way of doing a DIY active solar heating system, check out this very detailed guide at http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm. If you would like to compare prices through a dealer, shop.solardirect.com will show you reputable companies and sample pricing.
Passive Solar Heating
The best time to design a passive solar home is before the house is built. Retrofitting a house to a passive solar system is quite an undertaking, and may even be impossible depending on your site design and house orientation. If you are still in the planning phases of building your home, consult recently published passive solar design books like “The Solar House: Passive Heating and Cooling” by Chiras.
If you have already built your home, look into thermosiphon systems. A thermosiphon is a large sealed box with vents at the top and the bottom. Cold air inside of the house enters the bottom vent and gets hot in the box then circulates out through the top back into the home. A properly designed thermosiphon can last decades, and won’t need any fans or electricity. They can also be quite cheap, under $500 to install!
However, if you want to use passive solar heating to heat up your water supply that is quite possible as a DIY project. The principles are the same as an active collector. The challenge is to build your own solar collector, make sure it gets enough heat to heat your water, and then plumbing it up to your existing water system in such a way that the water can flow right without a pump. There is a free book online that can teach you how to do this at http://www.builditsolar.com/Projects/WaterHeating/ISPWH/IPSWH.pdf.
Questions to Ask before Starting
Using the sun to heat your home and your water can really cut your energy bills or even remove your ties to the grid, but there are some questions you have to ask before starting:
I live in a forest but I’m in the South. Does my site get enough sun?
Do the solar survey mentioned earlier before starting anything! If you don’t get enough sunlight due to obstacles or location, you’ll need to take care of those first or find another method to heat your house and water.
Should I choose an active or a passive system?
This depends on your budget, your house design, whether you plan to go DIY or not, and how much you want to be reliant on parts. Bear in mind that a solar heating system can last decades before needing maintenance. It really depends on your resiliency philosophy.
Should I choose a direct or indirect system?
For cold climates, it’s best to use an indirect system in case the pipes freeze. The heat exchanger is kept indoors so the water supply won’t be interrupted if a pipe bursts in the collector. In temperate climates, you can use either system.
Flat panel collector vs batch processor vs evacuated tube collector?
Each one has its advantages and disadvantages. Batch processors are a common type of collector used in DIY setups, but they are extremely heavy. Don’t install these on a roof unless it is engineered for the weight. There’s a battle in the industry about whether flat panel collectors or evacuated tube collectors are better. Do your research if you plan to use pre-built panels in your DIY project.
Where can I go for DIY plans?
The best place to go for DIY solar plans on the web at this time is Builditsolar.com. There is a massive amount of information here for home builders that want to convert their homes to use solar power.
Hopefully you’ve found this article on solar heaters to be informative and inspirational. Consider building one over the next year. You could save a lot of money on your energy bill, get free hot water, and disengage a little more from fossil fuels.