Residential Solar Thermal Systems
Of the many solar options, solar thermal is the most popular because it is the most cost effective choice to reduce your fuel bills. After incentives the pay back time is often 5-6 years, vs. Photovoltaic (PV) systems that can take closer to 10 years.
Solar thermal systems create hot water that can be used to meet domestic needs, space heating, pool heating, air conditioning, clear icy walkways, greenhouse heating, garden soil pre-warm, and more.
Solar thermal systems come in many different varieties, many of which are not appropriate for year round homes in New England climates. Normally we install pressurized systems which heat glycol antifreeze that is passed through a heat exchanger to heat the water in the storage tank. Evacuated tube systems, like the Thermomax model, show right, give better winter performance and flat plate systems (such as the Velux flat plate system at left) are less expensive.
Solar thermal systems will work with your existing systems and can be sized to meet 70% or more of you annual needs. It is not practical to attempt to meet 100% of demand, a backup system is necessary.
Basics of Solar Thermal Hot Water Systems
An average solar thermal system consists of one or more collector panels, a hot water storage tank, a pump, controller, and a heat exchanger. Since the system is pressurized it takes only a small, low energy pump to move the fluid through the system.
The basic concept is simple:
- Glycol antifreeze is pumped up to the collector manifold
- The sun’s rays hit the collector
- The liquid inside the tubes is heated
- The heat is released to the glycol pumping through the manifold
- The hot glycol is pumped down to the tank and gives off its heat to the water
- The cooled glycol is pumped back up to the collector to repeat the process
When the temperature in the glycol loop is less than the hot water tank, the controller shuts of the flow so no liquid is routed at night or times when solar radiation is insufficient.
A solar thermal system collects the sun’s radiant energy, not the heat you feel on a sunny day. Radiant energy is available even when it is cold or cloudy. The short winter days here in the Northeast prove to be the biggest hurdle because we do not have many daylight hours in which to gather the energy we need. In December and January you will find your system will struggle to meet all your demand, and your backup system will come on.
Some brands are better at collecting diffuse solar radiation (what you get when air moisture causes the direct sun radiation to diffuse into different directions) and are the best fits for space heating applications. Since we have plenty of daylight hours in summer, ideally we install collectors at about 55° to gather the most winter sun possible. Excess heat can be dumped outside, into a pool, into your heating system, a radiator in the basement, used as a soil pre-warm in the garden, and there are many other options.
Systems can be designed to meet part (expandable later) or most of your needs. When we quote your system we will tell you the “solar fraction”, the percentage of annual need meet by solar, for each of your different options.When evaluating systems it is important to compare winter outputs in Maine. Any reputable manufacture can make a collector that performs well in summer, or in Florida.
Types of systems: Evacuated tubes vs. Flat plate collectors
There are many types of solar thermal systems and we are only addressing those suitable for year round homes in New England. If you have a seasonal home you have additional options.
A Thermomax glass tube contains a specially coated copper fin to collect the sun’s radiant energy. Bonded to the reverse is a narrow copper pipe that is sealed with a liquid inside. The liquid, also under a vacuum, quickly boils and heats the copper tip that passes the heat to the manifold.
The majority of systems that we install are evacuated tubes because they provide superior winter performance. A collector panel generally consists of a group of 10-30 tubes connected to a manifold.
A true evacuated tube, such as Thermomax, is a single wall of glass with the copper collector heat pipe inside. All the matter is evacuated from the tube. Since temperature requires matter to move, having a vacuum is a perfect insulator. An evacuated tube will function just as well when it is –10° outside as it does at 60°. Hybrid, or Sydney tubes, such as Apricus, have a double wall of glass. Between the layers is a vacuum. The copper collection heat pipe sits inside in an air chamber protected by this vacuum blanket. These types of tubes, primarily from China, are cost effective but their winter output is slightly less than a true evacuated tube.
The majority of systems use a dry fit heat pipe that just plugs into the manifold. Inside the glass is the liquid filled copper collection pipe. It boils quickly, heating the copper tip. The heat is passed to the glycol; the steam cools, condenses, and starts the process again.
Double walled tube. A copper heat pipe would sit inside.
Flat Plate Systems
Flat plates are less expensive but since they are protected from outdoor temperatures by high quality insulation, not a vacuum, their winter performance is not as good as evacuated tubes. In addition, some people like the look of flat plates better. Velux, the premier manufacture of skylights, makes an excellent flat plate systems (see left) that can be flashed into the roof and comes in the same sizes as their skylights. This is often a great choice when aesthetics are a concern. To capture the winter sun, flat plates still ideally like to point south and here in New England, need to be at close to 55° so these systems perform best when mounted on highly slopped roofs.
Four Velux flat plate collectors and 2 skylights all flashed into the roof.
The underlying design of a flat plate is similar to an evacuated tube. Beneath the glass glazing is a series of copper collection pipes, like those found in a evacuated tube. The metal frame has high quality insulation inside to help protect these copper collection tubes from the outside temperatures. Flat plates come in many sizes but most commonly about 4x6ft.