How does solar thermal work?


In my earlier post I discussed how we decided to select solar hot water in our new house, but before we made the decision we had to understand something about the technology so that we could evaluate the options available to us.  In this post, I will be describing the components involved in solar thermal systems in very general terms and I will wrap it up with a discussion of the “gotcha” points we ran into when implementing our solution.

 

The general concept

Most current implementations of solar thermal in Canada involve pre-heating water before it goes into the normal hot water tank.  This means that the energy source for the normal tank does not have to be activated as often, thereby saving energy.

The technology usually involves installation of an array of solar collectors, a second hot water tank, a pump, a controller board and two fluid pathways and a bunch of other components for temperature and pressure management. The basics of solar thermal hot water technologies are well described in an Ontario Government publication.

Solar collectors are usually installed in a rooftop array on a South facing roof slope, but they are occasionally installed in a self-standing array in a sunny part of the yard. There are several configurations of solar collectors, but one of the most popular is evacuated collector tubes. Given that the tubes are evacuated their efficiency is only slightly dependent upon ambient temperature. Their efficiency is highly affected by the strength and duration of the sun to which they are exposed. This means that the efficiency of the unit will fall on short winter days or on days with lots of cloud cover.  I am going to focus on systems that use arrays of evacuated solar collectors.

 
Array of 7 sub-arrays, each with 10 evacuated solar collection tubes

The solar arrays

 Each solar array is mounted on a rack and includes a header into which a number of solar tubes are attached.  The rack is mounted on a roof or exterior wall with a southerly exposure or on a flat area next to the house.  The rack is installed at an angle that optimizes the sun that will fall on the array in a given latitude.  Solar arrays may consist of a single unit or multiple sub-units mounted in series.  The cold glycol comes up through the roof in insulated copper pipes, enters the header at one end of the array, is warmed while traversing the header and exits via the other end where it is directed back down through the roof in insulated copper pipes.

The solar array that we installed was purchased from CarEarth Inc. of Ottawa, and consists of three sub-arrays, two with 10 tubes each and one with five tubes.  It is installed on the south face of the roof.

The solar collectors

The solar collectors installed in each array are in the form of double walled evacuated glass tubes that vary in number, length and diameter. Given the same duration and strength of sunlight, the surface area covered by an array is directly proportional to the volume of water than can be heated by the array.

Inside each tube, and running the entire length of the tube, is a conducting rod (normally made of copper) to which is attached one or more solar collectors (configured either as wings or as a cylinder themselves within the larger glass walled cylinder).  As the solar collectors are heated by the sun’s rays they transfer their heat to the copper core. The copper core conducts this heat to the glycol loop running through the array’s header unit.  When this heated glycol arrives at the heat exchange unit in the solar hot water tank, it will release its heat to that water, raising the water’s temperature. 

As the space between the walls on the tubes is a near vacuum, very little heat is lost during the transfer from the collectors and copper core to the glycol, even on the coldest days.

In our implementation we used 25 tubes of 5 ft by 4 inch, arranged in three arrays. 

The upper fluid path (“glycol loop”)

Each of the collection tubes in the array is connected at the top to the array’s header.  This header is connected to the house’s upper fluid path or glycol loop.  In Canada this loop is normally filled with propylene glycol … hence the name “glycol loop”.  Because glycol is an antifreeze, the collectors can be used even during the coldest part of the winter.

The glycol runs in a loop between the solar collectors on the roof and the hot water tank heat exchange unit(s) in the basement.  An in-line pump moves the fluid around the loop sending cold glycol to the solar collectors on the roof and warmed glycol back to the exchange units in the solar tank. 

The solar hot water tank

The solar hot water tank is a water storage tank with a stainless steel insert and one or two

Tank and Heat Exchange Unit
Tank and Heat Exchange Unit

 heat exchangers.  The tank functions like traditional tanks insofar as it has a cold water input (city water) and a hot water output.  The difference is that the tank’s heat generation is not electric, oil or gas, but solar.  The glycol loop described above is connected to the tank’s heat exchange unit.  Cold water (city or well source) enters the tank at the bottom, receives heat from the heat exchange unit, and this pre-heated water is sent to the cold water intake of the traditional water heater.  In this way, the traditional hot water heater can respond to demands for hot water using only a fraction of the energy it would otherwise use.

In our implementation, we chose a 200 imperial gallon tank for storage of solar pre-heated water.

Controller board and other components

The controller is a small computer that determines when it the glycol in the solar collector is hotter than the temperature of the glycol in the heat exchange unit.  The controller is parameter driven and can be adjusted in a variety of ways, but in our case, when the glycol temperature on the roof is 6 degrees warmer than the glycol in the heat exchange unit, the pump is turned on and the glycol is pumped up to the roof to be heated.

Controller
Controller

Other components of note include the pump, the insulated copper pipe runs throughout the house, back-flow protectors (that isolate the house from the neighbourhood), pressure management equipment, and heat and pressure release valves.  I will be dealing with these items in greater detail in future posts, but suffice to say, the system is not without its complexities. 

Certification woes

These days, the controller and other components all come nicely packaged in a well-engineered, small case, but when we had our unit installed, the newly packaged items were not yet CSA approved.  For that reason, we opted to have the entire system engineered out of components commonly used in household plumbing.  As each of these individual components is CSA approved, their safe operating limits are well documented. 

Unfortunately, according to the newly modified Ontario Building Code, it is not sufficient for the components in the system to be CSA approved.  The entire system must also be certified by a civil engineer.  When we started, the code had no provisions for Solar Thermal equipment and we originally tried to have Municipal inspectors approve the installation, but as they were unfamiliar with the technology the would not approve the installation without certification by an engineer.  Since that time this decision has been codified in the Ontario Building Code.  Finding an engineer to certify our system proved impossible and while the system has operated flawlessly for 4 years, its installation has never been formally approved.

I will speak more about the certification process in a future post, but for now you should just be aware that certification and approval will not be an easy hurdle to jump.

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