Solar Heating

Technologies to Cut Energy Use

by Hugh Perry

Environmental groups were promoting the benefits of solar heating long before the federal government introduced its solar program in 1975. Over the next few years, domestic solar systems appeared on many buildings across Canada. But the program soon died from lack of commitment and maintenance. The infrastructure had not been as well established as it is now, 30 years later.

It was during that initial period of interest that CANSIA [Canadian Solar Industries Association, originally CSIA] was formed in 1978. With the assistance of NRCAN [Natural Resources Canada], the country’s first commercial passive exterior wall solar-conditioned fresh air system - went into production in 1977, and, to date, there are installations in 30 countries. Industry pioneers, like Solcan the first Canadian solar panel manufacturer, along with other leaders, have helped keep the industry alive.
In 2006 NRCAN assisted the Enerworks company in achieving CSA approval for their domestic solar package, which became available this year. Solar thermal heating is being proven practical in residential construction with programs such as CMHC’s EQuilibrium endeavour. NRCAN’s Residential Pilot Initiative for solar domestic systems is expected to place 1,200 new units in Canadian homes by December 2010.
The technology is the same for commercial and residential, with commercial seeing increased savings due to the greater economies of large scale installations Manufacturers are simplifying installations and decreasing retail cost, and demand is steadily growing. CANSIA is working with colleges for training and presently there are 66 certified installers/maintainers. This time around the infrastructure is in place to support the growth of this promising industry. This momentum has created a new threshold of cost effectiveness, and a new design logic that supports making all new southerly-sloped roofs into free energy collectors.

Collecting Solar Energy

Passive: Optimization of passive collection comes through good design incorporating window locations, exterior shading, and thermal mass construction for radiant heating.
Panel Collectors : Systems are dedicated either for preheating fresh air, reheating forced air, air conditioning, preheating domestic hot water, or for space heating. Each uses various applications of metal and glass to form absorption panels. The angle to the sun of these panels varies based on the application and latitude. For heating purposes, the angle from the horizontal plane is generally greater than 45 degress, and for cooling the angle is set at less than 45 degrees.
Plate Solar Panels: Liquid enters the bottom of the panel and exits the top picking up heat as it travels through copper tubing under glass. The panels visibly adapt themselves to the roof appearance, are heavy to install and can be custom-made to any configuration.
Vacuum Tube Panels: The vertical evacuated tubes are enclosed within two layers of glass with a vacuum between. A small amount of liquid is contained inside and rises and falls in response to solar heat. That heat is transferred to the system liquid from the top of the tubes only. Panel heights vary between manufacturers and the length depends on the required output. They are light weight and their performance can deteriorate if left empty for long periods.
Air Collectors: There are two types of air collectors, those that use exposed metal only, and those that use metal under glass. The first known as solar siding or solar roof uses dark surfaced perforated metal. Outside air passes through and rises behind the metal surface to enter the building at the top. This heated air is fed directly into the mechanical heating system. The second uses dark-surfaced thin metal covered with glass. Panels and tubes made from recycled pop cans connected to each other are most common. Like the thermal panel, air enters the bottom and exits into the building at the top. Preheated outside air or reheated inside air is fed into the mechanical system.
Solar Furnace: One other collection method seen in France and Italy employs an array of mirrors to focus sunrays either directly or indirectly onto water storage tanks generating temperatures between 1000 and 3000°C. This heat is used to generate electricity, melt steel, or to make hydrogen fuel. The technology is also known as a Solar Concentrator and could soon find practical application in Canada.

Applications for Solar Thermal
Solar for Air Conditioning: Cooling building spaces using heat seems counter intuitive, yet the principle was discovered in France as far back as 1858. It is known as Absorption Refrigeration and the process uses an Absorption Chiller.
The system eliminates the conventional compressor, which uses electricity, and can be unreliable, costly and noisy. An absorption chiller system has no moving parts and uses no HCFCs. Instead the absorption chiller uses the solar heat to provide the energy needed to drive the cooling system. In large installations the solar panels become the evaporator replacing the conventional cooling tower.
The process is similar to conventional cooling in that the liquid used for cooling will vaporize at low temperatures allowing it to take away additional heat, thereby providing the cooling. This process replaces the more familiar compressor and mechanically driven evaporator.
Solar for Domestic Hot Water: Because solar energy is not constant a primary/ secondary source of heat is required to bring the water up to the required temperature. That source can be a heated storage tank, which can have the advantage of offering additional solar storage capacity, or an on-demand boiler, which uses less energy. There are three methods for storing solar heated water: direct thermosiphon, drainback and closed loop.
Direct Thermosiphon pass through System: is used mainly where there is a large demand during daylight hours for car washes, laundromats, food processing or sports facilities. Domestic cold water passes directly through the panel en-route to the fixtures that are using the water. Since the demand is constant there is no requirement for protection against excessive heat and no additional pumping is required. An insulated storage tank holds heat that protects the water from freezing when no heat is being produced. Placing the tank above the panels allows natural thermo siphoning to occur.

Storing Solar Energy
Drainback System: can be used for large and small installations yet is not popular. No freeze protection is required because water only enters the panel when the panel sensor indicates that temperatures within the panel are sufficient to generate heat. When the system is in operation and the temperature drops below the panel temperature, the water drains back to a storage tank waiting for its next cycle. Drainback systems are compatible with plate solar panels, but problems are  associated with the system being open to air.
Closed Loop with Storage System: is the most common and is the only system in Canada with CSA approval as a complete package. The domestic water never comes in contact with the liquid in the closed loop circulating through the solar panel. Instead food-grade polypropylene glycol remains in the panel portion and is circulated via a pump between the panel and heat exchanger. The heat exchanger may consist of a coil inside a hot water tank or as a separate plate heat exchanger outside the tank. Sensors measuring the differential temperatures between the panel and storage tank regulate the flow.
Since water demand is not constant, heated water is stored in tanks and then supplied to fixtures. Both plate and vacuum tube panels can be used for collection.
Solar for Heating Buildings: The average heating demand in Canada is typically 50 to 60% of the maximum demand at the coldest time of the year. For that reason solar heating systems are often sized to deal with those average temperatures rather than the coldest temperatures, which occur for 15 to 20% of the heating season. To satisfy the demand during the coldest weather, a secondary system is tied into the primary solar heating plant.
Geothermal heat pumps work well with solar heating and also offer air conditioning. Projects such as Drake Landing in Alberta store solar heat in the earth below the development for later use in the winter. Heat pumps draw the stored heat from the ground as in a typical geothermal system for distribution throughout the project.
The systems used for heating a building are similar to those used for heating domestic water, where either drainback or closed-loop methods are used along with either vacuum or plate panels. The vacuum tubes decrease the amount of glycol required in the system, while the drainback method eliminates it entirely.
The only major differences between domestic water heating and building heating  are the quantity of panels, the quantity of storage, and the distribution of heat, which is accomplished using radiant floor heating [RHF], hot water radiators, or fan coil units.

Distributing Solar Heat

Radiant floor heating is an ideal method for distributing solar heat where concrete floors are used. RFH requires relatively low temperature water of about 43°C and, as a result, there are fewer requirements for safety valves and controls. RFH on any heating system can save up to 25% in energy and even more when combined with passive solar.

Hot Water Radiators: Radiators work well with medium-to-high temperature water which generally means a requirement for a primary or secondary heating source to raise the temperature of the water above 60c+- in cold weather.
Fan Coil Units: Fan coils are forced air units with a coil connected to the solar system that can be zoned to meet the specific needs of certain areas of the building. They are most often used where cooling is required and/or there is a significant requirement for the introduction of fresh air.
Free solar energy is available whether we use it or not, a simple fact that makes solar heating the most sustainable system possible. In Canada, the technology is available and there now exists an infrastructure for installing and maintaining it.
With the widespread availability of financial incentives, Canada is beginning to follow the global trend toward solar thermal. SABMag [and our new SAB Homes magazine] will continue to publish projects, describe new improved products, and discuss best installation practices and feedback on system success to help publicise solar technology.

Hugh Perry is associate editor of SABMag.

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