Our large solar thermal systems for institutional, commercial and industrial applications are designed by engineers using a mix of long years of experience along with a variety of analysis tools. Programs like TRNSYS, Fchart, WATSUN and RETScreen allow for simulations, predictions and optimization of system performance – particularly important for large installations.
Tools are readily available for pre-feasibility studies on solar air heating projects. Natural Resources Canada has developed RETScreen, a series of Excel based worksheets covering a range of renewable applications.
Presented below are some general considerations that a facility owner or manager might want to know to facilitate later discussions with our staff (compilation source: CANSIA Application briefs, various authors).
The first step in the design process is identifying the hot water demand of the particular application. Hot water consumption for the building can be calculated from utility bills or water bills, or can be estimated from the ASHRAE Applications Handbook on Service Water Heating, which gives average figures for hot water use per person for a given building. Solar design objectives are typically that the system be able to provide one quarter of the total annual water heating energy.
A rule-of-thumb approach is a good way to get a first approximation for the size of a solar system. It begins with an assumed volume of hot water per sunny day per solar collector. Based on experience, a reasonable average is between 100 and 200 liters of hot water from a solar collector of 3 sq.m area (4 x 8') per sunny day. This method is suitable for a small system such as a residential application but may also provide a ballpark figure for larger systems.
There is no code regarding tank sizing but a rule of thumb for residential applications is one gallon of storage per square foot (45 litres/sq.m) of collector area. In Canada, tanks range from 40 to 120 US gallons (150 to 450 liters) or custom-built tanks of larger sizes. They can be glass-lined in the smaller sizes and stainless steel in any size. They are also available with various linings including concrete.
The solar fraction is the fraction of the sun's energy that is converted into useful form, such as hot water. When a large solar system is properly designed based on the energy demand and the cost savings, the solar fraction is normally one quarter of the total hot water load. The solar heated water may not rise to the final desired temperature but may still produce half of the desired temperature rise, offsetting fuel expenses considerably.
Storage tanks vs. an on-demand heat exchanger
In many applications, it may be unnecessary or unwise to store energy as hot water in a tank. Storage tanks are expensive and, if the load matches the energy available from the sun, the cost for these tanks can be eliminated.
Instead of storing energy in the conventional way, an on-demand heat exchanger can instantaneously heat to the incoming cold water to the user set, desired temperature. A car wash is a good example because, in most cases, people wash their cars on sunny days and need only warm water. On-demand water heaters are popular in Europe for their energy savings, but have yet to heavily influence North American markets.
Other solar thermal design considerations
A major design consideration is in the positioning of collectors to get the most heat from the sun. The collectors are usually mounted on a roof or on a rack above the roof, at an angle to the horizontal called the tilt angle, and at an angle to South called the azimuth angle. Flat roofed buildings most often use rack-mounted panels that are well secured against exposure to the elements. Tilt angle is typically sufficient to give the panels some snow shedding ability, however snow can accumulate around panels and cause an extra load on the roof.
For a year-round solar system in Canada, the best performance is from collectors facing due south, tilted at an angle equal to the latitude of the location and never shaded. For a summer-only system, such as pool heating, the tilt angle will be at a shallow angle. If most of the use is in the winter, the panels will be a steeper angle, perhaps 65 degrees, to capture the lower angle of the sun’s movement.
Based on this movement of the sun from summer to winter, solar system design can be optimized for year round or seasonal use. For example, a canning factory uses more hot water in summer during the harvest season and should be designed to supply hot water for that skewed load.
©2006 Nexus Solar Corporation.