|
|
|
|
Basic Terminology Basic Solar Terminology We have included a list of terms you may encounter when reading through our web site. If you have a question on a particular term or you do not see it here, please email us at *info@trimlinedesigncentre.com and we will add it to our growing list. *Due to the huge amount of unwanted spam mail, please copy and paste the contact email address into your email program as it is not linked. (please remove the * or the email address will not work) Sorry for the inconvenience. Absorber: The part of the collector that actively absorbs the light rays. For flat plate finned collectors this is defined as the cross-sectional area of the selective surface parallel riser fins connected to the top and bottom headers. For flat plate non-finned collectors this is defined as the cross-sectional area of the single selective surface fin connected to the serpentine integral copper tube. For double glass evacuated tube collectors this is defined as the cross-sectional area of the inner tube (selective coated) measured using the outside diameter. For double glass evacuated tube collectors with reflective panels, the entire circumferential surface area of the inner tube is often used when calculating absorber area, as the reflective panel is supposed to reflect light onto the underside of the evacuated tube. Active systems: Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but generally more efficient. Active systems are often easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. If installed using a PV module to operate the pump, an active system can operate even during a power outage. Aperture: The part of the collector through which light enters. For flat plate collectors this refers to the cross-sectional surface area of the clear glass plate measured using the internal frame dimensions. For double glass evacuated tubes this refers to the cross-sectional surface area of the outer clear glass tube measured using the internal diameter, not the outside diameter. For single glass evacuated tubes this refers to the cross-sectional surface area of the clear glass tube measured using the outside diameter. BTU: Stands for British Thermal Units. This is an imperial unit of measurement for heat widely used in the US and also in the UK. The conversion to the metric unit kWh is: 1 kWh = 3412Btu, and for surface area values, 1kWh/m2/day = 314Btu/ft2/day. Celsius: The metric unit for temperature measurement. Convert as follows: Fahrenheit = (oC x 1.8) + 32 Celsius = (oF - 32)/1.8 Closed-Loop Active Systems: These systems pump heat-transfer fluids (usually a propylene glycol/water antifreeze mixture) through the solar water heater. Heat exchangers transfer the heat from the fluid to the water that is stored in tanks. Single or double-walled heat exchangers or twin coil solar tanks prevent contamination of household water. Some standards require a double walled heat exchanger when the heat-transfer fluid is anything other than water. Closed-loop glycol systems are popular in areas subject to extended subzero temperatures because they offer good freeze protection. However, glycol antifreeze systems are typically more expensive to purchase and install due to extra valves and heat dissipators (heat dump) required and the glycol must be checked each year and changed every few years, depending on glycol quality and system operating temperatures. Coefficient of expansion: The fractional change in volume (or sometimes in length, when specified) of a material for a unit change in temperature. Concentrating Collectors: These are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid, or the water itself. Delta-T Controller: Delta-T refers to the difference in two temperatures. This term is often use in relation to a solar controller. In such case the Delta-T is the difference between the solar collector temperature and the temperature of the water in the solar storage tank. A Delta-T controller can be configured to turn on the pump when the Delta-T difference exceeds a certain level (Eg.7oC / 12.7oF) and off again when the temperature difference drops below another setting (Eg. 2oC / 3.6oF). The controller turns on the pump when there is heat potential in the manifold. A Delta-T controller can also be used to provide freeze protection by circulating warm water from the tank through the manifold when the manifold temperature drops below 5oC. Drainback systems: A closed loop system that typically uses water as the heat-transfer fluid in the solar collector loop. The water is pumped through the collectors when on, and then is drained by gravity to the storage tank and heat exchanger when off. These systems have no valves to fail and when the pumps are off the collectors are empty, thereby assuring freeze and over temperature protection. They auto shut-off if the water in the storage tank becomes too hot, thereby eliminating extra valves and heat dissipators (heat dump). A problem with drainback systems is that the solar water heater installation and plumbing must be carefully positioned to allow complete drainage.(Gravity must be allowed to work) The pump must also have sufficient head pressure to pump the water up to the collector each time the pump starts. Electricity usage is therefore slightly higher than closed loop active system. The system is much simpler with fewer parts, no check valves, no air vents, no pressure gauges, and no expansion tanks. These systems are immune to power outages and pump failures. Draindown systems: An open loop system in which the collectors are filled with domestic water (household potable water) under house pressure when there is no danger of freezing. Once the system is filled, a differential controller operates a pump to move water from the tank through the collectors. Draindown systems have proven to be the most problematic of all freeze protection systems. They are vulnerable to frozen vacuum breakers and air vents, damaged sensors or wiring, lack of proper pipe drainage, and malfunctions with the draindown valve. This type of system is rarely installed new any more, and is not recommended. Many were converted to drainback or closed loop antifreeze systems. A draindown valve, invented in the 1970s exclusively for these systems, provides the freeze protection function. When the collector inlet temperature falls to 40°F, the draindown valve, activated by the controller, isolates the collector inlet and outlet from the tank. It simultaneously opens a valve that allows the domestic water in the collector to drain away. A vacuum breaker is always installed at the top of the collectors to allow air to enter the collectors at the top so the domestic water can drain out the bottom. Right next to the vacuum breaker, you'll find an automatic air vent to allow air to escape when the system fills. There can also be health concerns arising due to long periods of stagnant water breeding algae and other harmful bacteria. Draindown Valve: A device invented in the 1970s to provide freeze protection exclusively for draindown systems. When the collector inlet temperature falls to 40°F, the draindown valve, activated by the controller, isolates the collector inlet and outlet from the tank and simultaneously opens a valve that allows water in the collector to drain away. Efficiency: Solar collector efficiency is usually expressed as a percentage value, or in a performance graph. When assessing a collector's performance make sure it is based on absorber area. Flat plate collector's absorber area and gross area is almost the same, whereas evacuated tube collector absorber area is usually only around half of the gross area. When comparing two collectors, not only the performance graph need be considered. IAM values have a significant influence on actual heat output throughout the day. Looking at just the percentage efficiency value will not give a true indication of daily heat output. Energy: 1 kWh/m2/day = 317.1 Btu/ft2/day 1kWh = 3412Btu = 859.8kcal = 3.6MJ Approximately 1 kcal of energy is required to raise 1 liter of water by 1oC Evacuated Tube Collector Double Glass Tube: (Glass-Glass) tubes consists of two glass tubes which are fused together at one end. The inner tube is coated with a selective surface that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes perform very well in overcast conditions as well as low temperatures. Because the tube is 100% glass, the problem with loss of vacuum due to a broken seal is greatly minimized. Glass-glass solar tubes may be used in a number of different ways, including direct flow, heat pipe, or U pipe configuration. Evacuated Tube Collector Single Glass Tube: (Glass-Metal) tubes consist of a single glass tube. Inside the tube is a flat or curved aluminum plate which is attached to a copper heat pipe or water pipe. The aluminum plate is generally coated with Tinox, or another selective coating. These types of tubes are very efficient but can have problems relating to loss of vacuum. This is primarily due to the fact that their seal is glass to metal. The heat expansion rates of these two materials are different and so after a few years of daily contraction and expansion the seal can fail and the vacuum lost. Glass-glass tubes although not quite as efficient glass-metal tubes are generally more reliable and much cheaper. Flat-Plate Collector: Comprised of an insulated, weatherproof box containing a dark absorber plate under one or more transparent or translucent covers. Water or heat transfer (HTF) fluid passes through pipes located below the absorber plate. As the HTF flows through the pipes it is heated. This collector, although it is less efficient in some ways to evacuated tube collectors, is still the most common type of collector used in many countries. Flow Rate: The volume of water flowing through plumbing in a given period of time. Usually measured in volume/minute or volume/hour. 1 Litre/min = 0.264 US Gallon/min. Freezing point: The temperature below which a liquid turns into a solid. Gravity Feed Tank (GFT): A small tank, located above the level of the solar collector, which provides low pressure water supply. The feed tank is generally fitted with a float valve, fed by mains pressure cold water. Gross Area: The total surface area of the collector including the frame, manifold and absorber. Heat Dump: Used to prevent overheating. Depending on the type of system, it could be a finned radiator, heat dissipator, radiant heat loop or an extra storage tank. Heat Pipe: An evacuated rod or pipe used for heat transfer. Heat Transfer Fin: This serves two purposes; firstly to hold the heat pipe in place against the side of the tube, secondly to enhance heat conduction to the heat pipe. Heat Transfer Fluid (HTF): A liquid used to move heat energy from one place to another, such as through solar collectors and heat exchangers to the heat storage tank in solar water heating systems. Examples of heat-transfer fluids are water, glycol/water mixtures and hydrocarbon oils. Incidence Angle Modifier (IAM): Refers to the change in performance of the sun's angle in relation to the collector surface changes. Perpendicular to the collector (usually midday) is expressed as 0o, with negative and positive angles in the morning and afternoon respectively. Collectors with a flat absorber surface, which includes some types of evacuated tubes, only have 100% efficiency at midday (0o), whereas Apricus solar tubes provide peak efficiency mid morning and mid afternoon, at around 40o from perpendicular. This results in good stable heat output for most of the day. Insolation: Don't confuse this with insulation - the one letter change makes a big difference. The total amount of solar radiation direct, diffused and reflected-striking a surface exposed to the sky. Insulation: The ability to protect against transfer of heat/cold. Solar collectors use either, compressed glass wool, compressed fiberglass, or some other type to insulate the header/absorber from heat loss. Glass wool has excellent insulation properties, is very light and can withstand high temperatures, making it an ideal choice for a solar collector. Integrated Batch Collectors (IBC): Batch collectors are simple passive systems consisting of one or more storage tanks placed in an insulated box that has a glazed side facing the sun and heats household potable water. Batch collectors are inexpensive and have few components, but only perform well in summer when the weather is warm. Net Billing: Net billing is the process whereby solar power is produced and sold to the utilities. The associated fees associated with producing the power are not paid for by the utilities. A meter hooked up to the electrical grid measures electricity produced. So during the hottest, brightest part of the day, solar arrays can pump electricity back into the grid generating electricity and lowering electricity bills. Net Metering: Net metering is the process whereby solar power and associated fees are sold to the utilities. A meter hooked up to the electrical system measures electricity going both ways - power coming in from conventional power utilities, i.e. oil or coal, and solar power being generated. So during the hottest, brightest part of the day, solar arrays can pump electricity back into the grid, spin the electric meter backward and lower electricity bills. Open-Loop Active Systems: Open-loop active systems use pumps to circulate household potable water through the collectors. This design is efficient and lowers operating costs but is not appropriate if water is hard or acidic because scale and corrosion will gradually disable the system. Open-loop active systems are popular in regions that do not experience subzero temperatures. Flat plate open-loop systems should never be installed in climates that experience sustained periods of subzero temperatures. Passive systems: Passive systems move household water or a heat-transfer fluid through the system without pumps. Passive systems have the advantage that electricity outage and electric pump breakdown are not issues. This makes passive systems generally more reliable, easier to maintain, and possibly longer lasting than active systems. Passive systems are often less expensive than active systems, but are also generally less efficient due to slower water flow rates through the system. Potable Water: Water of sufficiently high quality that it can be consumed or used without risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually consumed or used in food preparation Pressure: Refers to the fluid pressure in the system. The conversions for the most commonly used units are: 1 bar = 1.02kg/cm2 = 14.5psi = 100kPa = 0.1Mpa = 10m. Solar Constant: The average intensity of solar radiation reaching the earth outside the atmosphere; accounting to two Langley’s or 1.94 gram-calories per square centimeter, equal to 442.4 BTU/hr/ft.², or 1395 watts/m². Solar Fraction: Solar fraction is the ratio of solar energy used divided by total energy used in the same application. It cannot possibly be more than 1.0 (or 100%). Note that solar fraction is distinctly different from efficiency. Tilt Angle: The angle that a collector or module surface forms with the horizontal plane. Thermal capacity: The ability of matter to store heat. Thermal Mass or Thermal Inertia: The tendency of a building with large quantities of heavy materials to remain at the same temperature or to fluctuate only very slowly; also the overall heat storage capacity of the building. Thermistor: A sensing device which changes its electrical resistance according to temperature. Typically used in control systems to generate input data on collector and storage temperatures. Thermosyphon Systems: A thermosyphon system relies on warm water rising, a phenomenon known as natural convection, to circulate water through the solar absorber and to the tank. In this type of installation, the tank must be located above the absorber tubes/panel. As water in the absorber heats, it becomes lighter and naturally rises into the tank above. Meanwhile, cooler water in the tank flows downwards into the absorber, thus causing circulation throughout the system. Vacuum Breaker: A device installed at the top of the collectors in a draindown system that allows air to enter the collectors at the top so water can drain out the bottom. Viscosity: Resistance of a liquid to flow. (and hence to sheer forces). |
Welcome |