Five Solar Thermal Principles

1. heat gain
2. heat transfer
3. heat storage
4. heat transport
5. heat insulation

WHAT IS HEAT? Heat is the measure of the amount of thermal energy an object contains. It is the product of temperature and mass. This means that a large mass contains more heat than a small mass at the same temperature. Some people feel that concentrating collectors are better than flat plate collectors because they get hotter. Actually flat plate collectors covering the same surface area harvest more heat than concentrating collectors because they transfer and conserve heat better.

HEAT GAIN: Heat gain refers to the heat accumulated from the sun: Solar-thermal-heat is trapped using the greenhouse effect. Heat and IR (infrared radiation) are produced when short wave radiation such as visible and UV (ultra violet) light strikes the blackened absorber plate of and become trapped inside the collector. The ability of a glazing surface to transmit short wave radiation and reflect long wave radiation is known as the greenhouse effect. Water tubes that run through the absorber plate collect this trapped heat and transfer it to a heat storage vault.

HEAT TRANSFER: Heat is transferred by conduction or convection. When a pot of water is boiled kinetic energy is transferred by conduction to water molecules at the bottom of the pot. These fast moving molecules spread their thermal energy to adjacent molecules by conduction and tend to occupy more space than the cold slow moving molecules above them. Since hot water occupies more space than cold-water, hot water will rise and the cold water above will sink to the bottom of a pot. This convection process is how heat is transferred uniformly into a pot of water. Uniform heat transfer in a pot of thick soup is more difficult. Solid soup particles interfere with the convection process and insulate above layers from heat transfer. Mom stirs thick soup because she knows the convection process does not work as well as it does
inside a pot of thin soup.
Heat is transferred from the absorber plates of the collector into the collector fluid by the process of conduction. From here the collector fluid is circulated through the carrier pipes to the bottom water layer of the heat transfer vault. Once inside the vault heat is transferred to and concentrated near the top layer of water. This top layer of hot water is where heat is transferred into the domestic heating and domestic hot water system.

HEAT STORAGE

Rate of heat transfer has to do with the conductive and convective medium and it also has a lot to do with the temperature differences. Objects with large temperature differences will transfer heat faster than objects with low temperature differences. If we used only one tank to transfer heat tank water temperature would rapidly reach a saturation temperature close to the collector fluid temperature. At this time very little additional heat would be transferred. Collector efficiency would also be compromised. If the collector fluid temperature is 180 F and the absorber plate temperature is 200 F the rate of heat transfer would be negligible. Heat transfer can be maximized with the aid of a multi tank heat storage vault system.

Tank 3 is the warmest because this is the first tank used to transfer collector heat. Tank 2, the warmer tank, will receive heat left over from tank 3’s transfer process. Tank 1, the warm tank, is designed to suck the last bit of heat from the already cooled collector fluid. This cooled fluid is then returned to hot collectors to gather more heat from the sun.
Heat extraction takes place at the hot, top layer of the tanks by preheating water. Heat from the storage vault is transferred in successive stages from the warm tank to the warmest tank. Tank 3 preheats water for tank 2 and tank 2 preheats water for tank 1. This is how a temperature gradient is maintained between tanks.

HEAT TRANSPORT: Heat transport refers to the manner of transferring heat from solar collector to heat storage vault. In cold climates it is important to separate the heat collection area from heat storage area. Closed loop systems filled with propylene glycol or antifreeze are popular in cold climates. Closed loop systems use a circulator pumps to transport heat from sun to storage vault. Warm climates use simple batch heaters, water tanks enclosed inside glazed boxes. Domestic well water is circulated directly through batch heater tanks.

HEAT INSULATION: Last but not least is insulation. A solar home is worthless without adequate insulation: You should be thinking about R25 in the walls and R35 in the roof/ceiling area. Heat storage vaults and heat transport tubing should also be well insulated. Sunrooms must be isolated from living quarters with doors or drape to prevent heat loss at night.

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