Solar collector

Solar collector is a device to transform the sun energy into more usable or storable form. The energy of sunlight is transformed into electromagnetic radiation from infra-red (long) waves into ultraviolet (short) ones. The solar energy striking the earth's surface depends on many factors such as weather conditions, location and orientation of the surface. It averages about 1000 watts per square meter under clear sky with the surface directly perpendicular to the sun's rays.

The solar heating system consists of the collector described above; a heat transfer circuit that includes the fluid and the means to circulate it; and a storage system including a heat exchanger (if the fluid circulating through the collector is not the same liquid being used to heat the object of the system). The system may or may not include secondary distribution of heat among different storage reservoirs or users of heat. The system can be used in a variety of ways, including warming domestic hot water, heating swimming pools, heating water for a radiator or floor-coil heating circuit, heating an industrial dryer, or providing input energy for a cooling system. The heat is normally stored in carparks. Heat storage is usually intended to cover a day or two's requirements, but other concepts exist including seasonal storage (where summer solar energy is used for winter heating by raising the temperature by a few degrees of several million liters of water).

There are two common systems of solar heating of domestic hot water: thermo siphon and pumped. In the thermo siphon system a storage tank is placed above the collector. As the water in the collector is heated, it will rise and naturally start to circulate around the tank. This system is self-regulating and requires no moving parts or external energy that is why it is very attractive. Its main drawback is the need for the tank to be placed at the level higher than the collector, which may prove to be physically difficult. A pumped system uses a pump to circulate the water, so the tank can be positioned independently of the collector location. This system requires external energy to run the pump. It also requires control electronics to measure the temperature gradient across the collector and modulate the pump accordingly.

Solar collectors can be mounted on a roof or wall, facing the equator. So a north-facing roof in the southern hemisphere and a south-facing roof in the northern hemisphere are ideal. Collectors are usually angled at the latitude of the location. Depending on sunshine available, 2 to 10 square meter array may provide 50 - 60% of the hot water heating required for a typical family house. Such systems are a secondary feature of sustainable housing, since space heating and hot water are usually the largest single consumer of energy in households.

A solar thermal collector that stores heat energy is called a "batch" type system. Other types of solar thermal collectors do not store energy but instead use fluid circulation (usually water or an antifreeze solution) to transfer the heat for direct use or storage in an insulated reservoir. Water/glycol has a high thermal capacity and is therefore convenient to handle. The direct radiation is captured using a dark colored surface which absorbs the radiation as heat and conducts it to the transfer fluid. Metal is a good thermal conductor, especially copper and aluminum. In high performance collectors a "selective surface" is used in which the collector surface is coated with a material having properties of high-absorption and low-emissivity. The selective surface reduces heat-loss caused by infrared radiant emission from the collector to ambient.

As it heats up, thermal losses from the collector itself will reduce its efficiency, resulting in increased radiation, primarily infrared. This is countered in two ways. First, a glass plate is placed above the collector plate which will trap the radiated heat within the airspace below it. This exploits the so-called greenhouse effect, which is in this case a property of the glass: it readily transmits solar radiation in the visible and ultraviolet spectrum, but does not transmit the lower frequency infrared radiation very well. The glass plate also traps air in the space, thus reducing heat losses by convection. The collector housing is also insulated below and laterally to reduce its heat loss. The second way to improve efficiency is by cooling the absorber plate. This is done by ensuring that the coldest heat transfer fluid is circulated through the absorber and with a sufficient flow rate. The fluid carries away the absorbed heat, thus cooling the absorber. The warmed fluid leaving the collector is either directly stored, or else passes through a heat exchanger to warm another tank of water, or is used to heat a building directly. The temperature differential across an efficient solar collector is usually only 10 or 20°C. While a large differential may seem impressive, it is in fact an indication of a less efficient design.

Solar collectors transform solar radiation into heat and transfer that heat to a medium (water, solar fluid, or air). Then solar heat can be used for heating water, to back up heating systems or for heating swimming pools.

Ex. 3 Answer the following questions:

1) What is solar collector? 2) What does solar energy striking the Earth’s surface depend on? 3) How can the solar heating system be used? 4) Why is the thermo siphon system considered to be more attractive? 5) Are there any drawbacks? 6) Why is water convenient to handle? 7) What is a good thermal conductor? Ex. 4 Make up special questions to the following statements:

1) The selective surface reduces heat-loss caused by infrared radiation emission from the collector to ambient. (What ...?) 2) A glass plate is placed above the collector plate. (Where...?) 3) The fluid carries away the absorbed heat. (What...?) 4) There are two common systems of solar heating of domestic hot water. (How many...?) 5) Solar collectors can be mounted on a roof or wall, facing the equator. (Where...?)

Ex. 5 True or false? If the sentence is false correct it. 1) The energy of sunlight is transformed into electromagnetic radiation from ultraviolet waves into infra-red ones. 2) The solar energy striking the Earth’s surface doesn’t depend on the location and orientation of the surface. 3) The system can’t include secondary distribution of heat among different storage reservoirs. 4) There are two common systems of solar heating of domestic hot water: thermo siphon and pumped. 5) The pump in a pumped system can be positioned independently of the collector location. 6) Copper and aluminum are bad thermal conductors. 7) The next way to improve efficiency is by heating the absorber plate.

Ex. 6 Insert the omitted words:

1) Solar collector is a ... to transform the sun energy into more usable or storable form. 2) Heat ... is usually intended to cover a day or two’s requirements. 3) This system is self-relating and ... no moving parts or ... energy. 4) Its main ... is the need for the tank to be placed at the level higher than the collector. 5) Other types of solar thermal collectors use ... circulation (usually water or antifreeze ...) to transfer heat. 6) In high performance collectors a “selective surface” is used in which the collector surface is … with a material having properties of ... and... . 7) This exploits the so – called ... effect.

(fluid, greenhouse, storage, low-emissivity, device, requires, high-absorption , external, drawback, coated, solution).

Ex. 7 Find the English equivalents of the following words and word combinations:

Инфракрасный недостаток жидкость теплообменник парниковый эффект поддерживать коэффициент излучения интенсивность подачи кислорода поглощённое тепло теплоёмкость

To back up flow rate absorbed heat infra-red thermal capacity drawback heat exchanger greenhouse effect emissivity fluid

Ex. 8 Find the Russian equivalents of the following words and word combinations:

Southern hemisphere conduct copper household self-regulating system transmit absorber plate storage tank ambient

Домашнее хозяйство окружающая среда накопительная цистерна поглощающая пластина проводить медь южное полушарие передавать саморегулируемая система

Ex. 9 Make up5 sentences of your own with words from ex-s 1, 7 and 8.

Ex.10 What does the word in italic mean in the following sentences? Consult the text.

1) It requires control electronics to measures the temperature gradient across the collector. 2) It is a good thermal conductor. 3) This exploits the so-called greenhouse effect. 4) This is countered in two ways. 5) It has a high thermal capacity and is therefore convenient to handle. 6) It can be mounted on a roof or wall, facing the equator.

Ex.11 Find in the text sentences with the verb in Passive Voice and translate them.

Ex. 12 Retell the text.

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