Solar water heating system

Transcription

1 Solar water heating system Perhaps the most popular application of solar systems is for domestic water heating. The popularity of these systems is based on the fact that relatively simple systems are involved and solar water heating systems are generally viable. This category of solar systems belongs to the low-temperature heat applications. The world s commercial low-temperature heat consumption is estimated to be about 10 EJ per year for hot water production, equivalent to 6 trillion m2 of collector area. In 2005, about 140 million m2 of solar thermal collector area were in operation around the world, which is only 2.3% of the potential. A solar water heater is a combination of a solar collector array, an energy transfer system, and a storage tank. The main part of a solar water heater is the solar collector array, which absorbs solar radiation and converts it to heat. This heat is then absorbed by a heat transfer fluid (water, non-freezing liquid, or air) that passes through the collector. This heat can then be stored or used directly. Because it is understood that portions of the solar energy system are exposed to weather conditions, they must be protected from freezing and overheating caused by high insulation levels during periods of low energy demand. Two types of solar water heating systems are available: Direct or open loop systems, in which potable water is heated directly in the collector. Indirect or closed loop systems, in which potable water is heated indirectly by a heat transfer fluid that is heated in the collector and passes through a heat exchanger to transfer its heat to the domestic or service water. Systems differ also with respect to the way the heat transfer fluid is transported: Natural (or passive) systems. Forced circulation (or active) systems. Passive Systems A special type of passive system is the Integrated Collector Storage (ICS or Batch Heater) where the tank acts as both storage and solar collector. Batch heaters are basically thin rectilinear tanks with glass in front of it generally in or on house wall or roof. They are seldom pressurized and usually depend on gravity flow to deliver their water. A step up from the ICS is the Convection Heat Storage unit (CHS or thermo siphon). These are often plate type or evacuated tube collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from collector to tank. Neither pumps nor electricity are used to enforce circulation.

2 Active Systems Active solar hot water systems employ a pump to circulate water or HTF between the collector and the storage tank. Like their passive counterparts, active solar water heating systems come as two types: direct active systems pump water directly to the collector and back to the storage tank, while indirect active systems pump transfer fluid (HTF), the heat of which is transferred to the water in the storage tank. Because the pump should only operate when the fluid in the collector is hotter than the water in the storage tank, a controller is required to turn the pump on and off. The use of an electronically controlled pump has several advantages: The storage tank can be situated lower than the collectors. In passive systems the storage tank must be located above the collector so that the thermo siphon effect can transport water or HCF from collector to tank. The use of a pump allows the storage tank to be located lower than the collector since the circulation of water or HCF is enforced by the pump. A pumped system allows the storage tank to be located out of sight. Because of the fact that active systems allow freedom in the location of the storage tank, the tank can be located where heat loss from the tank is reduced, e.g. inside the roof of a house. This increases the efficiency of the solar water heating system. New active solar water heating systems can make use of an existing warm water storage tanks ("geysers"), thus avoiding duplication of equipment. Reducing the risk of overheating. If no water from the solar hot water system is used (e.g. when water users are away), the water in the storage tank is likely to overheat. Several pump controllers avoid overheating by activating the pump at night. This pumps hot water or HTF from the storage tank through the collector (that is cold at night), thus cooling the water in the storage tank. Reducing the risk of freezing. For direct active systems in cold weather, the pump controller can pump hot water from the water storage tank through the collector in order to prevent the water in the collector from freezing, thus avoiding damage to the system Active systems can tolerate higher water temperatures than would be the case in an equivalent passive system. Consequently active systems are often more efficient than passive systems but are more complex, more expensive, more difficult to install and rely on electricity to run the pump and controller. Indirect water heating system The model base on which we focus our attention is concerning the Indirect water Heating system for home service purpose and home heating system.

3 In the follow we will describe the models of Indirect Water Heating System: Array of solar collectors, storage tank, controller, pumps as illustrated in Figure. SOLAR COLLECTOR: Solar energy collectors are special kinds of heat exchangers that transform solar radiation energy to internal energy of the transport medium. The major component of any solar system is the solar collector. This is a device that absorbs the incoming solar radiation, converts it into heat, and transfers the heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy collected is carried from the circulating fluid either directly to the hot water or space conditioning equipment or to a thermal energy storage tank, from which it can be drawn for use at night or on cloudy days. Conventional simple flat-plate solar collectors were developed for use in sunny, warm climates. Their benefits, however, are greatly reduced when conditions become unfavorable during cold, cloudy, and windy days. Furthermore, weathering influences, such as condensation and moisture, cause early deterioration of internal materials, resulting in reduced performance and system failure. Evacuated heat pipe solar collectors (tubes) operate differently than the other collectors available on the market. These solar collectors consist of a heat pipe inside a vacuum-sealed tube, as shown in Figure

4 Evacuated tube collector (ETC) has demonstrated that the combination of a selective surface and an effective convection suppressor can result in good performance at high temperatures. The vacuum envelope reduces convection and conduction losses, so the collectors can operate at higher temperatures than flat-plate collectors. Like flat-plate collectors, they collect both direct and diffuse radiation. However, their efficiency is higher at low incidence angles. This effect tends to give evacuated tube collectors an advantage over flat-plate collectors in terms of daylong performance. High efficiency solar collector using heat pipe evacuated tubes Able to be used in all climates Reliable and efficient with twin-glass solar tubes Copper heat pipes for rapid heat transfer Easy plug-in installation for mounting on the roof or at ground level Maintenance free Suitable for mains pressure water (up to 8bar / 116psi) Corrosion resistant silver brazed copper header Frame material: stainless steel, casing material: stainless steel, excellent insulator in glass wool Collectors may be connected in series to increase water heating capacity Tubes easily replaced if broken - can be used with broken tubes

5 STORAGE TANK: Thermal storage is one of the main parts of a solar heating, cooling, and power generating system. Because for approximately half the year any location is in darkness, heat storage is necessary if the solar system must operate continuously. For some applications, such as swimming pool heating, daytime air heating, and irrigation pumping, intermittent operation is acceptable, but most other uses of solar energy require operating at night and when the sun is hidden behind clouds. A storage tank in a solar system has several functions, the most important of which are: Improvement of the utilization of collected solar energy by providing thermal capacitance to alleviate the solar availability and load mismatch and improve the system response to sudden peak loads or loss of solar input. Improvement of system efficiency by preventing the array heat transfer fluid from quickly reaching high temperatures, which lower the collector efficiency. The location of the storage tank should also be given careful consideration. The best location is indoors, where thermal losses are minimal and weather deterioration will not be a factor. In our project we consider the Storage tank inserted in existing water circuit having an instantaneous heater device as figure ST.1 or having an auxiliary heater with exchanger as figure ST.2, Figure ST.1

6 Figure ST.2 Outlet obturates in design, cold water inlet connect with water pipe, supply the hot water by the pressure of tap water, with enough hydraulic pressure Equip temperature controller, P/T Valve, unilateral safety valve etc. automatic controller and protection Mild steel sheet coated with special porcelain enamel, install long anode magnesium rod, soften the water and lengthen the service life of inner vessel. Equip assistant electric element, safety and convenient. Can equip heating exchanger in tank based on requirement, bulky heating exchange; economize the running time of the whole system. Thickened high-density environment-friendly polyurethane thermal insulation layer, high efficiency and economize energy sources. High effect galvanized sheet shell, with outdoor coating technology, anticorrosive and rust-resistant, make the service life longer. Can connect multi-units in parallel, install total, and satisfy more hot water requirement. SOLAR PUMP STATION: Solar pump stations are used on the primary circuit of solar heating systems to control the temperature of the hot water storage. The pump inside the unit is activated by the signal from a differential temperature controller. The units contains the functional and safety devices for an optimal circuit control, and is available with both flow and return connection or with return connection only. The solar pump station is a pre-installed and leak-tested unit with fittings for transferring heat from the collector to the storage tank. It contains important fittings and safety devices for the operation of the solar thermal system:

7 Ball valves in flow and return in combination with check valves to prevent gravity and thermo circulation. Ports for flushing, filling and emptying the system. Air vent for manual bleeding of the solar thermal system. Flow meter for displaying and setting the flow rate. Thermometer in flow and return for displaying both temperatures. Pressure gauge for displaying the system pressure. Safety relief valve to prevent overpressure. DIFFERENTIAL TEMPERATURE CONTROLLER: One of the most important components of an active solar energy system is the temperature controller because a faulty control is usually the cause of poor system performance. In general, control systems should be as simple as possible and should use reliable controllers, which are available nowadays. One of the critical parameters that need to be decided by the designer of the solar system is where to locate the collector, storage, overtemperature, and freezing-temperature sensors. The use of reliable, good-quality devices is required for many years of trouble-free operation. As was seen in the previous sections of this chapter, the control system should be capable of handling all possible system operating modes, including heat collection, heat rejection, power failure, freeze protection, and auxiliary heating. The basis of solar energy system control is the differential temperature controller (DTC). This is simply a fixed temperature difference (ΔT) thermostat with hysteresis. The differential temperature controller is a comparing controller with at least two temperature sensors that control one or more devices. Typically, one of the sensors is located at the top side of the solar collector array and the second at the storage tank. The differential temperature controller monitors the temperature difference between the collectors and the storage tank. When the temperature of the solar collectors exceeds that of the tank by a predetermined amount (usually 4 11 C), the differential temperature controller switches the circulating pump on. When the temperature of the solar collectors drops to 2 5 C above the storage temperature, the differential temperature controller stops the pump. Instead of controlling the solar pump directly, the differential temperature controller can operate indirectly through a control relay to operate one or more pumps and possibly perform other control functions, such as the actuation of control valves.

8 Project Type TS1 Typical medium size family home : 300L suitable for 4/5 persons Composition: No.1 Living room No.3 Bed Room No.2 Bath room No.1 Kitchen No.1 Garage

9 The Kit have the following main components: No.5 mq solar heater panel No.1 compact working station No.1 Differential controller Safety devices No.2 3way valves