Solar thermal water heating

Solar Thermal Collector A device designed to receive solar radiation and convert it into thermal energy. Normally, a solar thermal collector includes a frame, glazing and an absorber, together with the appropriate insulation. The heat collected by the solar thermal collector may be used immediately or stored for later use. Typical use is in solar hot water heating systems. Also, see Passive Solar and Concentrated Solar Power (CSP). ... 

There are significant problems for one-step thermal water spHtting. In future nuclear and solar facihties, about 927°C is considered the upper temperature range, which is not sufficient for this reaction. Even if high temperature heat sources were available, materials of constmction would present difficulties. There would also be separation problems (155). 

Solar-thermal technology uses tracking mirrors to concentrate sunlight onto a receiver. In turn, the receiver absorbs solar energy as heat, warming a fluid that then drives a turbine generator. Most solar-thermal plants requite cooling water. 

Direct use of the Sun s heat energy for supplying hot water for homes by special designs of rooftop units (Also known as Solar Thermal)... 

A solar thermal electric-generation unit. A pipe containing synthetic oil is positioned along a mirror-coated trough. Sunlight hitting the mirror is reflected onto the pipe and heats the oil to 370°C. The hot oil is then pumped out and used to convert water to steam in a turbine in an electric power plant. 

A visitor to our solar system is likely to be struck with the abundance of life on Earth, the mild surface temperatures, and the presence of liquid water. Every year our planet receives from the Sun more than enough radiant energy to supply all our energy needs. About 55% of solar radiation is reflected away or used in natural processes. The remaining 45% is converted to thermal motion (heat), most of which escapes as infrared radiation with wavelengths between 4 and 50 pm. 

Passive Solar A system in which solar energy (heat from sunlight) alone is used for the transfer of thermal energy. Heat transfer devices that depend on energy other than solar are not used. A good example is a passive solar water heater on the roof of a building. 

Figure 2.126 illustrates a "passive" solar hot water collector. In this system there is no pump, and the water moves only when somebody in the house asks for hot water by opening a user valve. At that point the cold water inflow through the valve (V4) moves cold water into the collector, transfers the heated water from the solar collector into the solar hot water tank (V5), and sends the hot water from the tank to the user (V7). This system has no controls as such. If there is no usage and the sun is out, it is possible for the water to overheat in the collector or the tank. In that case, thermal expansion will increase the pressure, and when it reaches the settings of the pressure safety valves (PSVs), these safety valves will open, and both water and heat will be relieved and wasted. 

A further special area of propulsion systems is Chemical Thermal Propulsion (CTP). CTP is defined in contrast to STP (solar thermal propulsion) and NTP (nuclear thermal propulsion). In CTP, in a very exothermic chemical reaction in a closed system, heat but no pressure is generated since the products of the reaction are solid or liquid. The heat energy is then transferred to a liquid medium (the propellant) using a heat exchanger, which is responsible for the propulsion of for example, the torpedo. Suitable propellants are e.g. water (the torpedo can suck it in directly from its surroundsings) or H2 or He, due to their very low molecular or atomic masses. The basic principles of CTP can also be used in special heat generators. A good example for a chemical reaction which is suitable for CTP is the reaction of (non-toxic) SF6 (sulfur hexafluoride) with easily liquified lithium (m.p. 180 °C) ... 

Thermochemical splitting of water involves heating water to a high temperature and separating the hydrogen from the equilibrium mixture. Unfortunately the decomposition of water does not proceed until temperatures around 2500 K are reached. This and other thermal routes are discussed in Chapter 5. Solar thermal processes are handicapped by the Carnot efficiency limits. On the other hand, solar photonic processes are limited by fundamental considerations associated with band-gap excitation these have been reviewed in Refs.32 and 33. 

---