Heat Transfer Fluids for Solar Water Heating Systems
Heat-transfer fluids carry heat through solar collectors and a heat exchanger to the heat storage tanks in solar water heating systems. A heat transfer fluid is a gas or liquid that takes part in heat transfer by serving as an intermediary in cooling on one side of a process, transporting and storing thermal energy, and heating on another side of a process. Heat transfer fluids are used in countless applications and industrial processes requiring heating or cooling, typically in a closed circuit and in continuous cycles.
Cooling water for instance cools an engine, while heating water in a hydronic heating system heats the radiator in a room. Water is the most common heat transfer fluid because of its economy, high heat capacity and favorable transport properties. However, the useful temperature range is restricted by freezing below 0 °C and boiling at elevated temperatures depending on the system pressure.
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The heat transfer fluids are the special fluids that are specifically manufactured for the transmission of heat from one source to the other. For example, in the solar collector, the heat transfer fluid is heated up in the collector due to the sun and then is passed through the heat exchanger that warms up a separate tank of water that results in providing hot water for domestic use. The heat exchanger fluid is estimated to have a high demand in the coming years due to the increasing applications in various end-use industries.
There is a strong motivation to explore the possibility of harnessing solar thermal energy around the world, especially in locations with temperate weather. This review discusses the current status of heat transfer fluid, which is one of the critical components for storing and transferring thermal energy in concentrating solar power systems. Various types of heat transfer fluids including air, water/steam, thermal oils, organic fluids, molten-salts and liquid metals are reviewed in detail, particularly regarding the melting temperature, thermal stability limit and corrosion issues. Stainless steels and nickel based alloys are the typical piping and container materials for heat transfer fluids. Stability of the stainless steels and alloys while in contact with heat transfer fluids is very important for the longevity of concentrating solar power systems. Corrosion properties of stainless steels and nickel based alloys in different heat transfer fluids are discussed in terms of corrosion rates.
Types of Heat-Transfer Fluids
The following are some of the most commonly used heat-transfer fluids and their properties. Consult a solar heating professional or the local authority having jurisdiction to determine the requirements for heat transfer fluid in solar water heating systems in your area.
- Air Air will not freeze or boil, and is non-corrosive. However, it has a very low heat capacity, and tends to leak out of collectors, ducts, and dampers.
- Water
Water is nontoxic and inexpensive. With a high specific heat, and a very low viscosity, it's easy to pump. Unfortunately, water has a relatively low boiling point and a high freezing point. It can also be corrosive if the pH (acidity/alkalinity level) is not maintained at a neutral level. Water with a high mineral content (i.e., "hard" water) can cause mineral deposits to form in collector tubing and system plumbing. - Glycol/water mixtures
Glycol/water mixtures have a 50/50 or 60/40 glycol-to-water ratio. Ethylene and propylene glycol are "antifreezes." These mixtures provide effective freeze protection as long as the proper antifreeze concentration is maintained. Antifreeze fluids degrade over time and normally should be changed every 3–5 years. These types of systems are pressurized, and should only be serviced by a qualified solar heating professional. - Refrigerants/phase change fluids
These are commonly used as the heat transfer fluid in refrigerators, air conditioners, and heat pumps. They generally have a low boiling point and a high heat capacity. This enables a small amount of the refrigerant to transfer a large amount of heat very efficiently.