Compact Solar Water Heating Systems (Passive Systems)
A compact system also known as a monobloc has a tank for the heated water and a solar collector mounted on the same chasis. Typically these systems will function by natural convection (thermosiphon) or heatpipes to transfer the heat energy from the collector to the tank.
A special type of compact 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 pressurised and usually depend on gravity flow to deliver their water. They are simple, efficient and less costly than intense plate and tube collectors but only suitable in moderate climates with good sunshine. A step up from the ICS is the Convection Heat Storage Unit. These are plate type intense collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from heater to tank. Neither pumps or electricity are used. It is more efficient than an ICS as the intense collector heats a small(er) amount of water that is constantly rising to the tank. It can be used in areas with less sunshine than the ICS.
A integrated collector storage system
Direct ('open loop') compact systems, if made of metals are not suitable for cold climates. At night the remaining water can freeze and damage the panels, and the storage tank is exposed to the outdoor temperatures that will cause excessive heat losses on cold days. Some compact systems have a primary circuit. The primary circuit includes the collectors and the external part of the tank. Instead of water, a non-toxic antifreeze is used. When this liquid is heated up, it flows to the external part of the tank and transfers the heat to the water placed inside. ('closed loop'). However, direct ('open loop') systems are slightly cheaper and more efficient.
A compact system can save up to 4.5 tonnes annually of greenhouse gas emissions. In order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in six years, and achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems do not use any external source for power (as water flows due to thermosiphon principle).
Flat solar thermal collectors are usually used, but compact systems using vacuum tube collectors are available on the market. These generally give a higher heat yield per square meter in colder climates but cost more than flat plate collector systems.
A passive open loop system
Pumped Solar Water Heating Systems (Active Systems)
How the solar water heating system is pumped and controlled determines whether it is a zero carbon or a low carbon system. Low carbon systems principally use electricity to circulate the fluid through the collector. The use of electricity typically reduces the carbon savings of a system by 10% to 20%.
Conventional low carbon system designs use a mains powered circulation pump whenever the hot water tank is positioned below the solar panels. Most systems in northern Europe are of this type. The storage tank is placed inside the building, and thus requires a controller that measures when the water is hotter in the panels than in the tank. The system also requires a pump for transferring the fluid between the parts.
The electronic controllers used by some systems permit a wide range of functionality such as measurement of the energy produced; more sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display or transfer of error messages or alarms; remote display panels; and remote or local datalogging.
Newer zero carbon solar water heating systems are powered by solar electric (photovoltaic or PV) pumps. These typically use a 5-20W PV panel which faces in the same direction as the main solar heating panel and a small, low power diaphragm pump or centrifugal pump to circulate the water.
The most commonly used solar collector is the insulated glazed flat panel. Less expensive panels, like polypropylene panels (for swimming pools) or higher-performing ones like evacuated tube collectors, are sometimes used.
Schematic of an active solar heating system
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For More Information: Solar Collectors: K-12 Projects, Experiments & Background Information