The simplest system, which is found in most countries with a latitude of within 20° from the equator, is to have
Figure 3.24 Schematic
diagram of an active direct system.
Source: Government of Western Australia
Storage tank
Cold water inlet
Forced circulation (pumped) system Pump
Figure 3.25 Schematic diagram of an active,
indirect system.
Source: US Department of Energy
Flat-plate collector Hot water to house Cold water supply Antifreeze fluid in
collector loop only
Pump Double-wallheat exchanger Solar storage/ backup water heater Active, Closed Loop Solar Water Heater
Figure 3.26 Thermosiphon solar water heating kit with
integral tank used in the Cirque de Mafate, Réunion.
Source: Wikimedia Commons
Figure 3.27 Thermosiphoning batch system on the roof of
a house in Spain.
Source: Wikimedia Commons
the storage tank directly above the collector on the roof of the house. Water circulates by natural convection because hot water rises and cold water falls. This method is called thermosiphoning. It does not need a pump. The system is known as passive (or ‘compact’) for this reason. The tank can be an integral unit with the collectors or it can be located away from them, inside a roof space to help the water stay hot for longer. It doesn’t matter as long as it is positioned higher than, and close to, the top of the collectors, allowing the heated water to rise by natural convection into the tank.
This type of system is robust and long-lasting. There is little to go wrong. Systems that were installed in Colombia in the 1970s and 1980s are still working. Here, a man called Paolo Lugari developed designs under the company name Las Gaviotas, adapting the best systems from Israel. Colombia’s Banco Central Hipotecario (BCH) stipulated that the system must be operational in cities like Bogotá, where more than 200 days in the year are overcast. The systems came with a 25-year warranty and over 40,000 were installed. For places like Bogotá, where not every day is sunny, solar water heating cannot provide all of the hot water needed throughout the year. In these cases, the solar heated water is stored in a tank, which is connected to another heating source, such as an electric immersion element or a wood burning stove, for example. The sun heats the water up to a certain temperature, and so the auxiliary heating source has less work to do to bring it up to the desired temperature.
Figure 3.28 A thermosiphon system for a hospital in Wasso,
Tanzania. This tank is separate from the collectors, but notice how it is completely above them to enable the thermosiphon effect. It is tall and narrow in order to promote temperature stratification: the hottest water naturally convects to the top ready to be drawn off.
used. A thermostat can be installed to limit the temperature the solar tank reaches for safety purposes. The cold water feed enters the tank at the very bottom. From here it is drawn to the collector to be heated. It then returns to the tank one-third of the way down from the top. The water to be used is drawn from the very top of the tank; thus the water in the tank is heat-stratified. Heat is not being wasted by already heated water being circulated back to the collector, and only the hottest water is being used, which naturally rises to the top of the tank.
Figure 3.29 Schematic diagrams for an indirect, active solar water heating system. As the tank is below the collectors,
a pump must be used. The pump in the building on the left must be more powerful as it has further to pump the water, therefore it is better to position the tank closer to the collectors, as on the right, and let gravity feed the water to the point of use. It is called ‘closed loop’ because the liquid is returned to be heated again, after transferring its heat to the water in the tank.
Source: Energy Saving Trust
Solar thermal collector Primary circuit Second Floor First Floor Ground Floor Water storage tank Pump, actuator controller and other parts Water heater
The closed loop active system is the most commonly used in cooler countries. It consists of the following elements:
The solar collector absorbs the incident solar energy. The heat is conducted 1
into a heat-carrying fluid, usually water with antifreeze – propylene glycol (this is non-toxic, unlike the ethylene glycol that is used in vehicles) – that passes through the collector.
The primary loop circulates the heat down to the heat exchanger coil in the storage 2
tank and back up. The water in the tank absorbs the heat from the heat exchanger coil and the cooled fluid returns to the collector to start the process again. The heated water is stored in an insulated tank.
3
A controller controls the pump to only come on when the collector is hotter 4
than the tank. This is to prevent the pump from circulating the heat from the tank back up to the collector at night. A one-way valve also prevents it thermosiphoning back up with the same result.
Auxiliary heating, such as a gas or biomass boiler or electric coil, can be 5
used to top up the heat in the tank if required.
In some countries, there is a choice between two common primary system layouts: fully-filled and drainback.
Fully-filled systems
A sealed system must include an expansion vessel that takes up the expansion of the fluid when it gets hot. The pipework may undulate as long as air can be released at high points and fluid drained at low points. Undesirable heat loss during the night is prevented by a one-way check valve positioned after the pump and before the expansion vessel and the solar collector.
Figure 3.30 Direct system design for areas where freezing
is rare. The controller compares readings from the two temperature sensors and switches on the pump when the collector water is hotter than the return from the tank. The water heated is the water used.
Temperature sensor Temperature sensor Collectors Pump
Cold water supply
Hot water supply
Electronic controller
Drainback systems
With a drainback system, air is present in the circuit and any expansion of the fluid is contained by the drainback vessel with an air pocket. Undesirable heat Figure 3.31a An indirect, sealed solar thermal system
layout. It includes an expansion vessel to contain any expansion of the fluid when it gets hot.
Figure 3.32 A drainback,
sealed solar thermal system layout. Air is present in this sealed circuit, so when not being pumped the water falls back into the drainback vessel.
Figure 3.31b The behaviour of the expansion tank in an
indirect, sealed solar thermal system. The vessel is not insulated because, if hot fluid expands into it, it is desirable for it to cool down as quickly as possible.
connection to closed loop
loss is prevented by switching the pump off, which means that the fluid drains back from the collector into the drainback vessel positioned after the collector in the circuit. It requires a more powerful pump than a fully-filled system.
A drainback system has two main advantages over a fully-filled system: There is no risk of overheating because when the collector gets too hot the 1
pump switches off and the fluid drains back out of the collectors and into the drainback vessel.
There is no risk of freezing because, again, the pump is off when the 2
collector gets too cold.
Repeated overheating of the glycol fluid causes it to break down chemically and produce a sludge which can quickly block the system. Propane fluid might be used instead of glycol, which might corrode plastic gaskets over time.