Understanding basic Solar water energy systems

Introduction to solar energy collection
Systems - thermo siphon - forced circulation
Using solar energy - direct circulation - closed loop
Covers
Case

Note: references to compass bearings and installation directions apply to the northern hemisphere - in particular the UK. Local variations will need to be applied to other locations/latitudes.

Introduction

No matter how well a solar energy installation is designed, it is unlikely that solar energy alone will be able to supply all the heat energy demands of a building at the latitude of the UK.

Apart from solar energy collectors, a well designed, solar efficient building will include features which take advantage of natural solar heating and the conservation of the heat generated.

These features will include:;

1. Careful orientation of the building to enable the maximum use of natural solar energy. The rack and shape of the roof to suit the requirements of an externally mounted solar collector.
2. Small double (or triple) glazed window on the north facing wall and relatively large areas of double glazed windows on the southern wall (or even a conservatory along the southern side). These features reduce the heat loss from the northern wall (the cold side of the building) while maximising the natural solar heating effect on the southern side.
3. A large overhanging eve on the southern side of the building prevents excessive heating in summer when the sun is high in the sky by casting a shadow on the southern windows. In the winter, when the sun is low, the eve will not cast any shadow and maximum natural solar heating will be achieved.
4. Good insulation throughout the building, including roof, floor and wall.

Passive solar heating is the optimum starting point for maximum solar efficiency, but it can only be fully exploited where a new building is being designed.

When using solar energy in an existing building the use of an active solar energy system is necessary. The solar energy being collected by heating a transfer fluid in an external collector which is then used for internal space heating and/or domestic water heating purposes. By positioning the collector in an unshaded position and at a suitable angle to the horizon, the transfer fluid will be heated by the sun falling on it. The heated fluid then moves, either by either natural circulation (thermo siphon) or pumped circulation (forced circulation), into the building through pipes or ducts where it can be used for the required application.

Various designs of systems are possible using either water or air (or other special fluids) as the transfer medium, and using either natural thermo siphon circulation or forced circulation. Details are given below for water systems using thermo siphon and forced circulations.

Solar heating systems and components

As the solar energy is being radiated from the sun, it is necessary to collect the energy externally and then move it into the building where it can be used. To achieve this, water is circulated around the system to absorb the energy in an externally mounted collector and move into the building. There are two basic methods of achieve this circulation within a system; natural thermal siphon circulation or forced pumped circulation.

Thermo siphon solar water heaters

In thermo siphon circulation systems, the water is circulated by natural convection resulting from the difference in water density due to temperature difference of the water in different parts of the system. Just as hot air rises, hot water (at the temperatures we are interested in) also rises above cooler water.

When the water in the collector becomes warmer than the water in the storage tank, it will rise to the tank thus drawing cooler water from the bottom of the tank into the bottom of the collector (left). The circulation in thermo siphon systems is thus self starting and self controlled as it only occurs when the water in the collector is warmer than the water in the higher storage tank. In this type of installation, the solar collector must be mounted lower than the storage tank.

Advantages:

1. The installation is straight forward to design and build, and does not require any controls or conventional energy input to circulate the water, thus both the installation and running costs are minimal.

Disadvantages:

1. The circulation of the water within the system can be rather sluggish, especially where there is a small temperature difference between the collector fluid and storage tank. This reduces the amount of energy which can be usefully collected.
2. The layout of the system is fairly critical as the collector must be positioned below the storage tank and all interconnecting pipes must slope up from the collector to the storage tank. In practice this prevents the collector being mounted on the roof, and the necessarily low collector position can reduce performance due to shadows and obstructions caused by nearby buildings etc.

Design and construction considerations for a thermo siphon system

With a thermo siphon circulation, the collector must be mounted below the tank storing the heated water.

To ensure efficient thermal siphon circulation, the internal size of the interconnecting plumbing must be relatively large (28mm diameter minimum), this enables free circulation of the fluid. This requirement does increase the volume of fluid in the system which is undesirable but is unavoidable. All pipes must slope upwards from the collector to the storage tank to allow the free flow of water and to prevent any air locks.

When positioning the collector, the top of the collector must be at least 600mm below the bottom of the storage tank to ensure that reverse circulation does not occur when the temperature of the water in the collector is below the temperature of the water in the storage tank.

The pipe carrying the heated water from the collector should enter the storage tank at a point where between 25% and 33% of the tanks capacity is above the entry point.

The heated water should be drawn from the top of the storage tank to ensure that the cooler water remains in the tank.

Design and construction considerations for a forced pumped circulation solar water heater

Forced systems which use a mechanical pump to circulate the water, are more flexible in layout and offer improved efficiency especially at low levels of solar energy. As the whole principle behind the use of solar energy is to reduce/replace conventional sources of energy, the use of a circulating pump may appear to be undesirable. However the increase in the amount of solar energy which can be extracted should more than compensate for the conventional energy required to control the system.

Forced pumped circulation is more flexible than thermo siphon as the collector can be mounted either below or above the storage tank (above is usual to avoid ground level shadows). To ensure that air locks do not occur in the collector mounted above the storage tank, an air vent must be connected to the outlet from the collector. The top of the vent must be above the natural water level of the complete system and should be arranged so that any flow back up the vent is directed into the cold water header tank.

The positioning of the collector in the system is limited by the natural water level of the system which is determined by the level of the ball cock in the cold water header tank. It is necessary that the top of the collector is below this level to ensure that air is not drawn into the system as this would cause both operational and corrosion problems. The cold water feed tank should include an overflow pipe vented to the outside of the building so that an overflow does not cause damage within the building.

The use of a pump does complicate both the design and construction, not only by the pumps connection into the plumbing but also by the requirement for an effective temperature controlled switching unit. The control unit must monitor the water temperature at two points, in the storage tank just above the level of the inlet connection and in the top of the collector. The control unit must only switch on the pump when the temperature of the water in the collector is above the temperature of the water in the storage tank.

Advantages:

1. The design is very flexible as the positional relationship between each component is not critical. The solar collector can be positioned above the level of the hot water storage tank thus allowing the collector to be mounted on a south-facing roof away from lower level shadows.
2. The amount of solar energy which can be collected by a pumped system is higher than thermal siphon systems especially under low solar energy levels.

Disadvantages:

1. The inclusion of a pump and control unit complicate the design and construction of the installation, and increases the initial costs. (The running costs of the system should be more than offset by the increase in efficiency).
2. An electricity supply is required to operate the pump and control unit.

Using the collected solar energy

Having heated the water in the collector, it is necessary to store it for later use when it is required. As the volume of water being heated while it flows through the collector is relatively small, the water must be continually recirculated (by either pump or thermo-siphon) around the circuit to obtain a useful volume of water at a useful temperature.

As the temperature of the water from the collector may not be sufficient for direct use, it is recommended that an additional tank be installed to preheat the water entering a conventional hot water tank.

The use of a combined hot water cylinder is possible and more compact but the efficiency of the system will be far less than with a preheat tank. The water in the combined storage tank will need to be at a relatively high temperature to be useful, and the solar heated water will not contribute any energy while the water from the collector is below that of the water in the tank.

The heated water from the collector can be used in one of two ways to heat the preheat storage tank:

• the heated water from the collector can feed directly into the preheat tank (direct)
• the heated water can be passed through a heat exchanger in the preheat tank (closed loop) to heat the water in the tank.

Direct use of solar heated water:

The water in the preheat tank and the collector form the circuit and the heated water is stored in the tank. The heated water is then drawn off from the top of the storage tank as and when it is required.

Advantage:

• Easier to design and install than a closed loop installation.

Disadvantage:

• Water from the collector mixes with the water in the preheat tank so inhibitors/antifreeze cannot be added. In the UK, this means that the collector will need to be isolated and drained during periods of frost. Stop cocks will need to be designed into the system so the collector can be isolated and drained while still allowing the rest of the hot water system to be used.

Closed loop use of solar heated water:

The water circulates around a closed loop formed by the absorber, the heat exchange mounted within the storage tank and the interconnecting pipework. The heat collected at the absorber is transferred to the water in the storage tank as the heated water from the collector is passed through the heat exchange within the tank. The water in the closed loop does not mix with that in the tank. A separate cold water header tank is needed to supply the closed loop to cope with expansion and to replace evaporation loses. The header tank can be fed from the existing cold water header tank, or if it needs to be positioned above the existing tank, it can be fed directly from the cold water mains.

Care must be taken in the design to ensure that the closed loop is completely self-contained so that any inhibitor/ antifreeze used cannot be drawn back into the main water circuit.

Advantage:

• Water from the collector does not mix with water in preheat tank - thus allowing suitable inhibitors/antifreeze to be added to the collector water circuit to prevent corrosion and possible damage due to frost.

Disadvantage:

• More complicated to design and install.

The transparent covers on a solar collector

The transparent covers are an important feature of all collector designs as the type and number of covers can have a major affect on the overall performance. When solar energy hits a sheet of glass at right angles, as much as 90% of it is directly transmitted through the glass, the remainder is lost through reflection and absorption by the glass. The reason for this high efficiency is that the wavelength range of incoming solar energy is mostly (about 98%) less than 3µm, and the transmittance of glass is very good for wavelengths under 3 µm. Even the energy absorbed by the glass is not all wasted as it does raise the temperature of the glass which then re-radiates the energy from both of it's surfaces, so some of it heats the inside of the case. As the directly transmitted solar energy reaches the absorber, it is absorbed and raises the temperature of the absorber, this in turn heats the water. As the temperature of the absorber rises it starts to re-radiate energy, however the majority of this re-radiated energy (over 99%) is at a wavelength over 3µm. The transmittance of glass for energy over 3 µm is practically zero, so the majority of this re-radiated energy does not pass back to the atmosphere but is contained within the collector.

Plastics generally have a higher transmittance than glass for all wavelengths. While this would allow more of the direct solar energy to pass through the cover to the absorber, it also means that more of the reflected energy will be lost by transmission back through the front cover into the atmosphere. Most plastics are therefore not suitable as an outer glazing cover but can be beneficially used as an inner cover on multi-glazed units.

Some energy will be lost from the front cover by radiation, convection and external air movements cooling the outer surface. These losses increase as the temperature difference between the inside of the collector and the outside air increases and also with increased external air movements.

Losses from the front cover can be reduced by additional transparent covers that will reduce air movements within the collector and the amount of re-radiated energy that reaches the outer cover. However with each additional cover, the amount of solar energy reaching the absorber is reduced by the reflection and absorption losses of each additional cover. As an example of such losses, the transmittance losses for solar energy with an incidence angles up to 35° to the collector are approximately 10% for single glazing, 18% for double glazing and 25% for triple glazing (where all covers are glass). The use of plastics for the inner covers will reduce the loss figures for double and triple glazed units.

The distance between covers and between the inner cover and the absorber plate is not very critical, a spacing of between 10 and 25 mm is suggested.

The solar collector case

The design of the case is not really critical as it's only real purposes are to protect the absorber from the rigors of the weather, to secure the complete collector and to reduce back and side heat losses. The case should be insulated internally behind the absorber to avoid loss of useful heat.

The materials used in the construction of the case must stand up to both the hottest, driest and the coldest, wettest conditions, so must be robust and well protected by paint etc. If timber is used for the case, it should be pressure treated with a good quality timber preservative.

The case must also provide suitable anchorage points for securing the collector to the exterior of the building.

Introduction to solar energy collection
Systems - thermo siphon - forced circulation
Using solar energy - direct circulation - closed loop
Covers
Case