The following components make up a complete solar thermal hot water system.
Solar thermal collectors
Solar thermal collectors are responsible for converting the available solar energy into usable hot water and are a key component of any solar hot water system. There are different types of collectors such as flat plate, evacuated tube, Integrated Storage Collector (ICS or batch collectors), parabolic collectors etc. Out of which popular and best available technologies for building integrated hot water systems are flat plate collectors and evacuated tube collectors.
| Flat plate collectors | Evacuated tube collectors |
|---|---|
| This type of collectors consist of flat absorber plate made up of stable heat absorbing polymers or metals like steel, aluminium or copper with inbuilt flow tubes for heat exchanging medium (typically air, water or antifreeze) to circulate (see figure below). The absorber plate collector is backed with high insulating material to avoid conduction heat loss and housed in a metal framing on all sides. It is covered with a glass cover on the top of flat plate to provide a weather tight enclosure and prevent convection and radiation heat loss (greenhouse effect). Flat plate collectors are suitable are suitable for residential applications. Flat plate collectors are find applications in both commercial and residential buildings to meet the hot water demand. They can rise water temperature to approximately 70 °C. The flat plate is made up of copper, aluminium or steel. The flow tubes are made of copper and are fused with the flat plate tightly. When prone to freezing a mixture of water and glycol is used as heat exchange medium that runs in the flow tubes within the collector plate. | The evacuated tube collector (see figure below) consists of closed glass tubes, a diffusion plate, a manifold header, and a supporting frame. Individual tube collector consists of an elongated glass outer tube and a black metal inner tube. The black metal inner tube contains a heat pipe made of absorber metal like copper. The heat transfer medium in heat pipe collector incorporates a special working fluid, which begins to vaporize even at low temperatures. The steam rises in the individual heat pipes and warms up the carrier fluid (typically water or a mixture of water and glycol) in the manifold header. The condensed liquid then flows back into the base of the heat pipe and the cycle starts again. Evacuated tubes can be ideal for residential buildings located in high latitudes and cold climates with low level diffuse sunlight. They are superior to flat plate collectors and can produce water/steam at temperatures as high as 175 °C and could be used for DHW, space heating as well as heat source for solar thermal systems which require hot water temperature in the range of 55 to 180 °C. |
Heat transfer mechanism
Solar collector is connected to a hot water storage tank. The heat is transferred between the solar collector and water in the tank either by direct or indirect methods also characterized as open loop or closed loop. An open loop (direct) system circulates the household (potable) water through the collector while a close loop (indirect) system uses a heat transfer fluid (e.g., water or diluted antifreeze) to accumulate heat and a heat exchanger to transfer the heat to the household water. Flat plate collectors can employ both direct and indirect heat transfer whereas advanced technologies like evacuated tubes work only with indirect heat transfer technology.
| Direct heat transfer (open loop) | Indirect heat transfer (closed loop) |
|---|---|
| Direct heat transfer consists of water as the medium of heat exchange and water is directly circulated between the tank and collector. Though this system is efficient it has two disadvantages over indirect methods. One is the problems with freezing, if water is let stagnant in the collector and the other is due to new water entering the flow tubes all the time it is prone to furring | Indirect heat transfer technology consists of a heat exchange medium between solar collector and hot water. The main advantage of this system is that it can employ a wide range of heat transfer fluids which are not prone to freezing. However, this system more often than not needs auxiliary pumps to drive the heat transfer fluid. |
Circulation systems
Water circulation systems in solar thermal hot water systems occur in two ways. I.e., by passive means and active means. Passive circulation systems use thermosyphon effect or batch systems where as active methods use electrical pumps to circulate water.
| Passive/Thermo siphon system | Active system |
|---|---|
| Passive circulation involves the principle of “thermo siphon” based on the principle of natural convection. In this system the storage tank is always placed on the top of the collector. The water in the storage tank is flown into the collector gets heated up and is flown back into the top of the tank pushing the cold water at the bottom into the collector and the cycle repeats till an equilibrium is achieved. | Active circulation is done by the use of a pump to circulate the heat exchange fluid between the collec-tor and the storage tank. This is typically done when the heat exchange method is indirect. Pump automat-ically shuts off when the fluid temperature in the col-lector is less than the hot water temperature. It is ideal for hybrid systems in cold regions. |
| Advantages: • Passive systems are cost effective systems • Works excellent in locations with high solar radia-tion • Generally more reliable, easy to maintain and longer lasting because of no electric/moving parts |
Advantages: • Prevents the fluid from freezing • Effective distribution of heat • Storage tank can be located within the insulated environment |
| Disadvantages: • Not suitable to use with hybrid systems with connection to central heating systems • Passive systems are only suitable for anti-freeze climates because the outdoor pipes could freeze in sever cold weather |
Disadvantages: • Complicated systems which needs to be strictly controlled • Auxiliary energy is needed for pump operation • More expensive than passive systems |
Storage tank
The main challenge with the solar thermal systems is that the energy produced might not be used im-mediately but should be kept for later use. Hot water produces can be stored in storage tanks for later use. Stratified storage tanks are considered the best available technology. Stratified water tanks have special flow controller pumps and pipe connections. The flow regulators control the velocity of outgoing hot water and incoming cold water so that the water in the tank is not disturbed and the stratification is maintained. Typically in a tightly stratified tank the temperature of water at the top of the tank reaches about 95 °C while the water temperature at the bottom of the tank remains at 20-40 °C. Since still water is relatively bad conductor of heat the heat loss is minimum in an undisturbed stratified tank.
| Passive systems | Active systems |
|---|---|
| Storage tanks in the case of passive systems are typically located on top of the thermal collector. The water is heated through passive convection and stratification of hot and cold water is significant. This makes it important for the tank to be located just on top of the collectors and eventually outside the building’s thermal environment. The tank should be insulated tightly to prevent any loss of heat. This configuration works well in hot climate where the requirement of hot water is limited and the external temperatures do not fall very low after sunset. | Active systems need pumps to circulate water/heat exchange fluid. This makes it convenient to remotely locate the hot water tank, typically within the building’s thermal environment. Although the tank is located within the building the tank needs to be insulated well. This makes it suitable to integrate the tank with both solar thermal system as well as auxiliary backup heating system. |
| Standalone hot water system | Hybrid hot water and space heating systems |
|---|---|
| Solar thermal hot water systems that cater to only Domestic Hot Water systems are located in climatic regions with no or less requirement for space heating and continuous demand for hot water for domestic usage like washing, bathing etc. These kind of systems are popular in tropical countries and southern European counties. | Solar thermal hot water systems in cold regions could cater fully or partially to both domestic hot water as well as space heating requirements. Hybrid systems typically consist of a storage tank and are powered by auxiliary heat source such as a natural gas or an electric boiler. Solar hot water system however, has limitations on heading demand and the heat distribution systems when it is combined with space heating system. It is most often successfully integrated with space heating in highly insulated buildings with heat distribution through passive means such as radiation. Typically forced convection systems need much higher water temperature than that can be produced and met by conventional building integrated solar hot water systems for longer periods. |
The performance efficiency of a typical solar collector is determined for different values of solar irradiances E and a variety of temperature difference s between collector TC and ambient ait TA. The commonly used empirical equation for the collector efficiency etaC is:
etaC = eta0-(a1.(TC-TA)+a2.(TC-TA)2)/E
The three parameters eta0, a1 and a2 are constants for individual collectors and are estimated by collector test measurements. eta0 is also referred to as optical efficiency. Optical efficiency depends on the factors such as the transmission efficiency of the glass covering and the amount of solar radiation being absorbed. The following figure shows typical collector efficiencies for a flat-plate collector. The efficiency of a thermal collector for a given value of irradiance decreases as the temperature difference between collector and ambient increases. The efficiency decreases at a faster rate when the solar irradiance value is low. E.g., at a solar irradiance value of 200 W/m2, the output in the sample collector becomes zero even at a relatively lower temperature difference of about 40 °C. The efficiency for different solar thermal collectors can thus be determined using similar graphs.
Solar thermal systems trap and use the solar heat freely available. Solar collectors form the core element of any solar thermal heating system. Heat absorption, retention and transmission to the heat exchange medium are important features of any solar collector. For installations in the buildings for domestic hot water purpose small-scale building mountable solar collectors are available. They typically include flat plate collectors, evacuated tube collectors, parabolic collectors among others. Efficiency parameters of different types of typical building integrated solar collectors can be summarized in the following table.
| Collector type | Life time | Hot water/steam temperature °C | Solar radiation to useful heat - Efficiency % |
|---|---|---|---|
| Flat plate | 20 years | 70 °C | 80% |
| Evacuated tubes | 25-30 years | 55 to 180 °C | 90% |
| ICS | 55-65 °C | 90% |
Requirement of the quantity of domestic hot water is highly dependent on the climate. Hot water requirement in buildings in cold climate varies significantly from that of buildings in hot climate. In addition, usage of domestic hot water is usually a function of time. Therefore, it is important to design a system that will ensure sufficient storage and supply of hot water whenever required. Amount of hot water required and at specific intervals give an estimate of the daily hot water requirement. A suitable collector area and tank size is then designed to meet this load.
Since hot water collectors are typically located on the roof of the buildings their size is also limited to the carrying capacity of the roof. Solar collector area is an indicator of how large the system is and also decides other parameters like storage tank capacity, auxiliary heating required etc. On a thumb rule basis it is estimated that solar collector area in residential buildings is in the range of 3.5 m2 – 8 m2 and do not exceed more than 12 m2 or 50 % of the roof area. Application of solar hot water systems in multi-family dwellings and high rise buildings is constrained by the fact that less collector area is available per dwelling unit due to less roof area available. In such cases, either few dwelling units could benefit from such a facility or solar hot water system could support a large central hot water plant.
Solar thermal systems are typically installed on the roof of buildings. As discussed in the earlier section an appropriate orientation and angle of tilt has to be worked out to optimize the potential of the system. In existing buildings with sloping roofs orientating is largely predetermined determined and the angle of tilt has to be worked out appropriately. In the case of buildings with flat roofs both orientation and angle of tilt can be optimized. Collectors when placed on sloping roofs typically take the slope of the roof as their tilt angle.
When more than one solar thermal collector is to be placed then it has to be ensured that there is now overshadowing between the collectors due to each other.
Due to their location on the outside of the building solar thermal systems are subject to external elements such as snow, wind etc. Of these wind is of prime concern and the system must be duly anchored for the winds loads for the location. During the installation of solar thermal systems in existing buildings it has to be ensured that the building can take structural load of the system and definite anchoring mechanism. Newly designed buildings should take care of this aspect in the early stages of design itself. The spacing between the solar thermal collectors should also take human movement around the collectors for service purpose into consideration.
Mechanical systems integration
The whole solar thermal system would incorporate mechanical and other ancillary components for the system to generate and distribute hot water. Key components include
System maintenance
The system once made operational should regularly be subjected to operation and maintenance (O&M) checks. The thermal collectors should be checked for any accumulated dirt or dust, shadow casting elements such as vegetation growth etc. O&M should ensure smooth operation of the system, detect faults in the piping system, pump operations, valves and should rectify the same. All metal components in contact with water such as pipes and heat exchangers should be checked for furring and corrosion. The mounting component should be checked for any loose connections. The energy storage tank should be checked for sediments and any found needs to be removed. The controls should be checked for correct calibration and consistent reporting.
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