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Even if high quality components are used, the SWHS must be designed well to achieve a good performance and don’t lose money by inefficiencies
The experiences of the SoPro project indicate, that most SWHS for industrial processes (SHIP) in India are operating with an acceptable, but lower than possible efficiency. By improving the design of new systems, the performance and the value for money could be significantly increased
Therefore, it is recommended to develop a design guideline for SHIP to support manufacturers, planners and installers by optimising their SHIP designs. Indian and international experiences on system design should be used to develop recommendations for Indian SHIP applications and conditions. In addition, a planning software for SHIP should be developed to ease and professionalise the design process for manufacturers, planners and installers.
In meantime, general recommendations on the design of SWHS are made in the following based on SoPro India experiences.
1. Keep the hydraulic design of the SHIP system simple
1. Thermosiphon SWHS should be preferred, if:
2. Forced circulation SWHS should be used in any other case.Forced circulation SWHS need a pump, temperature sensors and a controller, which causes additional costs. The system works well, if the sensors are installed and the control parameter are set correctly.
Both collector types are proven to work well (about 90% of collectors in Europe are FPCs, about 90% of collectors in China are ETCs).
1. Choose the collector type by evaluating following criteria’s:
1. The size of the collector field must be adapted to the heat demand of the process and to the available space for installation.1. The amount of heat, which is demanded by the process and could be delivered by solar heat, should be calculated and compared with the possible expected solar yield delivered to process (20% - 40% of the solar irradiation) of the maximum size of the collector field (according the available roof space or ground space, if this kind of installation is possible). If the possible expected solar yield is much lower than the heat demand (which is usually the case for applications in industry), the size of the collector field is not depending on energy aspects, but either limited by the available space for installation, or by limitations on investment costs. If the possible expected solar yield is higher than 50% of the heat demand, which could be delivered by solar heat, detailed yield calculations should be made taking into account the variations of the demand from day to day and during the year, the variations of the solar irradiation during the year, and variations of storage tank size to identify the optimal size of the collector field.
2. The hydraulic of the collector field with several parallel collector rows of serial connected collectors must be optimised taking into account the pressure drop per row and the temperature to be reached. An equal flow of the heat transfer fluid through the entire field must be assured.2. The number of parallel rows should be limited, because the risk of an unequal flow of the heat transfer fluid through the collector field is higher at a higher number of parallel rows. To assure a similar pressure drop in each parallel row, all rows should have the same number of serial connected collectors. Since the power of the pump needed is increasing with the pressure drop, the number of serial connected collectors in one row is limited. In addition, the distribution of the collectors on the roof should be taken into account as well to limit the pipes needed.
1. The volume of the storage tank should follow the philosophy of having minimum storage time of solar hot water. It is defined by the maximum time with no heat demand by the process, but also by the amount of solar energy generated at that time and the demand, which happens after this period.1. In India, the volume of the storage tank is often dimensioned according to the water volume, which is expected to be solar heated, e.g. a 5000 LPD System (5000 Litre of water is expected to be heated by the SWHS to e.g. 60°C per day) often has a storage tank volume of 5000 Litre. However, if the solar heat is used continuously by the process, e.g. if the feeding water of a steam boiler is pre heated or the solar heated water reaches the temperature of 60°C for the washing process already at noon, the solar hot water volume, which cannot be used directly and must be stored until the next time of demand is much smaller. Sometimes the storage tank volume should be higher than the water demand of one day, e.g. if the heat demand is varying from day to day or if the generation varies a lot from day to day.
2. A cylindrical tank can be installed horizontal or vertical. The vertical installation should be chosen, if a stratification of the water temperature in the tank is desired.2. If the system design doesn’t requires a stratification of the temperature in the storage tank, the horizontal installation of the storage tank leads to same performance and is often simpler to realize. If stratification of the temperature is desired, the charging and discharging devices must be designed accordingly.
1. If the solar heated water is used in the process (open loop)
2. If the solar system is closed-loop (and the storage tank always full)
1. Manual operation should be only chosen, if the demand is very constant and the manual operation is very reliable.
2. Automatic control should be the standard for forced circulated SHIP systems.2. If an automatic control is used, the temperature sensors must be placed carefully (directly at the outlet of the collector and at the bottom of the storage tank), the control parameters must be set correctly and a hysteresis must be implemented in the controller (pump switched-on at a higher temperature difference between the collector and the bottom-of-the-tank temperature and switched-off at a lower temperature difference between the same two sensors).
1. Non-pressurised SWHS should be preferred1. Non-pressurised means no pressure in addition to the static pressure. Non-pressurised systems are simpler and cheaper, since they don’t have to withstand a high pressure and components like membrane expansion vessels can be avoided. If water is used as heat transfer fluid, losses of the water by evaporation through the open make-up water tank and the air valves are unproblematic (in contradiction to the case, where anti-freezing heat transfer fluid is used).
2. Pressurised SWHS could be advantageous for freeze-protected systems and systems with a large or complex hydraulic network.2. If freeze protection is required, a glycol-water mixture can be used, however the concentration of the mixture should stay at a specific level. Since losses of the fluid and uncontrolled refill with water in open, non-pressurised systems would dilute the mixture, pressurised systems avoids this problem. Also if the SHIP hydraulic network is of bigger dimension (longer pipe distances) and winding, a pressurised system could be the better solution, to avoid bottlenecks created by air in the pipes.
1. If there is no risk of freezing, water is the cheapest and best heat transfer fluid. Appropriate action against corrosion of the components, depending on the corrosiveness of the water must be taken.
2. If hot air is used in the process (e.g. for drying), the use of solar air collector systems should be considered.
3. If the temperature can fall below 0°C, a freeze protection is necessary by using a water-glycol-mixture as heat transfer fluid.
1. It is not necessary to install a second pump in parallel just for the case that the pump could break down.1. Today, water pumps do have a long lifetime and can be easily replaced, if they broke-down. To install a second pump in parallel to be able to switch to it, in case of a problem with the first pump occurs, causes unnecessary additional costs and needs additional effort to operate the pump regularly.
1. he most relevant temperatures of the system should be displayed: collector outlet temperature, temperature at the top and at the bottom of the storage tank, and temperature of the water delivered to the process.1. Based on these temperatures and the knowledge, that the system is operating (e.g. pumps are running), the operator can evaluate roughly, if the system is operating well.
2. A heat meter should be installed to measure the solar heat delivered to the process.2. If the solar heat delivered to the process is measured, the owner and operator can assess, if the expected solar yield is getting delivered and inform the solar company, if improvements are desired. The fuel savings can be calculated by dividing the solar yield by the efficiency of the boiler, which allows the investor to check the profitability of the SWHS.