Design recommendations for SWHS for industrial processes (SHIP)

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.

General hydraulic design principles

1. Keep the hydraulic design of the SHIP system simple

2. Minimize the number of heat exchangers in the system No heat exchanger is necessary, if the water consumed in the process can be used as heat transfer fluid, the water supply system doesn’t stand under pressure and the requirements on water purity of the process and corrosion protection allows it.
2. If heat exchangers are used, their number should be minimized, since each heat exchanger causes losses.
Simple hydraulic design

3. Use rather internal (in the storage tank) than external heat exchangers An external plate heat exchanger requires an additional pump and controller and holds the risk of mismatch between the heat flows on both sides of the heat exchanger.
Simple hydraulic design

4. Don’t split the water storage volume into several tanks, if possible
One big storage tank with the entire water volume has lower heat losses through its surface than two or more separate storage tanks with the same water volume, since the surface-volume-relation is lower. In addition, the risk of unintentional hydraulic effects due to the combination of several storage tanks is reduced if less storage tanks are used.
External plate heat exchanger
One big storage tank instead of several smaller ones
Internal heat exchanger

5. Deliver the solar heat directly to the process for use or minimize the storage time if it has to be stored Storing heat causes losses, this means that it is more efficient to use the solar heat directly after generation, if there is a demand. There is no advantage of heating up water by solar energy on one day and store it for use on the day after. If the solar heat is used to preheat water, e.g. as feeding water of a steam boiler, the solar heat can be used directly at any temperature. If a minimum temperature is required, e.g. for washing or production processes, the water must be stored until the desired temperature is achieved and should then be used, if a demand is there.

Thermosiphon or forced circulation

1. Thermosiphon SWHS should be preferred, if:

  • the storage tank(s) can be installed above the collectors and
  • the distance between the collectors and the storage tank is low        and the pipes can be installed without barriers in between.
Thermosiphon SWHS are simpler and cheaper. If they are designed well and installed correctly, e.g. if the dimension of the pipes is sufficient and the water can circulate unhindered, they operate very reliable.
Thermosiphon SWHS

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.
Forced circulated SWHS

Evacuated tubes or flat plate collectors (ETC / FPC)

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:

  • Demand temperatures: ETCs are less efficient at lower demand temperatures and more efficient at higher temperatures than FPCs,
The threshold demand temperature above which the ETCs are more efficient depend on the collector efficiencies and the type of use, it is typically between 60°C and 80°C.
  • Only collectors should be used with a certificate issued by an accredited test centre, showing their efficiency and their reliability,
  • Weight of the collector, if it is filled with water,
The roof has to bear the weight of the collectors
  • Corrosion risk,
If aggressive water is used, perhaps glass tubes (ETCs) are better protected against corrosion than metal tubes of FPCs (but only if the manifolds and pipes are also less sensitive against corrosion)
  • Reliability of the collector, and
Glass tubes of ETCs can break. Usually they can be easily replaced, but it must be assured, that new glass tubes are delivered in short time, the replacement is simple due to easy access to all collectors, the possibly broken glass doesn`t harm the environment and can easily be removed, and the heat transfer fluid (usually water), doesn’t cause problems if it leaks out of the broken tube. Flat plate collectors show usually no leakages since the heat transfer fluid is flowing only through metal tubes, which are welded or screwed together. However, a corrosion protection must be given.
  • Price per square meter collector area, taken into account the solar yield expected.
One should be aware, that it is difficult to compare the solar related areas of ETCs and FPCs, since different areas are defined: the gross area (including the frame of the collector), the aperture area (where the sunlight can enter the collector, for FPCs: the area without the frame), and the absorber area. The area definitions are for ETCs more complicated than for FPCs, since the area between the glass tubes are not regarded as aperture area, if there is no reflector installed behind the glass tubes, but must be regarded, if a reflector is mounted behind the glass tubes. Regarding the absorber area there are sometimes misunderstandings, since the entire cylindrical surface of the inner ETC glass tubes is coated as absorber. However, according to international standards, it is not the cylindrical surface, but the section through the tube, which is used as absorber surface (diameter of the tube multiplied with the length). Since the efficiency depends on the definition of the reference area, only efficiency values based on the same type of area should be compared based on measurements and certification by a test institute.
Evacuated tube collector ETC

Flat plate collector FPC

Size and hydraulic of the collector field

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.
Hydraulic scheme of a collector field with 4 rows in parallel with 2 times 5 collectors in parallel

Size and type of installation of the storage tank

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.
Example for water demand of a steam boiler during 6 am to 6 pm and time, which defines the storage tank volume

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.
Vertical installed storage tanks enable a better temperature stratification than horizontal installed tanks

Hydraulic connections of the storage tank

1. If the solar heated water is used in the process (open loop)

  • the hydraulic connection to empty the storage tank should be placed at the deepest point at the bottom, to make use of the entire storage volume and
  • the hot water from the collector field should, in this case, be fed into the storage tank at the bottom to avoid stratification of the water temperature.
Since the water is used from the bottom of the storage tank for the process (due to varying water levels in the storage tank), the stratification of the water temperature would be counterproductive, because the hot water would remain at the top and the colder water from the bottom would be used. Therefore, the water should be mixed and the temperature stratification destroyed, which can be achieved by feeding in the hot water from the solar collector field also at the bottom. Since it is hotter than the already existing water at the bottom, the water will be mixed up.
Hydraulic connections at a storage tank with varying water level

2. If the solar system is closed-loop (and the storage tank always full)

  • the hydraulic connection to transfer the heat from the storage to the process should be in the upper part of the storage tank where the water is warmer due to stratification of temperature, and
  • the hot water from the collector field should be fed in the storage tank also in the upper part of the tank to support the development of a temperature stratification in the storage tank.
Hydraulic connections to achieve temperature stratification at a storage tank which is always filled with water

Controlling forced circulation systems

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).
Hysteresis control of the water pump (switch-on above and switch-off below different temperature levels)

Pressurised or non-pressurised SWHS

1. Non-pressurised SWHS should be preferred

1. 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.
Non-pressurised SWHS with an open make-up water tank to allow expansion and contraction of water

Pressurised SWHS with a membrane expansion vessel to allow expansion and contraction of water without pressure changes

Heat transfer fluid

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.

Don’t install a second pump only for redundancy

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.

Basic monitoring integral with the SWHS

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.
Display of the collector temperature at a controller

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.
Heat counter (HC) at the loop to the process with fluid sensor (F) and two temperature sensors (T1 ,T2)