Monitoring Result Overview
HP Dairy, Rampur

Summary of the monitoring results of the Solar Water Heating System at HP Dairy, Rampur

OVERVIEW ON THE MONITORED PERFORMANCE OF THE SWHS

The HP Dairy SWHS is a non-pressurized, open system, composed of a Flat Plate Collector (FPC) field with 120 m2 total collector gross area and a water storage tank with 600 litre volume. The collectors are installed on the roof-top of the company building and the storage tank on the ground. The SWHS is used for pre-heating the feeding water of a steam boiler.

The system was monitored since 15 October 2014. Monitored data until 25 July 2015 are available and can be found under the tab weekly results, further data are not available due to data transfer problems. Due to different reasons (broken sensors, failures in data storage or data transfer, etc.) monitored parameters are for several days or weeks not available. However, since the heat meters sum up their values continuously, the energy generated could be calculated for each month.

The monitoring results of the HP Dairy SWHS, available for the period of 15 October 2014 until 25 July 2015 (9 months), are extrapolated to annual data and shown in table 1 and 2. Since the data of the summer period are missing, the real data can be expected up to 10% better than the presented ones.

Table 1: System performance values of the solar system (for the entire system with 120 m2 collector gross area, 111 m2 aperture area), annual values are extrapolated on measurements from Oct 2014 – July 2015.
Lines, formulas Description of parameter Annual values of the whole system Average daily values of the whole system
(1) Solar irradiation on collector area 155.9 MWh/a 427.0 kWh/d
(2) Solar yield in the collector circuit 46.3 MWh/a 126.9 kWh/d
(3)=(2)/(1) Solar collector efficiency 30%
(4) Solar energy delivered to process 30.4 MWh/a 83.4 kWh/d
(5)=(4)/(1) Solar system efficiency 20%
(6) Water volume pumped in circuit to process 2,037 m3/a 5.6 m3/d
(7)=(4)/((6)*1.14) Average temperature increase of water delivered to the process 13.1 °C
(8) Boiler efficiency assumed 70%
(9)=(4)/(8) Fuel saved (diesel), energy content 43.4 MWh/a 119.0 kWh/d
(10) Fuel saved (energy content 10.4 kWh/ltr) 4176 ltr/a 11.4 ltr/d
(11) Total annual fuel demand (diesel) 3,03,000 ltr/a 830 ltr/d
(12)=(10)/(11) Solar fraction 1.4%
(13) Fuel cost saving (52 Rs per ltr fuel) 2,17,000 Rs/a 595 Rs/d
(14) Investment (total system costs) Rs 1,667,000
(15)=(14)/(13) Simple pay-back without subsidy 7.7 years
(16) Central Financial Assistance (CFA)/Subsidy Rs 792,000
(17) Investment costs minus subsidy Rs 875,000
(18)= (17)/(13) Simple pay-back including subsidy 4.0 years
(19) Carbon emissions saved (2.68 kg CO2/ltr) 11.2 to CO2/a 30.6 kg CO2/day

To ease the comparison of different solar systems with different collector areas, the values per square meter aperture collector area are shown in table 2. The gross collector area is 120 m2, while the aperture collector area (area without the collector frame, this means the area, where the solar irradiation can enter the collector) is 111 m2.

Table 2: System performance values of the solar system (per square meter of collector aperture area)
Lines, formulas Description of value Annual values per square meter collector area Average daily values per sqm collector area
(1) Solar irradiation on collector area 1,404 kWh/(m2*a) 3.8 kWh/(m2*d)
(2) Solar yield collector circuit 417 kWh/(m2*a) 1.1 kWh/(m2*d)
(3)=(2)/(1) Solar collector efficiency 30%
(4) Solar energy delivered to process 274 kWh/(m2*a) 0.8 kWh/(m2*d)
(5)=(4)/(1) Solar system efficiency 20%
(6) Water volume pumped in circuit to process 18.3 m3/(m2*a) 50.3 ltr/(m2*d)
(7)=(4)/((6)*1.14) Average temperature increase of water delivered to the process 13.1 °C
(8) Boiler efficiency assumed 70%
(9)=(4)/(8) Fuel saved (diesel), energy content 390 kWh/(m2*a) 1.1 kWh/(m2*d)
(10) Fuel saved (energy content 10.4 kWh/ltr) 37.6 ltr/(m2*a) 0.1 ltr/(m2*d)
(13) Fuel cost saving (52 Rs per ltr fuel) 1,955 Rs/(m2*a) 5.4 Rs/(m2*d)
(14) Investment (total system costs) Rs 15,000 / m2
(15)=(14)/(13) Simple pay-back without subsidy 7.7 years
(16) Central Financial Assistance (CFA)/Subsidy Rs 7,135 /m2
(17) Investment costs minus subsidy Rs 7,883 /m2
(18)= (17)/(13) Simple pay-back including subsidy 4.0 years
(19) Carbon emissions saved (2.68 kg CO2/ltr) 100 to CO2/(m2*a) 0.3 kg CO2/(m2*d)

Description of parameters

Fig 1: Orientation of the collectors towards south-east due to installation area

The calculated solar irradiation (1) on the collector surface is about 1,400 kWh/m2 per year, which corresponds to an average daily irradiation of 3.8 kWh/(m2*d) collector area. The average global irradiation is 4.8 kWh/(m2*d) on horizontal area. In the sloped angle of 45° it should be slightly higher. There are three reasons for the lower value, firstly, the collector area is oriented in parallel to the long side of the building (see Fig. 1), this means towards south-east 30° while the optimal orientation would be towards south. Secondly, the collectors in the morning and especially during winter are shadowed by high mountains, since the dairy is located at the bottom of the valley. In addition, since the data of the summer months are missed, the real value can be assumed 10% higher than extrapolated.

Fig 2: Collectors partly shadowed in the afternoon

Solar yield collector circuit (2) is the heat generated by the flat plate collectors, which is delivered to the storage tank. Due to the very limited roof area, the flat plate collectors are partly shadowed in the afternoon by parts of the building, which reduces the solar yield (see Fig. 2).

By dividing the solar yield by the solar irradiation, the solar collector efficiency (3) results, which is calculated with 30%. During winter time, the efficiency is lower than during summer time due to the lower ambient temperature and higher shadowing by the lower angle of the sun.

Most relevant is the solar energy delivered to the process (4) and the related solar system efficiency (5), which is the energy delivered to the process divided by the solar irradiation. The HP Dairy solar system delivers about 30.4 MWh solar energy per year to the process (274 kWh/m2), which results in a solar system efficiency of 20%.

Fig 3: Boiler feeding water tank (right side) with pipe from the solar system (blue) and the pipe to the boiler (red)

The solar heat is transported from the heat exchanger to the feeding-water tank by pumping 2,037 m3 water through the circuit to process (6). The amount of solar energy transported by this water volume increased the water temperature in the average by 13.1 °C (7).

The temperature of feeding water in the boiler feeding-water tank is not known. A part of the solar heat is lost since the the pipe to the tank and the water tank itself are not insulated (see Fig 3). These losses are not taken into account in the calculation, since their amount is not known.

Only a part of the energy content of the fuel is converted into heat due to the efficiency of the boiler (8), which is assumed with 70%. Therefore, the solar energy delivered to the process (4) must be divided by the boiler efficiency (8) to derive the energy content of the fuel saved (9). The volume of the fuel saved is given in (10) and the saved fuel costs in (13). The simple pay-back without subsidy is calculated with 7.7 years in (15) considering the investment costs given in (14). The simple pay-back is 4.0 years (18) if subsidy (17) is factored.

The carbon emissions saved are shown in (19).

Summary of the system performance

In each SWHS, only a part of the solar irradiation can be converted into useful heat. The amount of losses depends on the quality and efficiency of the components used, the design and operation of the system, which influences e.g. the temperature of the water which flows into the collector and the temperature of the water storage tank, the water temperature needed by the process (as higher the temperature, as higher the losses), the ambient temperature, the fraction of the total heat demand covered by solar energy, and the maintenance of the SWHS. Therefore, there is a broad range of possible solar system efficiencies and it is difficult to fix a typical efficiency. However, for a SWHS like at HP Dairy a system efficiency of about 20% to 40% could be expected. This means that the performance achieved is with about 20% in the typical range, but there is room for improvement of the performance.

DISCUSSION OF THE MONITORED PERFORMANCE

In the following, the monitoring results and possible reasons for the measured values are described. Fig. 4 illustrates the measured efficiencies of the HP Dairy solar plant.

Fig. 4: Measured energy flow and losses of the Synthokem SWHS, percentages in relation to solar irradiation

Efficiency of the collector field and circuit:

The typical maximum efficiency of flat plate collectors is 70% - 80%, depending on the transparency of the glass and the absorber coating used. With increasing temperature difference between the absorber and the ambient the thermal losses are increasing depending on the insulation of the collector and coating of absorber. On sunny days, e.g. on 24 April, the collector field delivers a temperature of about 70°C to the storage, the collector circuit efficiency reaches about 40% and the system efficiency about 35%. However, on colder days and less solar intensity, the efficiency is significantly lower.

Following causes could be responsible for the 70% of energy losses from the solar irradiation to the heat delivered to the storage tank:

  1. At maximum, 70% - 80% of the solar irradiation is converted into heat, this means, that about 20% - 30% losses are caused by reflections on the glass and the absorber. The collector glass is usually cleaned by rain. But if there is soiling on the collector glass, the so called “optical” efficiency can be lower than 70%.
  2. The absorber is heated by solar irradiation, but is increasingly loosing heat to the ambient with growing temperature difference between the absorber and the ambient. The losses depend on the type of coating of the absorber, the insulation at the back of the collector box, the tightness of the collector box and the ambient temperature.
  3. Losses happen, if the collector field is not equally flown through by water, e.g. if air is trapped in some pipes, the water flow of parts of the collector field could be obstructed and this part of the collector field does not deliver heat to the storage or only an reduced amount.
  4. Heat losses occur on the way from the collector to the tank, by improper insulated pipes and fittings.
  5. The solar yield is also lowered if the collector circuit is not operated though the sun is shining, e.g. if the collector circuit pump is not switched on due to suboptimal controller parameter setting.
  6. If the heat delivered to the storage tank by the collector field is not handed over to the process efficiently, the storage temperature remains higher than necessary. Because then, the solar collector field has to operate at higher temperature than necessary and higher collector temperature results in lower efficiency.

Solar system efficiency:

Additional losses of 10% (in relation to the solar irradiation) between the collector circuit and the heat demanding process are caused by:

  1. Heat losses through the surface of the storage tank (taking into account, that the losses are already reduced by insulation).
  2. Heat losses of the pipes and fittings, which are not well insulated.
  3. Heat losses by unwanted circulation of the (hot) water from the storage to the collector field during night, due to natural convection by gravity.
  4. Losses by heat exchanger. Since heat can only be transferred if a temperature difference exists, the heat exchanger leads to a higher temperature in the storage tank then the return flow from the process circuit. If the heat exchanger and the related pumps are not designed optimal (heat transfer capacity and/or mass flow through the heat exchanger too low), the heat cannot be transferred properly and the resulting temperature in the storage is higher than possible (which leads to a higher temperature in the fluid delivered to the collector field, which results in a lower efficiency).

Based on the measured data, it is not possible to identify which cause is responsible for which share of the losses. Some of the losses cannot be avoided, e.g. the reflection on the glass pane or the heat losses of the pipes and the storage tank, but losses can be reduced by a good quality of the products used. However, the effort to reduce the losses and increase the performance e.g. by using components with higher quality, is only reasonable, if the additional costs are lower than the cost savings which can be achieved by these measures.

Other losses can be avoided by a good operation and maintenance of the SWHS. A high performance requires that the fluid is pumped through the collector circuit whenever the temperature in the collector is higher than of the water to be heated in the storage. If an automatic controller is used like at the HP Dairy SWHS, the temperature difference between the collector outlet and the bottom of the storage tank (height of socket, where the pipe to the collector field is connected) is measured and if the difference is higher than e.g. 8°C the pump is switched on and if it falls again below e.g. 3°C, the pump is switched off. This hysteresis is necessary to avoid a permanent on-off-on-off operation as long as a stable operation mode is not achieved. However, it is important, that the temperature measurement is done properly, e.g. the sensors placed right, with sufficient accuracy and that the parameters of the controller are set correctly.

As long as the collector circuit pump is not operated, the water heated by the collector, if the sun is shining, must be transported to the temperature sensor by micro-circulation. Therefore, it is important that the collector outlet temperature sensor is placed close to the collector outlet, as it is at the HP Dairy System (see Fig. 5).

Fig. 5: Collector outlet temperature sensor is located in the pipe close to the collector outlet.

Further it is important to prevent that air is encased in the collector circuits. This can be assured by the installation of vent valves at the highest points of the collector circuits. Especially, in non-pressurized systems they are important since they are usually open to the ambient. In the case of the HP Dairy SWHS, the collector circuit is not under pressure, however, it is also not emptied if the pump is not running. A vent valve is installed at the highest point, but there are other local points in the piping system, where air could be accumulated, without a valve. If air is encased in a collector circuit, it can hamper the circulation of the water in the related collectors so that their solar energy yield is not transported to the water storage tank.

Fig. 6: Hydraulic of the collector field

Also if the water flow through the different parts of the collector field, which are connected in parallel, is not equal, it can happen, that solar energy yield of parts of the collector field are not or only partly delivered to the water storage tank. Fig. 6 shows the hydraulic scheme of the 60 flat plate collectors of the HP Dairy SWHS. Since several collector rows are connected in parallel with different number of collectors (and therefore different pressure losses), it can be assumed that the flow through the different collector rows is not equal. This leads to the risk that in one or several collector rows air is encased and hampering the water flow. This should be evaluated by comparing the temperatures of the different collector rows. Vent valves at the highest points of the circuit are necessary and should be checked regularly, to assure that they function properly.

Losses can also occur due to a suboptimal system design. The hydraulic design of the collector field shows the risk, that the parallel collector rows are not flown through equally. In addition, the design of an external heat exchanger is relatively costly and creates additional challenges to define the right dimension and run the pumps in both connected circuits with the right flow. How much the system efficiency could be increased by a different design cannot be assessed without additional investigation, however it can be stated, that the storage temperature is higher than optimal, since the return flow from the storage to process loop, which defines the cold water temperature in the storage, is typically at 40°C and should be lower.

Further it should be stated, that the solar system is delivering heat to the feeding-water tank, which is not insulated. Since the solar fraction is with 1.4% very low, the resulting temperature increase of the feeding water by solar energy is rather low and therefore also the losses are rather low. However, the integration of the SWHS into the conventional heating system is not designed optimally.

RECOMMENDATIONS

Recommended evaluation of the SWHS

Fig: 7: Temperatures of the storage to process circuit

The solar system is economical, since it is running with a system efficiency of 20% and a simple payback time of 4.0 years, taking into account the subsidies received. Therefore, there is no urgent need for changes in the system design.

However, the system performance could be increased by reducing the return flow temperature in the circuit of the storage to the process (this means to the heat exchanger), e.g. by optimizing the flow on both circuits of the heat exchanger (see Fig. 7). It should be proven as well, if the heat exchanger could be removed and the hot water of the storage tank could be directly pumped to the process feeding water tank, because then the cold temperature at the bottom of the hot water storage would be reduced very efficiently.

Fig. 8: Collector inlet and outlet temperature curves

The collector outlet temperatures measured in the collector circuit are typically between 50°C and 70°C, which is fine. But the collector inlet temperature is increasing immediately with the outlet temperature (see Fig. 8). It is expected, that the inlet temperature stays longer at a lower temperature, because it reflects the low temperature at the bottom of the water storage tank, which should increasing slowly due to the volume. At least it should be at 40°C according to the return flow from the process (see above). Therefore the heat flow within the water storage tank should be evaluated. If a heat exchanger within the water storage tank is installed, which is not exactly known, its function must be evaluated.

It should be further evaluated, if the flow through the collectors is equal. As part of this evaluation it should be checked, if air is enclosed in one or more parallel hydraulic branches and, if necessary, missing air vents should be installed.