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Scientific monitoring system to measure the yield of the solar water heating system
Each heat circuit has an own heat counter, which uses a flow meter F and the temperature difference between two temperature sensors T to calculate the thermal energy generated. Heat counter 1 measures the soalr yield of the collectors fields delivered to the solar tank, heat counter 2 measures the thermal energy delivered from the storage to the heat exchanger and heat counter 3 measures the thermal energy delivered to boiler feeding water tank. The pyranometer is positioned in the same angle than the collectors and measures the intensity of the solar irradiation. So the efficiency of each step of the energy chain can be identified: solar irradiation / conversion to heat in the collectors / losses of the storage / losses of the heat exchanger.
On 22 July there is almost no energy transfer to the process. But it seems that the reason is on the side of the process, because the storage temperature is as high as on other days.
One day is sufficient, to increase the storage temperature from about 20°C to about 60°C. But in the collector loop it can be seen that the temperature increase is not so steep than at other days with high storage temperature.
During monsun, the solar yield is zero on very cloudy days.
On sunny days, temperatures in the storage tank above 70°C are achieved. The temperature in the circuit is expected to decrease during discharching the storage tank. However, it is increasing during the operation of the collector circuit pump. It can be concluded, that during the collector circuit is operating, the solar yield is directly transfered to the process in the storage tank.
Due to technical problems in the monitoring system, there are no data on process loop.
The monitoring system didn't work on 6 days of this week.
At monsun time the solar radiation is much lower.
In summer the typical opration behaviour can be seen: in the morning, the temperatures in the collector circuit are increasing very fast. This is due to the relative high temperature of the storage tank of 60°C. A second reason is, that due to the high mountains around, the intensity of the sun is already very high, when the sun appears behind the mountains. During operation of the pump and the solar sytem, the temperature of both temperatures, collector inlet and outlet, is increased by about 10°C.
On some days, the solar energy delivered to process is very close to the solar yield at the collector loop (27, 28 April). On other days the difference is relative high, resulting in a low system efficiency (e.g. 30 April).
At higher radiation, the solar system efficiency achieves a level between 25% and 30%.
Again a day with very low solar radiation is reported at 16 April. On this day, the solar circuit is not operating. However, heat from the storage is transfered to the process.
There are some days with very low irradiation (7 April), which results in a storage temperature decrease from 60°C to 40°C.
With lower radiation not only the absolute yield, but also the solar system efficiency is decreasing to about 10% in this week.
At good solar radiation of about 6 kWh per m2 collector area, the overall system efficiency is between 25% and 30%.
It can be clearly seen, that after a day with low solar radiation (8 March), the storage temperature is decreasing strongly (see storage to process graph).
With increasing irradiation the temperature in the storage tank is increasing significantly from about 20°C on 3 March to about 60°C on 6 March, as it can be seen in the storage to process loop graph.
The solar irradiation and solar yield at the collector loop are improving, however, it is not known, why the solar energy delivered to process is in relation varying so much.
Obviously the weather is changing strongly, if the solar irradiation of the three days with data displayed are compared.
Due to technical problems of the monitoring system, only data of 2 days are available. These days show a much higher irradiation than the weeks before.
It is not known, why the energy delivered to the process has only one peak during the week. Unfortunately, temperatures and energy values are not measured at the heat demanding process.
At low solar irradition the system efficiency is with 13% better than the week before.
Again a week with low input and output values.
The temperature in the storage tank is further decreasing to less than 40°C. As a result the heat delivered to process is very low.
The solar irradiation and therefore the solar yield and the energy delivered to the process is very low.
During winter time the amount of solar energy falling on the collector (solar irradiation) is below 3 kWh/m2 and therefore much lower than during summer with abouve 6 kWh/m2 due to the shadow of the high mountains around the site of HP dairy plant.
The storage tank temperature and the energy delivered to the process are further decreasing.
In this week the solar energy delivered to the process is low, not only as absolute value, but also relative to solar irradiation and solar yield at the collector loop. The solar system efficiency is only at about 14% in the average of the week. Perhaps it is due to the lower temperature in the storage tank, that the heat of the storage tank cannot be transfered to the process.
Finally, what counts in a solar thermal system is, how much solar energy is delivered to the process. This value is shown in the table in total and per m2 collector area.
Since the pump of the circuit from the heat exchanger to the process (feed water tank) is operating irregular, the third graph is showing not temperatures, but the heat delivered to process per hour. It can be seen, that there are some peaks of demand, which are either at lunch time or in the evening.
There is a circuit from the storage tank to the heat exchanger and a circuit from the heat exchancher to the feed tank of the process (steam boiler). The second graph displays the circuit from the storage tank to the heat exchanger (toward the process). The mass flow is displayed in dark blue, however, the temperature difference is always in the same size of aobut 5-10°C. It can be concluded, that this is due to micro circulation in the tubes.
During collector loop pump operation, the collector outlet temperature (red) and inlet temperature (blue) are increasing very fast from about 20°C to about 60°C. The temperature of the storage tank is at about 50°C as it can be seen from the storage outlet temperature in the storage to process loop. The collector field is increasing the temperature from the storage (blue line in collector circuit graph) by 3 - 5°C and the collector outlet temperature is at about 55-60°C.
If the mass flow rate in the collector loop graph (dark blue) line is not zero, the pump is operating. If there is a mass flow, the energy transfer is proportional to the mass flow and the temperature difference. If the mass flow is zero, the temperature difference between collector outlet (red line) and return temperature (collector inlet, blue line) does not say anything about energy, because no heat is transported. Without mass flow, the temperatures should be close to ambient temperature in the tubes, where the temperatures are measured. However, the collector outlet temperature is higher. Assumably the reason is micro circulation of heat from the storage tank.