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The first two graphs display the measured data of the two collector circuits of the Synthokem Labs solar water heating system (SWHS), the Collector Loop "Day demand" and the Collector Loop "Night demand". The light blue line shows the temperature of the water before it enters the collector field (inlet temperature) and the red line shows the temperature of the water after it lefts the collector field (outlet temperature). The temperature scale is displayed on the left axis. The dark blue line at the bottom shows the mass flow rate of the water flowing through the collector field, which is zero, if the pump is not running. The yellow line shows the irradiation on the collector surface. Both scales are displayed on the right axis.
All data are measured in time steps of one minute. However, to delete short-time fluctuations of the measured values, only the hourly average of the data is displayed in the graphs.
The solar irradiation curve shows an ideal shape e.g. on 15 October without any disturbance by clouds. If clouds are moving over the collector field, the irradiation can be significantly reduced for that time, as it happened e.g. on 16 and 17 October. The maximum irradiation intensity is about 1000 W per m2 collector area.
The solar irradiation is prerequisite to harvest solar energy. However, only if the pump is running and water is circulating through the collector, solar energy can be harvested and the outlet temperature (red) is higher than the inlet temperature (blue). After the pump is switched off, the water temperature in the tubes, where the temperature sensors are installed, is declining to the ambient temperature and the red line has the same values than the blue line and is hidden by the blue line.
In the Collector Loop "Night demand" the water storage tank is filled with cold water in the morning and heated up by solar energy during the day. In the evening the solar heated water is pumped into the water storage tank of the collector circuit "Day demand". The temperature increase of the water flowing through the collector is almost proportional to the intensity of the solar radiation since the mass flow of the water is constant. With higher temperatures the thermal losses of the collectors are increasing and the temperature difference decreasing, but this is not a significant effect in the displayed temperature range.
The temperature increase around noon is typically about 12-15°C and the outlet temperature is increasing in parallel to the inlet temperature as long as the irradiation is in a similar range. The inlet temperature is increasing since the water temperature in the water storage tank is increasing by the solar yield. If the solar irradiation is declining significantly, the temperature difference between inlet and outlet is declining as well.
The operation time of the collector circuit pump is about 8 hours on sunny days. Only during pump operation the temperature values displayed are related to collector and storage temperatures, because they are measured in the pipes.
The water in the storage tank which is part of the Collector Loop "Day demand" is continuously used as feeding water for the process. Therefore, the water volume in the tank is decreasing if feeding water is consumed. If the water level falls below a specific level, additional cold water is pumped into the tank. This can be seen e.g. on 16 October, when the collector inlet temperature (light blue) dropped by about 10°C abruptly at around 16 h.
The solar preheated water from the storage tank of the "Day demand" collector circuit is pumped to the steam boiler (the process) typically a few times per hour. Since the duration of water pumping and the number of pumping periods are varying, the hourly values of the solar thermal energy delivered to the process are displayed as bars in the third graph.
The continuous variation of these values is induced by the irregular water demand of the steam boiler, as well as by the water temperature at the outlet of the water tank. Since the water outlet is at the bottom of the tank, first the cold water is used, if the water temperature is not completely mixed in the tank, but stratified. This can usually be seen in the morning between 9:00 and 13:00, when the solar energy delivered to the process is rather low since the cold water, which is pumped in the morning into the tank is delivered to the process whereas the solar heated water is stored at the top of the water tank.
In the afternoon the solar heated water is delivered to the process and the energy yield is increasing. The same happens in the evening, when the water of the storage tank of the collector circuit "Night demand" is pumped into the "Day demand" storage tank. This leads to high solar energy values delivered to the process before and after midnight.
The data table shows typical data of the system per day and in the last column as weekly average. Although both collector fields have the same collector area, the "Night demand" collector circuit has an up to 50% higher solar yield than the "Day demand" collector circuit. The reasons are that the "Day demand" collector field is partly shadowed, the hydraulic design is not optimal (some collectors have a double mass flow rate than the other part) and the water volume of the storage tank is continuously varying during the day. The impacts of the several factors are not known.
One would expect that the solar yield delivered to process of one day is the solar yield of both collector circuits of the same day minus some losses. But since the solar yield of the collector circuit "Night demand" is transferred in the evening to the storage tank "Day demand" and used during the entire night, the value of the solar yield delivered to the process includes partly the yield of the day before as well. At the same time a part of the solar yield of the collector circuit "Night demand" is included in the value of the following day.