General Explanation for Monitoring Results
HP Dairy, Rampur

How to interpret the graphs and monitoring results of the Solar Water Heating System at Synthokem Labs, Hyderabad

Temperature graphs displayed

The first graph displays the measured data of the collector circuit, which connects the collector field on the roof with the water storage tank on the ground. 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.

The second graph displays the measured data of the circuit from the storage to the process.

Temporal resolution of monitoring data

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.

Solar irradiation curve

The solar irradiation curve shows an ideal shape e.g. on 22 April without any disturbance by clouds. At a cloudy day, the irradiation can be significantly reduced, as it happened e.g. on 20 April. On 19 and 21 April the day was partly clouded. The maximum irradiation intensity is close to 1000 W per square meter collector area.

The solar irradiation is prerequisite to harvest solar energy. However, only if the pump is operating and water is circulating through the collector, solar energy can be harvested and the collector outlet temperature (red line) is higher than the inlet temperature (light blue line). After the (collector circuit) pump is switched off, the mass flow rate (dark blue line) is zero and the water temperature in the pipes, where the temperature sensors are installed, is slowly declining until they reach the ambient temperature. However, natural circulation within the tubes can be the reason, why the outlet temperature (red line) is declining very slowly in comparison with the inlet temperature (light blue line).

Temperature curves of the Collector Loop

If the temperature in the collector field is increasing due to solar irradiation, the collector circuit pump is switched on by the controller and is pumping the water through the collector field (mass flow of about 1000 kg/h, dark blue line).

The temperature increase of the water by flowing through the collector field is only about 4 °C (collector outlet temperature, red line, minus collector inlet temperature, light blue line, see graph). This is an indication, that the mass flow is relatively high.

There was no possibility to measure the temperature within the storage tank, however if the pump is operating, the collector inlet temperature is representing the storage temperature. During night, without mass flow, the light blue line shows the ambient temperature (see point (1) at the graph), after the pump is running, the storage water is reaching the temperature sensor, this is why the temperature is increasing very fast from point (1) to point (2), which is about the storage temperature. During the day, the temperature of the storage increases by about 20°C (from point (3) to point (4)) by solar energy on 26 April 2015 as shown in the graph.

The operation time of the collector circuit pump is about 8 hours on sunny days (see graph). There is only one mass flow value, if the pump is switched on, but since the average mass flow per hour is displayed, the values shown are varying.

Temperature curves of the Storage to Process Loop

The solar heated water is stored in the storage tank and then delivered to the process as pre-heating water for a steam boiler. The water from the storage is pumped through a heat exchanger and the heat is transferred to the water, which is pumped to the feeding water storage tank of the process on the roof of the building. The “storage to process loop” represents the data of the circuit from the solar storage water tank to the heat exchanger.

The pump is operating also in the evening and even by night, therefore the hourly average mass flow rate is almost always above zero (dark blue line). The difference between the temperature of the warm water sent to the feeding-water tank (red) and the return water temperature (light blue) is very stable (red and blue line is parallel), which shows, that the temperature of the feeding-water tank (which is not insulated), is always at a significantly lower temperature.

Solar energy delivered to process

The monitored results of the loop from the heat exchanger to the feeding water storage tank are not shown as temperature and mass flow values, but as resulting energy values delivered per hour to the process.

Since the operation of the pumps of the storage to process loop and the loop from the heat exchanger to the feeding-water storage tank is coupled, both graphs show very similar results. The energy is proportional to the temperature difference multiplied by the mass flow rate. The solar energy is mainly delivered to the feeding-water storage tank between 10 o'clock in the morning and 10 o'clock in the evening.

Result table: Energy data per day

The data table shows the most important data of the system per day for each day (if data are available) and in the last column as weekly average.

The energy values per day of the solar irradiation on the total collector area, the solar energy delivered from the collector to the storage tank (solar yield collector loop), and the amount of solar energy delivered to the process (this means to the feeding-water storage tank) are listed in the table.

To enable the comparison of data from solar systems with different collector areas, the same data are given per m2 collector area as well.

The solar system efficiency is calculated by dividing the solar energy delivered to process by the solar irradiation on total collector area. Since solar energy can only be delivered to the process, if the pumps are operating, the pump operation time of the collector circuit is displayed. The average temperature increase of the water pumped to the feeding-water storage tank (displayed as “delivered to process”) by solar energy is shown in the last line.