DE20000759U1 - Solar thermal control - Google Patents

Description

II. Description

Solar thermal control system

2. State of the art:

Control units for solar thermal systems of the usual design have a three-point temperature control and a display of this and the switching states of the pumps. The switching thresholds are also usually editable.

3. Statement of the effects to be achieved with the invention:

a) Increasing the overall effectiveness in systems with more than one collector

1. A solar system consists of one or more collectors. If there is more than one collector, it makes sense to divide the collectors into groups and to adjust the orientation of the individual groups to the path of the sun (Figure 1). This results in better use of solar radiation over the course of the day, as one group of collectors is always aligned with the sun and collector losses are thus minimized. In a standard system with only one heat transfer medium circuit, the different irradiation of the collectors over the course of the day results in a cooling function for the collector surfaces that are not exposed to the light.

2. Furthermore, the arrangement of the collectors to each other (series or parallel connection) should be able to be changed according to the irradiation intensity in order to better utilize the advantages of the individual circuit variants and mitigate the disadvantages:

The parallel connection (Figure 2) has the advantage that when there is high heat radiation, the heat can be dissipated with a low mass flow. The disadvantage is that the system cools down more quickly when there is low radiation.

The advantage of the series connection (Figure 3) is that the heat transfer medium runs through all absorbers. This creates a uniform flow and less rapid cooling of the system when there is little solar radiation. However, it must be ensured that the heat produced can be dissipated by an increased mass flow when there is strong radiation.

b) Optimisation of each individual plant according to its location factors

Although today's devices make it possible to change the limit values or switching thresholds, determining these is usually left to the operator. Determining the values is made more difficult by the fact that the operator does not have any location-based and system-specific data available either at the start or after a longer period of operation of the system. As a result, he often falls back on regional standards, which leads to losses in the economic efficiency of the system.

4. Solution:

a) Increasing the overall effectiveness in systems with more than one collector

1. Point one can be achieved by separating the entire cycle and the individual

Collector groups form autonomously running systems, so that the individual circuit paths are firstly shorter
and secondly, they must be switched separately. This solution also solves the cooling function problem
of the non-illuminated collector surfaces, since these can be switched off if necessary.
In order to switch the pumps on and off effectively, the following points must be observed when determining the switching difference between the collector temperature and the storage tank temperature measured at the return:

- On the way from the collector to the storage tank, the heat medium cools down by approx. 0.5-3 Kelvin, due to heat conduction and convection on the pipe walls.

- The tolerance of the temperature sensors must be set at approx. 1-2 Kelvin.

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Max. 3 Kelvin Max. 2 Kelvin min. 3 Kelvin Max. 2 Kelvin approx. 10 Kelvin

- In order to work effectively, there must be a difference of at least 2-3 Kelvin between the heat exchanger in the domestic hot water tank and the domestic water.

- When the pump is switched on, the system initially cools down briefly due to circulation, which is caused by the flow of the even colder heat transfer medium.

From these four points, the minimum "worst-case" differential temperature is calculated as follows:

Losses in the piping system:

Temperature sensor tolerance: +

Minimum temperature difference to storage: +

Cooling through circulation +

2. For the control implementation of point two, a two-stage pump is required. The

Criteria for levels one and two or for switching from one level to the other are explained below:

If the radiation on the collectors is low and the system has cooled down, the collectors are connected in series and the pump runs at speed level 1 (1200 rpm), as this gives the heat transfer medium more time to heat up in one cycle.

If the radiation is strong when the system is cooled down, the collectors run in parallel and the pump runs at speed level 2 (1850 rpm), as this is the fastest way to make the heat generated available to the consumer.

If the radiation changes from low to high when the system is heated, the system switches from series to parallel operation in order to dissipate the increased heat more quickly.

However, if the irradiation changes from strong to low, the system switches from parallel to series operation to prevent the system from cooling down more quickly.

In order to avoid excessive cooling of the system at night due to natural circulation in the collectors, series operation is selected for the night.
The following table shows a summary overview of the switching states:

Irradiation Plant status Collector connection small amount Cooled down Series, pump stage 1 strong Cooled down Parallel, pump stage 2 Change from
Low to high operating temperatur Change from
Series on Parallel Change from
Strong to low operating temperatur Change from
Parallel to series

In terms of circuitry, as shown in Figure 4 , two valves are required per collector pair to switch between series and parallel.

The valve paths associated with the circuit variants are shown in the table below. The paths marked in black are switched on and the white one is blocked.

Circuit type Vl V2 series * * Parallel

b) Optimisation of each individual plant according to its location factors

In order to obtain precise data from the system, these are stored in the control system for a maximum period of one week. Two data blocks are defined for this purpose. The status of the individual pumps and collectors, the temperatures and the associated time are stored in the first data block (measured value block):

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Byte No. Data content description l.Byte Minutes number 2. Bytes Number of hours 3. Bytes Day number 4. Bytes Month number 5. Bytes Year 6. Byte - Bit 1 Status of the east pump (on/off) 6. Byte - Bit 2 West pump status (on/off) 6. Byte - Bit 3 Status of the East collectors (row/parallel) 6. Byte - Bit 4 Status of the West collectors (series/parallel) 7. Bytes Flow temperature 8. Bytes Return temperature 9. Bytes Temperature of the east collectors 10. Bytes Temperature of the west collectors 11. Bytes Marks the end of a measurement block

The second data block (limit block) contains the current switching thresholds or temperature differences as well as the time of the change:

Byte No. Data content description l.Byte Marks the beginning of a limit block 2. Bytes Number of minutes 3. Bytes Number of hours 4. Bytes Day number 5. Bytes Month number 6. Bytes Month number 7. Bytes Year 8. Bytes &Dgr;&Tgr;- Switching from parallel to series 9. Bytes &Dgr;&Tgr;- Switching from series to parallel 10. Bytes Switch-on temperature of the system 11. Bytes Switch-off temperature of the system 12. Bytes Marks the end of a limit block

The measured value block is saved at intervals of 10 minutes and the limit value block only when the limit value data changes. In order to be able to carry out a meaningful evaluation of the data, the values are sent to the operator at least once a week via a remote data transmission using an analogue modem or the operator reads them using a direct PC connection. The data can now be evaluated on the PC using the software provided and the switching thresholds can be changed according to the requirements of the system. Season-dependent settings can now also be easily determined and adjusted if necessary.

Benefits achieved:

The previously described change in the connection of collectors makes it possible to increase the effectiveness of small and medium-sized systems with little additional effort and cost, as the relatively expensive stepless pump speed control can be dispensed with.

By storing the system data and transmitting it to the operator, it is possible to set location-specific limit values and create system-specific statistics in a cost-effective manner, which in turn leads to increased economic efficiency.

Claims (1)

Solar thermal control system - Collector field series-parallel switching - Optimization of each individual system through system data storage