EP1950499A2 - Method for operating a solar thermal array - Google Patents

Abstract

The method involves determining temperatures at a collector (1) and in a memory (2), respectively, and calculating a temperature difference between the temperatures. An auxiliary quantities value is compared with a predefined reference value. A pump (3) is switched on when the evaluated value is larger or equal to the reference value or the difference exceeds a preset threshold value. A temperature maximum is determined by monitoring a temporal process. The pump is switched off after determining a defined number of maximum, when the difference is smaller and equal to the threshold value.

Description

The invention relates to a method for operating a solar thermal system, in particular in a starting phase.

A solar thermal system basically consists of solar panels, which capture solar energy and deliver it to a heat transfer medium (water-glycol, water-ethanol), a reservoir and a closed circuit, which transports the heat absorbed in the collector to the storage tank, a regulation that controls the circulation the heat transfer medium, also called brine, controls at corresponding temperature differences from the collector to the memory. The brine is heated by the solar radiation in the collector and then fed through the brine circuit to the memory. Subsequently, the solar heat can be used, for example, for hot water preparation, heating support or swimming pool heating.

The collector type, which is relatively widespread in addition to flat-plate collectors, is the vacuum tube collector, in which the absorber surfaces are enclosed in evacuated glass tubes instead of in a housing. The advantage here is that the proportion of heat loss through convection is smaller and the tube is optimally adapted to the position of the sun can. The heat is transferred to the coolant, inter alia, by the direct flow of the liquid through the absorber.

In most cases, a simple temperature differential controller is sufficient for controlling a small solar system for hot water production. The controller uses two temperature sensors to determine when the temperature at the collector outlet is higher than the temperature in the storage tank measured at the height of the solar circuit heat exchanger and then starts the solar circuit circulation pump. Normally, the solar controllers are set so that a temperature difference of about 5 - 8 K is ensured between the collector and the pump for starting the pump. If this drops to 2 to 3 K, the circulation pump will be shut down by the solar controller. Despite this setting of the solar controller problems can occur when starting the system in which the system does not start or shut down too early. Thus, after the pump starts, cold liquid enters the collector, which flows through it and leaves it heated again. As a result, the temperature drops rapidly again. Due to the present temperature difference may occur according to the prior art for switching off the pump. If the pump continues to run, the temperature rises again because the hot liquid which remained in the collector at the start of the pump flows back into the collector after it has flowed through the reservoir. Only after some upheavals does a quasi-stationary state arise.

The invention has for its object to provide a method for a solar thermal system available, with a reliable pump start and operation of the system is made possible in the startup phase.

• die mittels des Temperatursensors (4) gemessene Temperatur T₁ am Kollektor (1) und die mittels des Temperatursensors (5) gemessene Temperatur T₂ im Speicher (2) werden erfasst,
• der Temperaturgradient $\frac{{\mathit{dT}}_1}{\mathit{dt}}$
  
  am Kollektor (1) wird berechnet,
• die Temperaturdifferenz ΔT zwischen der gemessenen Temperatur T₁ am Kollektor (1) und der gemessenen Temperatur T₂ im Speicher (2) wird berechnet,
• ein Hilfsgrößenwert d_Ist für die Pumpe wird in Abhängigkeit von der Temperaturdifferenz ΔT, einer Konstanten und dem Temperaturgradienten $\frac{{\mathit{dT}}_1}{\mathit{dt}}$
  
  berechnet und mit einem vordefinierten Sollwert d_Soll verglichen,
• wenn der berechnete Hilfsgrößenwert d_Ist größer oder gleich dem vordefinierten Sollwert (d_Soll) ist oder die Temperaturdifferenz ΔT einen vorgegebenen Grenzwert ΔT_Start überschreitet, wird die Pumpe eingeschaltet und der berechnete Hilfsgrößenwert d_Ist(t₀) zu diesem Zeitpunkt t₀ gespeichert,
• die mittels des Temperatursensors (4) gemessene Temperatur T₁ am Kollektor (1) wird weiterhin erfasst, wobei durch die Beobachtung des zeitlichen Verlaufs Maxima erkannt werden,
• nach dem Erkennen einer definierten Anzahl von Maxima, deren Anzahl 2 nicht unterschreiten darf, wird die Pumpe abgeschaltet, wenn die Temperaturdifferenz ΔT kleiner oder gleich einem vorgegebenen Grenzwert ΔT_Stop ist.

According to the invention this is achieved according to the features of claim 1, characterized in that a method for operating a solar system, in particular in a starting phase, in which the heat transfer medium to be heated by means of a pump (3) in a circuit between a storage device (2) and at least one collector (1) is conveyed, with a temperature sensor (4) arranged at the output of the collector (1) in the direction of the memory (2) and with a temperature sensor (5) in the memory (2) with the following Procedural steps are provided:

• the temperature T ₁ measured at the collector (1) by means of the temperature sensor (4) and the temperature T ₂ measured in the memory (2) by means of the temperature sensor (5) are detected,
• the temperature gradient $\frac{{\mathit{dT}}_1}{\mathit{dt}}$
  
  at the collector (1) is calculated
• the temperature difference ΔT between the measured temperature T ₁ at the collector (1) and the measured temperature T ₂ in the memory (2) is calculated,
• an auxiliary variable value d _is for the pump depending on the temperature difference ΔT, a constant and the temperature gradient $\frac{{\mathit{dT}}_1}{\mathit{dt}}$
  
  calculated and compared with a predefined setpoint d _set ,
• if the calculated auxiliary variable value d _{actual is} greater than or equal to the predefined setpoint value (d _setpoint ) or the temperature difference ΔT exceeds a predefined limit value ΔT _start , the pump is switched on and the calculated auxiliary variable value d _actual (t ₀ ) is stored at this time t ₀ ,
• the temperature T ₁ measured at the collector (1) by means of the temperature sensor (4) is still detected, whereby maxima are detected by the observation of the time profile,
• after detecting a defined number of maxima whose number may not fall below 2, the pump is turned off when the temperature difference .DELTA.T is less than or equal to a predetermined limit .DELTA.T _Stop .

• Figur 1 eine schematisch dargestellte Solaranlage,
• Figur 2 einen Temperaturverlauf am Kollektorfühler nach einem Pumpenstart und
• Figur 3 ein Ablaufdiagramm eines möglichen Regelungsvorgangs des Pumpenstarts.

Further advantageous embodiments of the invention will become apparent from the features of the dependent claims and the description. The invention will now be explained in more detail with reference to FIGS. Show here

• FIG. 1 a schematically illustrated solar system,
• FIG. 2 a temperature profile at the collector sensor after a pump start and
• FIG. 3 a flow diagram of a possible control process of the pump start.

At the in FIG. 1 simplified illustrated solar system pumps a pump (3) a heat transfer medium (eg water-glycol, water-ethanol) in a closed circuit between a collector (1) and a memory (2). A control (6) controls the circulation of the heat transfer medium at corresponding temperature differences from the collector (1) to the memory (2).

FIG. 2 shows by way of example a measured temperature profile (b) at the collector sensor (4) after a pump start, in which two measured temperature maxima T _max1 and T _{max2 are shown} after mixing a heat transfer medium from the collector T _K and the return T _R. With (a) the pump speed characteristic is designated.

In FIG. 3 the method steps of the control method according to the invention can be taken from a flow chart. Accordingly, a temperature measurement is first carried out by means of a temperature sensor (4) attached to the collector (1) and in the memory (2) by means of a temperature sensor (5) located on the storage bottom. The measured values are used to calculate the temperature gradient at the collector sensor (4) and the temperature difference (ΔT) between the collector (1) and storage tank temperature (2).

An auxiliary variable value d _Actual for the pump is defined by the following formula, where C is a constant (eg 10 K): $$d_{\mathit{1}\mathit{st}}=\left(T_1-{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0001.png"\; state="\{\{state\}\}">T</figure-callout>}}_2+C\right)\cdot \frac{{\mathit{dT}}_1}{\mathit{dt}}=\left(\mathrm{\Delta }T+C\right)\cdot \frac{{\mathit{dT}}_1}{\mathit{dt}}\mathrm{,}$$

The addition of the freely chosen temperature constant C in equation 1 has the following meaning: the value d _actual should only assume positive values. In order for the system to be able to start when the measured collector temperature (T ₁ ) is lower than the tank temperature (T ₂ ), the collector temperature (T ₁ ) must be adjusted. With this approximation, the temperature in the lower temperature range is further given a higher weight.

The calculated auxiliary variable value d _actual is compared with a previously defined setpoint value d _setpoint , which is preset in the factory during commissioning as start value d _start . The later determined target value d _set takes into account plant-specific settings during commissioning.

If d _actual > = d _setpoint , the pump is put into operation and the calculated auxiliary variable value d _actual stored at the time of pump start t ₀ . The pump runs at least until two temperature maximum values (T _max1 , T _max2 ) are detected at the collector sensor (4). After the second temperature maximum, the calculation of the temperature difference (ΔT) of the temperatures measured at the collector (1) and in the store (2) takes place. An adaptation of the setpoint value d _setpoint is preferably carried out as a function of ΔT according to the following table: ΔT after second Change of the d value Thermal maximum Less than 0K d _Soll = d _Ist (t ₀ ) + 0,2 Between 0 K and 3 K d _Soll = d _Ist (t ₀ ) + 0,1 Between 3K and 7K d _Soll = d _Ist (t ₀ ) Between 7K and 10K d _Soll = d _Ist (t ₀ ) - 0, 1 Greater than 10K d _Soll = d _Ist (t ₀ ) - 0,2

You can also use adjustment steps other than the change steps listed in the table. A change in the desired value does not take place when external influences (such as water spills or radiation decay) act on the system.

Subsequently, the pump continues to run under the condition that the temperature difference .DELTA.T is greater than a predetermined limit .DELTA.T _stop , preferably 3 K.

If the d _actual value does not lead to a start of the pump or the system (eg due to water tapping or error in the d value calculation) and the condition is satisfied that ΔT is greater than 7 K, then the pump should also start , In this case, however, there is no adaptation of the setpoint value d _setpoint .

From the DE 38 35 012 Among other things, a control system for a solar system is known. This control system includes a control device for switching on a pump, wherein the control device of temperature sensors, can be controlled. The temperature difference between the temperature sensor on the solar collectors and the temperature sensor for the flow temperature is determined. If this temperature difference is greater than a previously entered system-dependent value, the pump is turned on. After a certain period of time, after which a steady flow has settled, the temperature difference between the water flowing back from the collectors and the water leading to the collectors in the supply is determined. Then this temperature difference can either with a fixed temperature value or after Multiplication with the flow rate of the pump can be compared with a previously set power value. If this comparison is negative, the pump is switched off again. In the scheme described here, the starting conditions are regulated by a temperature difference but with the interposition of a waiting time.

In the method according to the invention is in contrast to DE 38 35 012 no waiting time provided. There are continuously determined temperature minima and temperature maxima at the collector sensor (4) or the temperature difference between the temperature measured at the collector and in the memory. Thus, a reliable pump start and operation of the system is ensured in the initial phase.

Claims (3)

Method for operating a solar system, in particular in a starting phase, in which the heat transfer medium to be heated by means of a pump (3) in a circuit between a storage device (2) and at least one collector (1) is transported, with a temperature sensor (4) at the output of the collector (1) in the direction of the store (2) and a temperature sensor (5) in the store (2) with the following method steps: the temperature T ₁ measured at the collector (1) by means of the temperature sensor (4) and the temperature T ₂ measured in the memory (2) by means of the temperature sensor (5) are detected, - the temperature gradient $\frac{{\mathit{dT}}_1}{\mathit{dt}}$

at the collector (1) is calculated the temperature difference ΔT between the measured temperature T ₁ at the collector (1) and temperature T ₂ in the accumulator (2) is calculated, an auxiliary variable value d _actual for the pump (3) becomes dependent on the temperature difference ΔT, a constant and the temperature gradient $\frac{{\mathit{dT}}_1}{\mathit{dt}}$

calculated and compared with a predefined setpoint d _set , - If the calculated auxiliary value d _{Ist is} greater than or equal to the predefined setpoint (d _Soll ) or the temperature difference .DELTA.T exceeds a predetermined threshold .DELTA.T _start , the pump (3) is turned on and the calculated auxiliary variable value d _Ist (t ₀ ) at this time t ₀ saved, the temperature T ₁ measured at the collector (1) by means of the temperature sensor (4) is further detected, whereby maxima are detected by the observation of the time course, - After detecting a defined number of maxima whose number may not fall below 2, the pump (3) is turned off when the temperature difference .DELTA.T is less than or equal to a predetermined limit .DELTA.T _Stop . Method for operating a solar system according to claim 1, characterized in that after the detection of a defined number of maxima whose number may not fall below 2, the predefined setpoint (d _setpoint ) as a function of the current temperature difference .DELTA.T and the auxiliary variable value d _actual (t ₀ ) is recalculated at time t _{0 of} the last pump start. Method for operating a solar system according to claim 1, characterized in that the desired value (d _setpoint ) during initial commissioning by a factory default value (d _start ) is specified.

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