ENERGY-EFFICIENT COOLING OF A PEROWSKIT SOLAR CELL

Description

Technical field

[0001] The invention relates to a method for controlling a cooling device for actively cooling a perovskite solar cell. Furthermore, the invention relates to a photovoltaic system comprising a perovskite photovoltaic module with at least one perovskite solar cell and a cooling device for actively cooling the perovskite solar cell. State of the art

[0002] It is known that the efficiency of perovskite solar cells typically decreases at high temperatures. This decrease, or deterioration, of efficiency at high temperatures is generally not only more pronounced than in conventional crystalline silicon solar cells, but can also be irreversible. The temperature at which there is a risk of permanent damage to the perovskite solar cell is also referred to as the critical temperature. It is therefore important that the temperature of the perovskite solar cell always remains below this critical temperature, even under conditions such as high solar irradiance. Document US10480730B1 describes a method for photovoltaic devices.

[0003] The critical temperature depends heavily on the structure, material composition, and processing of the perovskite solar cell. Under certain circumstances, the critical temperature of a perovskite solar cell can be as low as 50°C or 60°C – a temperature that the solar cell can typically reach quickly during operation.

[0004] One approach to addressing the challenge of degrading perovskite solar cells at temperatures above the critical temperature is to modify the structure, material composition, and / or processing of the perovskite solar cell. The modification aims to raise the level (in other words, the height) of the critical temperature of the perovskite solar cell.

[0005] A second approach involves finding ways and means to at least partially "cure" the deterioration in efficiency caused by the high temperatures to which the perovskite solar cell is exposed. In other words, this approach aims to at least partially reverse the supposedly irreversible damage to the perovskite material.

[0006] Neither the first nor the second approach has yet yielded fully satisfactory and practical solutions.

[0007] German patent DE 10 2007 055 462 A1 discloses a method for controlling a photovoltaic system, wherein the photovoltaic system comprises at least one solar cell and at least one Peltier element, which is thermally coupled to the solar cell and is used for cooling and / or heating the solar cell. The solar cells are fixed to a plate-shaped mounting element formed by an aluminum sheet. The temperature of the solar cells is monitored by means of a temperature sensor located on the back of the aluminum sheet. This monitoring is time-dependent and takes place exclusively during periods of sufficient solar irradiance. During the day, the temperature is measured at regular intervals and compared with a maximum limit temperature Tmax of 40°C.If, during monitoring, a temperature T is detected which is above the maximum limit temperature Tmax, a voltage is applied to the Peltier elements in such a way that the solar cells are cooled.

[0008] The object of the present invention is to develop an alternative concept to reduce the risk of permanent damage to the perovskite solar cell due to thermal effects. General description of the invention

[0009] This problem is solved by the method for controlling a cooling device and the photovoltaic system according to the independent claims. Advantageous further developments and embodiments are disclosed in the dependent claims, the description, and the figures.

[0010] The method according to the invention relates to the control of a cooling device for the active cooling of a perovskite solar cell, wherein the perovskite solar cell is part of a perovskite photovoltaic module. The method comprises the following steps: Determining a measure of the internal temperature of the perovskite solar cell and activating the cooling device if the determined measure of the internal temperature of the perovskite solar cell is greater than a corresponding measure of a predetermined temperature threshold, wherein the perovskite solar cell has a light-absorbing layer and the measure of internal temperature is the actual temperature of the light-absorbing layer of the perovskite solar cell.

[0011] The perovskite solar cell is cooled as needed, specifically when a certain temperature threshold is exceeded, which is advantageously lower than the cell's critical temperature. This prevents the perovskite solar cell from being exposed to temperatures higher than its critical temperature. Potentially irreversible damage to the solar cell can thus be avoided from the outset.

[0012] The cooling system is controlled based on the internal temperature of the perovskite solar cell, or a corresponding measure of that internal temperature. This internal temperature is measured continuously or at intervals. If the measured temperature exceeds a predetermined threshold, the cooling system is activated. Conversely, if the internal temperature falls below the threshold, the cooling system is deactivated. This enables energy-efficient and therefore efficient, demand-based cooling of the perovskite solar cell(s).

[0013] An important aspect of the present invention is that the cooling device is activated depending on the internal temperature of the perovskite solar cell. The internal temperature—also referred to as "bulk temperature" in technical terminology—is to be understood in contrast to the surface temperature. The internal temperature of the solar cell refers in particular to the temperature of the light-absorbing layer of the solar cell. In the case of a perovskite solar cell with a pin structure, i.e., comprising a hole-conducting layer, an intrinsic layer, and an electrically conductive layer, the light-absorbing layer comprises, in particular, the intrinsic layer. In a perovskite solar cell, the intrinsic layer has, for example, the (crystal) structure CH₃NH₃PbI₃.Since the intrinsic, perovskite-based layer of a perovskite solar cell can degrade irreversibly if exposed to temperatures above the critical temperature, the temperature of this area of ​​the solar cell, i.e., its internal temperature, is of particular importance. Conversely, if, for example, the temperature at the front contacts of the perovskite solar cell is above the critical temperature, but the actual temperature of the light-absorbing layer is below the critical temperature, the perovskite solar cell is still "in the green zone" and cooling of the solar cell does not yet need to be initiated.

[0014] Another important aspect of the invention is that the perovskite solar cell is actively cooled according to the inventive method. This is particularly important in contrast to permanent cooling. Since the internal temperature of the perovskite solar cell, or a corresponding measure thereof, is used as a control variable for the cooling device, it is ensured that cooling only occurs when there is a need for it. The inventive method is therefore energy-saving and thus conserves money and resources.

[0015] Active cooling offers the significant advantage of energy efficiency, as cooling only occurs when necessary. Furthermore, active cooling provides a relatively high cooling capacity to reliably prevent the internal temperature from exceeding the critical temperature of the solar cell with the highest possible probability. The method according to the invention thus represents a reliable and energy-efficient cooling mechanism for a perovskite solar cell, reducing the risk of permanent damage to the perovskite solar cell due to thermal effects.

[0016] Active cooling should also be distinguished from so-called back-ventilation. Back-ventilation of photovoltaic modules is common with conventional crystalline silicon solar cells. For this purpose, the photovoltaic modules are mounted at a certain distance from the roof. To achieve sufficient back-ventilation of the photovoltaic modules, it is recommended to maintain a distance of approximately 10 cm between the roof and the module. Back-ventilation of the modules works via convection air. A chimney effect is created in the space between the photovoltaic module and the roof membrane, with the strength of this effect (and thus the cooling) depending on the size of the distance between the roof and the module, as well as the material of the roof membrane and the underside of the modules. Height-adjustable roof hooks are available to facilitate maintaining the desired distance.

[0017] Naturally, a perovskite photovoltaic module typically contains more than one perovskite solar cell, namely a multitude of perovskite solar cells. A perovskite solar cell can also be part of a tandem cell, which consists of a perovskite solar cell and another solar cell, for example, a conventional silicon solar cell. Impressive efficiencies of 28% have recently been achieved for such tandem solar cells on a laboratory scale.

[0018] The first step of the process involves determining a measure of the internal temperature of the perovskite solar cell. If the perovskite photovoltaic module contains multiple perovskite solar cells, the first step can also include determining the measures of the internal temperatures of several perovskite solar cells within the perovskite photovoltaic module.

[0019] The term "measure of internal temperature" refers to any parameter suitable for characterizing the internal temperature of a perovskite solar cell. This can be the actual temperature in Kelvin (or degrees Celsius or Fahrenheit). However, it can also be, for example, a current in amperes or a voltage in volts, which cannot be directly converted into a specific temperature but can nevertheless be used to assess whether the current internal temperature of the perovskite solar cell is above or below a predetermined temperature threshold.

[0020] The predetermined temperature threshold is advantageously selected to be slightly below the critical temperature. Depending on the design of the cooling device, particularly with regard to its cooling capacity, the temperature threshold should be chosen to ensure that, after exceeding the temperature threshold and activating the cooling device, the temperature typically only rises to the point where it remains below the critical temperature of the perovskite solar cell.

[0021] A "predetermined" temperature threshold is understood to be a predetermined temperature threshold. Bearing in mind that the critical temperature of a perovskite solar cell, above which irreversible degradation of its efficiency threatens, depends heavily on the specific cell type, the corresponding temperature threshold should be individually defined.

[0022] There are various conceivable methods for determining the internal temperature or a measure thereof.

[0023] One method is based on impedance spectroscopy, specifically electrochemical impedance spectroscopy. For this, an alternating voltage is applied to the perovskite solar cell, and the resulting current flow is measured for different frequencies of the alternating voltage. Based on a suitable plot of the measurement results, for example in a Bode or Nyquist diagram, conclusions can then be drawn about internal parameters of the perovskite solar cell, such as charge carrier transport or the recombination rate of the charge carriers. In particular, the internal temperature of the perovskite solar cell can also be determined using impedance spectroscopy, as described, for example, in the international patent application PCT / EP2019 / 074317, filed on September 12, 2019.

[0024] A second method for determining the internal temperature of the perovskite solar cell involves the use of a thermocouple. A thermocouple consists of a pair of metallic conductors made of different materials, connected at one end, and suitable for temperature measurement due to the thermoelectric effect. In principle, the thermocouple generates electrical energy from heat when there is a temperature difference along the conductor. The resulting electrical voltage at the ends of the metallic conductors is relatively small, on the order of a few tens of microvolts per 1°C temperature difference. Advantageously, the two metallic conductors of the thermocouple are directly connected to the light-absorbing layer of the perovskite solar cell, for example, its intrinsic layer.Such a connection is usually permanently established so that the internal temperature of the solar cell can be continuously monitored during operation of the perovskite photovoltaic module.

[0025] Various implementations are also conceivable for the specific design of the cooling device for actively cooling the perovskite solar cell.

[0026] For example, solar cells can be cooled using a coolant. The coolant can be routed through pipes or hoses near the solar cells. In this case, the cooling system would ideally have an inlet and outlet, with the coolant temperature at the inlet being lower than the predetermined temperature threshold at which cooling is activated. The coolant can consist primarily of water, with optional additives to lower the freezing point and / or raise the boiling point.

[0027] Liquid-based cooling of perovskite solar cells is also possible, for example, using rainwater that is collected and directed in a controlled manner over an inclined surface of the perovskite photovoltaic module. In this case, when a predetermined temperature threshold is exceeded, rainwater is directed over the module surface. The water temperature is lower than the temperature of the photovoltaic module, thus cooling the module and the perovskite solar cells within it. The cooling control is based on the internal temperature of the perovskite solar cell or a corresponding measure of the internal temperature. The measure of the perovskite solar cell's internal temperature is therefore determined continuously or at intervals.If this value exceeds a predetermined threshold, the cooling device is activated; in the case of cooling using a rainwater reservoir, for example, by opening a valve at the reservoir's outlet. Using rainwater is advantageous because it prevents any limescale deposits from forming on the module surface. In principle, however, alternative cooling fluids are also conceivable. Furthermore, using rainwater in an open system is also possible, where it can evaporate and the heat of vaporization—also known as "evaporative cooling"—is used to cool the perovskite solar cell(s) of the photovoltaic module.

[0028] In another embodiment of the invention, the cooling device includes a radiator for dissipating heat by thermal radiation. The radiator is made, for example, of a highly thermally conductive metal and has a surface with a high emissivity.

[0029] The radiator – and other cooling device designs – are primarily located on the side of the perovskite photovoltaic module facing away from the sun. This has the advantage of preventing any shading of the solar cells. Furthermore, it minimizes the heating of the cooling device itself.

[0030] In a further embodiment of the invention, the cooling device includes a Peltier element. When the cooling device is activated, a current flow through the Peltier element reduces the internal temperature of the perovskite solar cell.

[0031] A Peltier element, in this context, is an electrothermal transducer that generates a temperature difference when an electric current flows through it, based on the Peltier effect. Peltier elements can therefore be used to cool an object to which they are thermally connected. In this application, Peltier elements are also referred to as Peltier coolers or TECs (thermoelectric coolers).

[0032] In a further embodiment of the invention, the cooling device comprises a plurality of channels in a cover plate of the perovskite photovoltaic module. The cover plate is, for example, a glass pane, in particular tempered safety glass. The channels are small in size compared to the other dimensions of the photovoltaic module and are also referred to as microchannels. When the cooling device is activated, a coolant flows through the channels. The coolant, at least in its upstream flow, has a lower temperature than the component to be cooled, in this case, the perovskite solar cell. Thus, the coolant absorbs some of the heat energy from the perovskite solar cell and thereby cools the internal temperature of the perovskite solar cell.

[0033] If, for example, the cooling device is arranged on the outside of the perovskite photovoltaic module, there is advantageously good thermal conductivity from the cooling medium to the perovskite solar cell to be cooled.

[0034] The invention relates, in addition to the described method for controlling the cooling device, to a photovoltaic system comprising a perovskite photovoltaic module and a cooling device. The perovskite photovoltaic module has one or more perovskite solar cells. The cooling device is suitable for actively cooling the perovskite solar cell. The photovoltaic system further comprises a control device for controlling the cooling device. The control device is specifically designed to execute a method according to one of the embodiments described above.

[0035] The active cooling of the perovskite photovoltaic module can optionally be combined with a passive cooling mechanism or a passive temperature stabilization mechanism. Known passive temperature stabilization mechanisms in this context include providing a large thermal mass, for example in the form of a water tank, on the back of the photovoltaic module, or using a latent heat storage system that stores a large portion of the supplied thermal energy as latent heat (e.g., for a phase change from solid to liquid).

Claims

1. A method for controlling a cooling device for actively cooling a perovskite solar cell, wherein the perovskite solar cell is part of a perovskite photovoltaic module, the method comprising the following steps: - determining a measure of the internal temperature of the perovskite solar cell, and - activating the cooling device when the determined measure of the internal temperature of the perovskite solar cell is greater than a corresponding measure of a predetermined temperature threshold, wherein the perovskite solar cell has a light-absorbing layer, and the measure of the internal temperature is the actual temperature of the light-absorbing layer of the perovskite solar cell.

2. The method according to claim 1, wherein the measure of the internal temperature of the perovskite solar cell is determined by means of impedance spectroscopy.

3. The method according to any one of claims 1 or 2, wherein the measure of the internal temperature of the perovskite solar cell is determined by means of a thermocouple.

4. The method according to any one of the preceding claims, wherein the cooling device is located substantially on the side of the perovskite photovoltaic module facing away from the sun.

5. The method according to any one of the preceding claims, wherein the cooling device comprises a Peltier element, and a current flow through the Peltier element, upon activation of the cooling device, generates a reduction of the internal temperature of the perovskite solar cell.

6. The method according to any one of the preceding claims, wherein the cooling device comprises a cooling liquid having, at least upon activation of the cooling device, a temperature in an inflow of the cooling device that is lower than the temperature threshold.

7. The method according to any one of the preceding claims, wherein the cooling device has a radiator for emitting heat by heat radiation.

8. The method according to any one of the preceding claims, wherein the cooling device has a plurality of channels in a cover plate of the perovskite photovoltaic module.

9. The method according to claim 8, wherein, upon activation of the cooling device, a cooling liquid flows through the channels and absorbs a part of the thermal energy of the perovskite solar cell.

10. The method according to any one of the preceding claims, wherein the perovskite photovoltaic module has a frame, and the perovskite solar cell is thermally conductively connected to the frame.