Hybrid solar panels in district heating

District heating are a very common type of installation in Northern Europe. Decarbonisation in Europe is a major challenge and district heating are one of the solutions that will accelerate this transition. To achieve this, these grids have to use renewable energy in the generation of the heat they supply through their network, and the integration of solar thermal technology in these grids can also make them interesting in southern Europe, extending their implementation in other countries where they are less common

Traditionally, urban or district heating  have operated in a high temperature range for thermal collectors (typical grid range 70/90). Under these conditions, solar thermal technology is not able to achieve sufficient performance to make the integration of thermal collectors into grids attractive.

The fact that the thermal collectors have lower production due to working at high temperatures can be solved by combining this technology with a water-water heat pump (CHP), which allows the collectors to work at a lower temperature and the CHP to supply heat to the grid at a suitable temperature range (70/90). However, the handicap of this combination is that the electricity consumption of the BdC entails an electricity consumption that did not previously exist, with its consequent associated economic cost. This electricity consumption could be covered by a photovoltaic installation, but the surface area needed for the installation of thermal collectors and the photovoltaic surface area needed to meet the BdC’s consumption requires a sufficiently large space, which is not always available. Therefore, the combination of PVT panels (which generate heat and electricity) with a BdC (see diagram in principle) achieves a triple advantage: the panels work at low temperatures, achieving good performance (both thermal and photovoltaic), on the other hand, the electricity consumption of the BdC is supplied by the photovoltaic generation of the PVT panels themselves and thirdly, the BdC receives heat from the panels at a higher temperature than if the heat comes from the environment or from the ground, improving its COP.

Fig. 1. Schematic diagram of the principle of the PVT + HP system for district heating.

When a thermal collector works in the temperature range of a district heating, its working point is around 0.07 of the horizontal axis of its performance curve (Tm=80 and Tamb = 10ºC and an irradiation of 1000 W/m²), which means that the collector has a thermal efficiency of 30% (see point 1 of fig. 2). However, when the PVT panel is combined with a HP, and within the different PVT typologies, when a PVT with a roof (also called ‘glazed-PVT’) is used, the working point is around 0.015 (Tm=25, Tamb=10 and 1000 W/m², see point 2 of figure 2) which means that the PVT has a thermal efficiency of 60%, i.e. an overall efficiency of 80% taking into account that a PVT generates photovoltaic energy at the same time. These working points can be identified in figure 2, which shows the performance curve of a thermal collector (red line) and the two performance curves of a PVT panel (its thermal efficiency and its total efficiency with the addition of PV production).

Fig. 2. Performance curve of a thermal collector and a glazed PVT with roof (glazed-PVT)

Taking as a reference the production of a thermal collector when working in a 70/90 district heating, connected as a preheating of the return, its annual thermal production in a location such as Madrid, where there is an annual horizontal irradiation of 1,785 kWh/m².year, is 525 kWh/m².year for each square metre of panel installed. In the case where PVT+HP is combined, the system will have a generation of 1,350 kWh/m².year to be fed into the grid. For this, the electricity generated by the panels (385 kWh/m².year) will be self-consumed by the BdC itself, which, in order to provide these 1,350 kWh/m².year, will have a consumption of 345 kWh/m².year). As a result, the PVT+HP system manages to multiply 2.5 times the potential energy generated on a given plot of land.

By performing the same analysis in a location like Würzburg where the available annual horizontal irradiation is 1,127 kWh/m².year, it can be seen that a thermal collector will generate 330 kWh/m².year for every square metre of panel installed. In the case of the PVT+HP system, its thermal contribution to the grid will be 850 kWh/m².year.

In most of the existing networks, the space available is limited, because even though there is plenty of space, the thermal contribution of the networks to the buildings to which they supply heat is very high. Adequate dimensioning so that the generation systems (such as boilers, etc.) are shut down during the summer months helps to be able to carry out the annual maintenance operations required by the installation.

In some district heating networks, especially in cold countries, geothermal ground source heat exchangers are used, which extract heat from the ground and feed it into the heating network. In these cases, the integration of PVT panels is interesting for two reasons: on the one hand, during the winter months the temperature of the ground is reduced by extracting its heat, reaching the operating limits of the HP, which negatively affects its COP, leading to higher electricity consumption. In this case, the fact that the PVTs can provide heat to the ground avoids this limitation by increasing the thermal capacity of the ground, even being able to accumulate surplus heat from the summer, depending on the type of ground. On the other hand, in the central months of the year or when the grid operates at a lower temperature, the heat carried by the panels can be sent directly to the grid, meaning that the HP has to provide less heat, which means fewer wells are needed, reducing the investment required. This heat input to the ground is called ground regeneration.

Currently there are already two district heating in Spain using this system: the network in Ólvega (Soria) with 1,082 m² of PVT panels and in Cuenca with 2,191 m² of PVT panels, the latter being the largest PVT panel installation in Europe.

1090 PVT installed for the Cuenca district heating.