Systems combining Hybrid Solar Panels (PVT) and Heat Pumps (BdC)

In the world in general and in Europe in particular, there is a trend towards the use of renewable energies and energy efficiency. Of all renewable energies, solar energy is one of those with the greatest potential to reduce emissions in cities and within the different solar technologies, hybrid solar panels (PVT), which generate electricity and hot water, are a clear disruption in the market, called to play an important role in decarbonisation.

On the other hand, the energy we demand in our buildings has to represent the lowest possible energy consumption, which means working on the energy efficiency of our systems. It should be clarified that the energy demand of a building is the energy needed to satisfy the comfort conditions we demand in our buildings (hot water, heating, cooling, etc.) and the building’s consumption is the energy coming from outside the building (and normally measured by electricity, gas, diesel, etc. meters). Therefore, to satisfy the energy demand of a building, we need the most efficient systems possible with the aim of consuming as little energy as possible from outside the building, as this consumption entails a cost not only economically, but also in greenhouse gas emissions. And one of the technologies that has gained prominence in recent years on the road to energy efficiency are heat pumps (CHP), and especially aerothermal energy.

Renewable energies, which have undergone a great penetration in the market during the last decade, are not enough in the short and medium term to satisfy all the energy demand of our cities and countries, so it is necessary that the additional energy that renewable energies are not able to provide, is supplied by systems that are efficient and low polluting.

When you want to reduce the energy consumption of a building it is necessary to take 3 consecutive steps: the first is to use passive measures to reduce energy demand (such as insulation, shading, etc.), the second step is to use renewable energies to provide part of the energy demanded by the building, and the third step is that the energy demanded by the building and which cannot be met by renewable energy must be supplied by an efficient system. In such a way that the energy dependence on the building’s exterior (consumption) is as low as possible.

The second and third steps are related to the building installations combining renewable energy and an efficient system such as PVTs and Heat Pumps (HP) respectively. There are numerous possible combinations between PVTs and CHPs with both air-to-water (aw) and water-to-water (ww) machines and each has its own most suitable application. This article aims to describe some of them.

• PVT + HP_aw as preheating: The first combination of PVT and BdC is simple and can be integrated into most buildings. This consists of cold water coming from the grid being preheated in the solar tank (itself heated by the PVTs) to a certain temperature and then heated to the consumption temperature in another tank (which in turn is heated by a BdC). This BoC can be any of its typologies: aerothermal, geothermal or hydrothermal. These systems can work in parallel with the existing boilers in the building, so that the boiler used for DHW can be replaced by this system.

• PVT + HP_aw with multi-energy tank: in many types of buildings it is necessary to prevent legionella and for this, systems that reach high temperatures are needed. In particular, in the BdC, specific equipment is used that uses refrigerants (such as CO2) to reach temperatures above 70ºC, whose characteristic is that they have a high thermal jump, so they are fed with cold water and make a high thermal jump in the BdC. In the hydraulic circuit of the PVTs, the lower the flow temperature to the panels, the higher their performance, so both technologies work better if they receive water at a low temperature. In order to combine both technologies efficiently, multi-energy tanks with stratification are used to provide water at low temperature in the lower part and high temperature in the upper part. This system is very efficient as the three components (PVT, BdC and tank) work in the right conditions to achieve good yields.

• PVT + compact HP_aw: is a system designed for small DHW consumption (such as single-family houses) when another independent installation is used for the heating and/or cooling system. This system consists of PVT panels connected in the same tank as the DHW DHW boiler. These DHW boilers have their compressor placed on top of the tank and heat the water tank with a coil (condenser) that surrounds the tank. This system has the advantage of being compact and in a single tank the thermal energies from the PVTs and the BoC are provided. This system can combine the renewable source and the auxiliary system in the same tank as long as the consumption in the dwelling is less than 100 l/day (3 people or less in the dwelling) as it is not within the scope of application of the CTE HE-4. The disadvantage is that the solar thermal production works on a tank that keeps the DHW warm, so the thermal and photovoltaic performance is lower than if a previous tank were preheated. As it works at a higher temperature, this system can only be combined with PVTs with a roof, as PVTs without a roof have practically zero performance in these working conditions. This system is interesting in small installations where simplicity and cost prevail over efficiency optimisation.

• PVT + multitasking HP_aw: in those cases of small size (single-family houses) where the DHW, heating and cooling system is integrated in a single system, it makes sense to use PVT with a BdC that can provide the 3 energy demands. This system can include the tank inside or outside the boiler, depending on the space available inside the house. This system has clear advantages such as compactness, since, in a space similar to a fridge, the whole system is included to provide the thermal energy demanded by the home. In addition, if the PVTs are sized so that during the periods of cooling demand (from May to September) the PVTs can supply 100% of the DHW, the machine does not have to change the cooling cycle to heating mode to supply the DHW, so the system, in addition to being more efficient, extends the durability of the machine.

• PVT + high temperature HP_ww: the previous combinations are designed for open circuits (normally DHW) but in closed circuits, and especially when working at high temperature, the combination of PVT with water-water BdC is very suitable. The reason is that the panels work as the cold focus (evaporator) of the BdC, achieving a double advantage: the PVTs work at a lower temperature, improving their performance (both thermal and photovoltaic) and the temperature of the cold focus of the BdC is higher, so the SCOP of the machine is higher and consequently its electricity consumption is lower. This means that the electricity consumption of the BdC is lower than the photovoltaic production of the PVTs and it is an installation that generates a lot of thermal energy at high temperature and has surplus electricity for other uses. This combination is suitable for district grids, industrial processes or even on a small scale for single-family houses. The advantage of using water-to-water BdC is that its cost is significantly lower than air-to-water, but its handicap is that it is not the only system and needs an auxiliary system (boiler, aerothermal, etc.) to provide thermal energy in those months when irradiation is not sufficient.

With the exception of the last case, as a general rule (with exceptions) the sizing of these systems is based on sizing the PVTs to cover between 60% and 70% of the annual DHW demand. Therefore, the BdC will have to provide the remaining thermal energy. This means that the compressor will only work for 30-40% of the demand, which means lower electricity consumption and a longer lifetime. Furthermore, in most cases, this electricity consumption is lower than the photovoltaic production of the PVTs, which means that systems are achieved that provide 100% of the DHW demand and also generate surplus electricity for the building’s consumption.

One of the advantages of using PVT panels compared to traditional photovoltaic panels is that the overall performance of PVT is between 3 and 4 times higher than photovoltaic panels. Therefore, the surface area required to generate the same energy using photovoltaic panels is between 3 and 4 times greater than with PVT (without taking into account the self-consumption factor), which is why in most buildings in our cities we do not have enough roofing to generate so much energy. Therefore, the use of PVT allows us to maximise the energy savings and consequently the economic and emissions savings of our roofs.

Since the entry into force of the Energy Saving Certificates (CAE) in our country, both technologies have an applicable token for the thermal savings achieved and the energy efficiency of the system, which is an economic incentive for this type of installation.