Photovoltaic and Heat pump. Is it true that you save?

In this article, we will try to explain in a specific sequence the most important aspects in choosing a heat pump. If you follow the reading of this article in numerical order, the picture will be clearer at the end.

We will therefore analyze in sequence:

1. What is a heat pump?
2. What are the advantages of a heat pump?
3. Why should I prefer it to a traditional boiler?
4. What types of heat pumps exist?
5. How do I know if a heat pump saves me money?
6. Is it suitable for my heating system with radiators?
7. Without a boiler, how is domestic hot water produced?
8. Does the envelope of my building affect its operation?
9. What would be the ideal system to combine with a heat pump?
10. And what advantage does photovoltaic give me?
11. Can heat pump and photovoltaic work together?

Let’s begin.

1) What is a heat pump?

A heat pump is a system that transfers heat from a low-temperature source to a high-temperature source, using a compressor and a refrigerant. To do this, it does not use any fossil fuel, but only electrical energy and the energy contained in the air or water.

2) What are the advantages of a heat pump?

Heat pumps are more efficient and environmentally friendly compared to traditional boilers, as they use renewable energy sources and have a low environmental impact. Moreover, they are positioned in the garden or outside the house, thus eliminating the traditional boiler from inside your home, with its noises and related dangers due to the presence of methane gas or LPG.

3) Why should I prefer it to a traditional boiler?

The advantages of heat pumps include greater energy efficiency, lower operating costs, higher reliability, and if used properly, a lifespan equal to that of a traditional boiler.

4) How many types of heat pumps exist?

There are different types of heat pumps, such as air-to-air, air-to-water and geothermal heat pumps.

How does an Air-to-Air heat pump work?

We can define an Air-to-Air heat pump system as the common “Air Conditioner” or a more complex system with fan coils powered by a hydronic coil. The heat pump system with fan coils and hydronic coil is a heating and cooling system that uses a heat pump to transfer heat between the water in a hydronic circuit and the building’s air. In this case, the system works as follows: The heat pump extracts heat from the outside air or releases it to the outside air depending on the operating mode (heating or cooling). The hot or cold water is transported from the heat pump to the fan coil’s hydronic coil through a series of additional system components such as inertial buffers and booster pumps. The hydronic coil, which acts as a heat accumulator (just like a car’s radiator), exchanges heat with the water circulating in the hydronic circuit, transmitting or receiving heat depending on the operating mode. To simplify, the hot or cold water circulating in the hydronic coil heats or cools the coil itself. The fan coils, which are air diffusion units present in each room, push hot or cold air onto the hydronic coil which, being hot or cold, consequently heats or cools the rooms. This system combines the flexibility and versatility of the heat pump with the ability to accumulate and distribute heat of the hydronic coil and the adjustable air diffusion of the fan coils, offering high energy efficiency and optimal comfort in the building.

How does an Air-to-Water heat pump work?

An air-to-water heat pump works by transferring heat from the outside air to a refrigerant (the gas contained in the heat pump circuit) which is then used to heat water for heating or domestic consumption, through a heat exchanger that can be located in the outdoor unit (thus a monobloc heat pump) or in the indoor unit (split heat pump). This process occurs thanks to the use of the compressor which increases the pressure and temperature of the refrigerant and the heat exchanger, so that this energy can be transferred to the water. The reverse process is used to cool the water in summer, in order to cool buildings equipped for this technology. In a radiant heating system, the air-to-water heat pump is used to produce hot water which is then sent to radiant floors, walls or ceilings through a network of pipes or pre-fabricated panels. The heat is transmitted to the environment from the warm surface of the radiators or floors, creating a comfortable and uniform heat source. The radiant heating system can be powered by different types of energy sources, but the air-to-water heat pump is particularly efficient because it uses the free energy present in the outside air to produce heat and if powered by a photovoltaic system, it allows to significantly reduce operating costs.

How does a Geothermal heat pump work?

A geothermal heat pump works by transferring heat from the underground to the water used for heating or domestic consumption. This is done through a system of pipes carrying refrigerant fluid, which are buried underground, usually beneath the foundation slab of a house under construction or in adjacent land where the indoor unit of the heat pump is located. Unlike an Air-Water heat pump, the refrigerant fluid is heated by the heat present in the subsoil. For example, if the outside temperature is 0°C, the underground temperature at 1 meter depth is about 12°C. It’s easy to understand that the ground, compared to the air, is giving us 12°C of pre-heating. At this point, the refrigerant fluid is compressed at a higher starting temperature compared to the air, which means that to bring it to the desired temperature, for example 30°C in the case of a radiant system, about 40% less energy is needed. The reverse process is used to cool water in summer. In a Fan Coil, Radiant, or Dehumidification system, there is a need for very cold water. In the case of a radiant system, the summer supply temperature is about 18 degrees, while in the case of a Fan Coil or Dehumidification system, the required water is usually in a range from 7°C to 10°C. If the ground temperature at one meter depth is about 18°C in summer, the starting temperature of the refrigerant fluid will be about 18°C. It’s easy to understand that the energy required to cool the water and bring it to 7/10°C will be much less than that of an Air-Water heat pump which has the air temperature as its starting point, which can be as high as 35°C. As with other types, the geothermal heat pump is electrically powered and uses energy to perform the work of compressing the refrigerant.

How can I know if a heat pump will save me money?

The COP (Coefficient of Performance) of a heat pump is a number that represents the ratio between the electrical energy used to operate the heat pump and the thermal energy produced by it. In other words, the COP indicates how efficient a heat pump is in converting electrical energy into thermal energy. The higher the COP, the greater the efficiency of the heat pump. For example, if a heat pump has a COP of 3, it means that for every unit of electrical energy used, the pump produces 3 units of thermal energy. Let’s suppose that in a given period of time a heat pump consumes 1000 kWh of electricity and produces 3000 kWh of thermal energy. The COP of this heat pump can be calculated as follows: COP = Thermal energy produced/Electrical energy used COP = 3000 kWh/1000 kWh = 3 So the COP of this heat pump is 3, which means that for every unit of electrical energy used, the pump produces 3 units of thermal energy.

But is the COP a fixed value that always applies to that heat pump?

No, the COP is not a fixed value. It depends on various factors, such as the outdoor temperature, the temperature required for heating or cooling, the thermal capacity of the building, and the quality of the installation. In general, the COP of a heat pump varies depending on the operating conditions and can be lower than the value declared by the manufacturer in adverse conditions. However, modern heat pumps have a fairly constant COP during their operating cycle, which means they maintain a high level of efficiency even in adverse conditions.

However, there is a way to evaluate the efficiency of a heat pump over a longer term, and it’s called SCOP, which is essentially the “Seasonal” COP.

The COP (Coefficient of Performance) and SCOP (Seasonal Coefficient of Performance) are both values that measure the energy efficiency of a heat pump. The COP measures the energy efficiency of a heat pump at a specific moment, that is, how much heat is provided for each unit of electrical energy used. The COP varies based on external conditions (outdoor air temperature, humidity, etc.) and the use of the heat pump. The SCOP, on the other hand, measures the average efficiency of a heat pump over an entire season, considering variations in climatic conditions. The SCOP provides a more accurate assessment of the heat pump’s energy efficiency throughout the year, and is therefore a more reliable value for evaluating its long-term energy performance.

Is it suitable for my heating system with radiators?

An Air-Water or Geothermal heat pump can be used to heat a radiator system, but it depends on the type of existing system and its specifications. Generally, a heat pump can be used to heat the water circulating in the radiators, providing heat to the environment, just like a traditional boiler, however, there are aspects to consider. For proper integration, it’s important to verify if the existing system meets the power and sizing requirements necessary for the heat pump’s operation. In some cases, it might be necessary to replace old radiators with larger ones so that they can allow the heat pump to work at lower flow temperatures. If the flow temperature of a heat pump in heating mode is between 25°C and 35°C, you can achieve maximum energy savings and comfort from this technology, as the COP at those temperatures, once the return water from the system has a temperature about 5°C lower than the heated water pushed towards the system (this aspect is called “Thermal Jump”), the heat pump will start its modulation, drastically lowering consumption and using only the energy necessary to recover the 5°C of the Thermal Jump.

Without a boiler, how is domestic hot water produced?

We have said that the heat pump must operate for prolonged periods for its electrical energy consumption to be in favor of the COP. So, for example, if we were to use the heat pump like a boiler to take a shower, to have an instantaneous temperature of 40°C, we would need a heat pump of about 20kW thermal, which with a COP of 3.5 translates to 5.71kWh electrical. At this point, it’s clear and transparent that this approach is impractical as a 15-minute shower would require 5.71kWh electrical and would make the heat pump work at 100%, without even considering that if for some reason, the user closes the water flow, the heat pump would turn off only to turn on again when reopened. For a heat pump, this is certain death in a very short time.

So, how is domestic hot water produced?

Heat is accumulated inside large quantities of water, within a tank. “Split” heat pumps typically come with an outdoor unit (the heat pump) and an indoor unit (which contains the water accumulation and everything necessary to move water in the system circuits). The operation is much smarter with this approach as to heat 200 liters of water from 40°C to 50°C degrees, about 2kWh electrical are needed. And considering that the indoor unit is positioned in a closed environment, the heat of the water will remain there for a long time as the 200-liter tank is very well insulated.

Does the envelope of my building affect its operation?

The envelope of a building consists of walls, windows, doors, and roof, and has a significant effect on the operation of a heat pump. Its quality can affect both the amount of heat that the heat pump needs to produce to maintain the internal temperature of the building, and the amount of heat that is lost to the outside. A well-insulated envelope helps to keep the heat inside the building, minimizing heat loss and increasing the efficiency of the heat pump. In particular, wall insulation and roof insulation help maintain the temperature inside the building, while the use of high-quality windows with good air tightness can help prevent heat loss through the windows. On the contrary, a poorly insulated envelope or low-quality windows can cause heat loss and reduce the efficiency of the heat pump. For example, cracks in walls or windows can cause heat dispersion, forcing the heat pump to work harder to maintain the desired temperature. In general, a well-designed and well-constructed envelope can help improve the efficiency and effectiveness of a heat pump in heating, reducing energy costs and increasing the comfort of the inhabitants.

So in practical terms, how does the heat pump behave if I have a highly insulated envelope?

With a highly insulated envelope, the heat pump will provide the minimum energy input necessary to maintain internal comfort. This translates into a lower flow temperature for the heating system and therefore lower consumption and fewer start-ups for the heat pump.

And what if I have a non-insulated house?

With a dwelling without additional envelope such as the one called “Thermal Coat” and without the presence of windows with “decent” air tightness, the heat pump will stay on longer, with a higher flow temperature to compensate for the building’s energy losses.

And what advantage does photovoltaic give me?

An enormous advantage, but only on one condition: the user’s habits and the parameterization of temperatures in the environments need to be re-evaluated based on the hours when the sun shines and allows us to self-consume our energy.

The use of a heat pump together with a photovoltaic system can bring several advantages, including:

1. Energy savings: The heat pump can use the solar energy produced by the photovoltaic system to heat the house or produce hot water, thus reducing the need to use electricity from the grid.
2. Greater energy independence: With the photovoltaic system producing solar energy and the heat pump using it, the user will be able to reduce their dependence on traditional energy sources.
3. Reduced emissions: The use of solar energy reduces greenhouse gas emissions, helping to protect the environment.
4. Return on investment: A photovoltaic system and a heat pump can offer a long-term return on investment, as energy costs will be reduced over time.

In summary, using a heat pump with a photovoltaic system can lead to greater energy efficiency, increased energy independence, reduced emissions, and a long-term return on investment.

Can heat pump and photovoltaic work together?

This is where it gets interesting. The answer is YES!

There are various approaches and these vary depending on the brands chosen to use. The “Analog” system provides that when there is energy production from the photovoltaic system, the inverter, through an Open/Closed dry contact (which physically is a connection between the heat pump and the inverter) tells the heat pump in a very elementary way.

1. Open contact, I’m producing energy.
2. At that point, the heat pump starts producing domestic hot water and, if it’s winter, also heating the inertial buffer of the heating circuit

The “intelligent” system ensures that the heat pump and the photovoltaic system communicate not through an “Open/Closed Contact” but through a dedicated language that includes many variables. Let’s take as an example the system between SMA inverter and Vaillant heat pump. Vaillant’s eBUS system with SMA photovoltaic works as an integrated energy management system. The SMA photovoltaic system produces electricity from the sun, which is used by the Vaillant heat pump to produce heat and hot water. The eBUS system manages the distribution of electrical energy within the building, optimizing the use of energy produced by the photovoltaic system. Furthermore, the eBUS system uses a communication network to connect all components of the system, including the heat pump, washing machine, dryer, and photovoltaic. This allows for optimal energy management and continuous monitoring of consumption.

For example:

1. There is high production and the energy demand of my building is low
2. The eBUS system automatically raises the temperature of our domestic hot water by 5°C
3. The result is that we have stored the energy that we would have fed into the grid, in our domestic hot water

Another example considering a washing machine and a dryer with eBUS protocol (BOSCH or SIEMENS):

1. I fill the washing machine with clothes
2. I press the Start button for washing
3. The eBUS system, based on the available energy from the photovoltaic and that currently used by the rest of the building, as well as based on weather forecasts, decides whether to start it immediately or postpone it to achieve the highest self-consumption rate for that wash
4. If you’re in a hurry, you can obviously force the start or disable the dedicated function.

In summary, Vaillant’s eBUS system with SMA photovoltaic is an integrated solution that allows for the best use of energy produced by the photovoltaic system, improving energy efficiency and reducing energy costs.