Decarbonisation and heat pumps

Keywords: Heat pump, Decarbonisation, Sustainability, Energy sources, Energy carriers

An analysis of the practical meaning of the decarbonisation shows that the mass solution will be insulating the buildings and using heat pumps for heating and domestic hot water. The challenge is renovating all existing buildings so that they reach the same performance of a new building (zero emission level).

Using heat pumps requires attention and low flow temperature heating systems.

The discussion and a simple example show that the right order is: first insulate the building, then install a heat pump for several reasons.

Finally, the process to renovate the whole building stock will require more time than planned due to the limited availability of trained professionals, materials, and finance. The easy renovations have been done in the last decades; the tougher steps are now due.

Why heat pumps

Our world and way of life are based on the availability of plenty of cheap energy. Energy is the ability to do work: if we had no energy doing a lot of work for us, we would still be ploughing our piece of land all day long to get some food on our table every day.

The word “availability” is stressed, because it is not enough to have a given amount of energy in kWh or Joule. You need that given amount of energy:

·         of the right type (eg. mechanical, electrical, heat or heat extraction at a given temperature);

·         at the right time (otherwise you must store energy);

·         at the right place (otherwise you must transport energy).

We often forget the importance of the availability because our current energy distribution system is so reliable that when we press the switch, we are confident that the light will turn on or a motor will start.

With the “Green Deal” EU plans to phase out completely fossil fuels by 2050. This has huge implications on the sources of energy and on energy carriers. Table 1 shows the list of the current sources of energy, their availability and the effect of decarbonisation: deleting the first three lines (natural gas, oil and coal).

Table 1. Energy sources and the effect of decarbonization.

From a global point of view, if you remove fossil sources, you are left with

·         nuclear power

·         hydropower

·         photovoltaic

·         wind power

·         biomass and biogas

It is evident that this is relatively easy in populated countries with a large nuclear capacity (e.g. France) or in low populated countries with huge hydropower resources (e.g. Sweden, Norway), given that biomass and biogas are available in limited amounts and photovoltaic and wind power have a limited time availability. This is a global issue at national level.

Considering buildings, the energy is supplied via an energy carrier. For each required service (e.g. heating, cooling, domestic hot water, lighting), “generation devices” will convert the energy carrier into the desired type of end-use energy.

For several services there is little choice.

·         If you need lighting, you use electricity in a LED. I doubt that you will use a flame to produce some light, like in the past.

·         If you need an air or water flow, you will use a pump driven by an electric motor.

·         If you need cooling, you will use a refrigeration cycle with a compressor driven by an electric motor.

There are indeed alternatives, such as a pump driven by a combustion engine, an absorption heat pump or a compression refrigeration cycle driven by a combustion engine. Their application is limited to special cases, such as emergency systems, isolated installations, heat recovery.

For heating, there are at least three local generation technologies:

·         combustion;

·         heat pumping;

·         electric resistance (Joule effect).

Table 2. Energy carriers and the effect of decarbonization.

As shown in Table 2, if you “decarbonise”, that is you remove fossil fuels from energy carriers, you are left with:

·         electricity;

·         district heating and cooling;

·         biomass;

·         biofuels;

·         other future manufactured fuels (e.g. hydrogen).

Manufactured fuels will very likely be reserved for transports and high power or high temperature industrial applications.

Biomass may be used locally, where it is produced, and the quantity is limited as for biofuels.

District heating is available in some areas.

For the buildings, electricity allows to produce all needed services and it is already available in all buildings. A heat pump is required anyway for cooling purpose and the cooling demand is growing.

The logical conclusion is that the heat pump will be the basic solution for heating, cooling and domestic hot water production. An electric resistance means an inefficient means to provide heat, except for small local energy needs (hand washing in the toilet of an office).

The evil is in the details

The big picture looks clear: the long-term implication of decarbonisation is that heat pump will be the basic heating generation device in the buildings, except some cases where (efficient) district heating or some biomass will be available.

This is not only because heat pumps use an available energy carrier: it is a machine that may transfer a lot of heat using little driving energy. Because of that, it is potentially much more efficient than an electric resistance or a combustion boiler, which are converting energy and thus are limited to 100% efficiency. An electric resistance is a heat pump with COP=1 (but it can achieve any temperature).

However, there are several consequences and practical issues to be considered.

All building professionals (designers, installers, O&M staff) have to switch from boilers to heat pumps. Heat pumps require a much higher level of expertise than boilers. To build a heating system you need a boiler, a pump, some pipes, some radiators and a thermostat. If it does not work, you are a champ. Boilers forgive nearly all mistakes because they are simple, powerful, oversized, low cost, small size. It is a mature technology. Heat pumps require space, they are often noisy, they introduce safety and environmental concerns and they are still much more expensive in €/kW compared to boilers. Additionally, for air source heat pumps their (low) capacity and efficiency decreases exactly when you need them the most, that is during the coldest days.

Adding-up to the required expertise of professionals, the efficiency of the heat pump is extremely sensitive to operating conditions. A poor set-up and commissioning means two digit decrease in efficiency. An example of such a common mistake is using a constant flow temperature instead of applying an (optimised) outdoor temperature reset. Another example are functional diagrams still showing mixing valves to reduce the flow temperature, like with boilers: this should be avoided with heat pumps or reserved for transients.

All end users must learn to use this machine. The heat pump works slowly but constantly at low temperature. No more hot radiators to feel the reassuring immediate warmth of the heating system.

Until full decarbonisation of the electricity production, heat pumps may still use large amounts of fossil fuels. This is true for any electric device. Too often “electric” is considered a synonym of “clean” and “carbon free”. It depends. If you are in France, where electricity is mostly nuclear and hydropower, then replacing a gas boiler with a heat pump (or a combustion engine car with an electric car) brings a near kill of carbon emissions. This is not true in most Europe where electricity is still produced from natural gas and even coal. Additionally, heat pumps run in winter when photovoltaic is quite low forcing to use fossil fuels.

Heat pumps require a heating system with a low flow temperature. One could say that a “high temperature heat pump” is an oxymoron. Each additional degree in flow temperature costs 2..3% in efficiency: it is worth minimising the flow temperature all along the installation. Going backwards along the installation, from the heating terminals to the generator, the flow temperature can only increase (i.e. it can only decrease going from the generator to the heating terminal). To reach the best efficiency you need to:

·         minimise the flow temperature at the heating terminals;

·         avoid any increase in flow temperature going from the heating terminals to the heat pump.

This means:

·         using embedded heating terminals (floor heating, ceiling heating) or radiators sized on purpose. Fan-coils are not optimal but can be of help in situations like historical buildings or when you need cooling and dehumidification;

·         avoiding any mixing of flow water with return water:

o   no 3-way mixing valve shall be used;

o   if an hydraulic separator is used, you shall ensure that the primary flow rate in the generator always exceeds the secondary flow rate in the installation, which means some balancing and using only 2-way control valves (avoid 3-way by-pass valves on terminals).

The standard EN 15450 (Heating systems in buildings. Design of heat pump heating systems) is currently being revised (the public enquiry is ongoing) and will cover in detail all these specific requirements of heat pumps and heat pump systems.

Do not be deceived by some test results. The “medium temperature application” SCOP, which is labelled as “55°C”, is actually the result of an average of COP values tested with a flow temperature of respectively 52, 42, 36 and 30°C according to EN 14825. A heat pump may reach medium to high temperature but this always implies a very low COP and operating with very high refrigerant pressure, which stresses the compressor and the refrigerant piping.

All the above is not to say that heat pumps are not efficient. They are, indeed, but any mistake will compromise their efficiency. The bad results often mentioned about heat pump performance are usually due to wrong design and/or commissioning.

Another concern is the electric grid. Switching from combustion boilers to heat pumps means transferring a huge load from the fuel distribution infrastructure to the electricity grid. The resulting increase of electric loads on the grid will all occur simultaneously in cold days. The grid will have to be reinforced in terms of generation capacity and transmission and distribution lines. If also transports are shifting simultaneously from combustion engines to electric drive, this adds again to the grid issue (not to mention the burgeoning of data centres to support IA use).

Insulating houses

There is some concern because of the recent slow-down of the heat pump market. Is this an unexpected issue?

Installing a heat pump on new buildings is not an issue. A new building is well insulated and can be designed according to the needs of the heat pump technology. However, new buildings are only a small share of the building stock, 1 %/year as an order of magnitude. It would take something less than a century to complete the decarbonisation. The EU target date, 2050 is in 20 years.

The big challenge is installing heat pumps on existing buildings.

The most recent existing buildings are often already equipped with low temperature heating terminals like floor heating. Again, installing a heat pump should not be too difficult. The issue can be finding a suitable location for the heat pump itself (the use of R290 is demanding about that) and for the domestic hot water storage. Then, the electricity supply may need an upgrade.

The real problem is the majority of the existing buildings, which are poorly or not insulated and still equipped with high temperature heating systems, typically radiators.

You may achieve the reduction of the flow temperature of your heating system in 2 ways:

·         by replacing the heating terminals;

·         by insulating the house

Replacing the heating terminals means installing embedded panels, floor heating or ceiling panels. This is quite invasive and expensive and may be not suitable for non-insulated buildings because low temperature heating terminals have a limited capacity in W/m².

Additionally, replacing the heating terminals lowers the flow temperature but keeps the high heating load and needs: you will have to install a large heat pump, requiring an increased electric connection, a lot of space, emitting a lot of noise. And you are still using a lot of energy, which is not compliant with the zero emission building objective.

The EPBD IV Directive not only asks to decarbonise the buildings, it also asks to reduce the heating needs (energy efficiency first principle). This means decently insulating all existing buildings, at least to a level comparable to a current new building. This makes sense, because renewable and carbon-free energy are still rare, poorly available and often expensive commodities.

Decarbonisation requires two main tasks: insulating buildings and installing heat pumps. So, which is the right order? What should be written in the Building Renovation passport?

If you start by insulating the building then:

·         you achieve a low flow temperature even with the existing heating terminals;

·         you only need a small heat pump.

It has also to be noted that insulating a poorly insulated house may cut the energy need by a factor 3 to 4. This means that even if the boiler is not replaced, the fossil fuel use will be reduced by the same factor.

On the contrary, installing a heat pump on the same uninsulated building, in the typical context of electricity produced mostly by fossil fuels (in winter, little PV is available) will reduce the fossil fuel use by some tens of percent. But this very limited result will cost an expensive oversized heat pump that will be useless when you will eventually insulate the building.

A simple example

A calculation has been performed on an actual existing building to show which should be the typical priority.

The building shown in the Figure 1 is located in northern Italy. Desing temperature is −5°C and the heating season is around 2,300 degree-days (20°C reference temperature).

The building was erected in 1920…1930. Walls are 2 lines of bricks (26 cm thickness plus mortar). Windows are single glazing wooden frame. The ceiling to the underroof is a light uninsulated panel hanging on wooden beams. The heating system is a conventional boiler with radiators and a room thermostat. The apartment on the last floor is being refurbished. The heat load is around 15 kW due to the high losses of the ceiling towards the under-roof. Some details are shown in figures 1 and 2.

Figure 1. The building and a detail of a radiator and window.

Figure 2. The underroof.

A low temperature floor heating systems cannot be installed, due to the limited height of the floors, uneven floorboarding and the limited weight bearing capacity of the structure.

Trying to install a heat pump on the uninsulated building would still require a very high flow temperature as shown in Figure 3: the required average flow temperature in January is around 70°C. Even accepting that the required huge heat pump (15 kW nominal size for 100 m² apartment) will not be able to provide comfort in the coldest days, the seasonal COP would be a disaster.

Figure 3. The required average flow and return temperature if the building is not insulated.

A staged deep renovation has therefore been designed.

The building is a condominium: there is one owner per each floor. It is difficult to find an agreement, so the renovation will begin with the easy insulation: the roof. The full sequence is:

1)    Insulation of the roof. This the easy, immediately doable, affordable and effective insulation work. It is relevant for the building unit on the upper last floor whilst, for the other floors there are no losses through the ceiling so it is like “already done”.

2)    Replacement of the generator: condensing boiler or heat pump. The option to install a boiler (the last one before natural gas distribution is cut?) is kept because it is economically effective and simpler than finding the space for the domestic hot water storage, the heat pump outdoor unit (hang on the external wall?), the indoor unit and the heating storage.

3)    Insulation of walls and replacement of windows. They are performed together to optimise the connection between walls and windows and avoid thermal bridges.

The intermediate results are summarized in Table 3.

Table 3. Intermediate results of the staged renovation.

This table shows that after the full insulation of the house, even without changing the heating terminals, the required flow temperature in January is only 32°C.

Even if a boiler is still used, the full insulation of the house would cut the energy need, hence the fossil fuel use, by a factor 6.

This type of results is quite common:

·         installing a heat pump on a non-insulated building with radiators results in a huge machine, high risk of discomfort in cold day and poor COP because of too high flow temperature;

·         Insulating the roof (and any other easily accessible walls) bring to non-optimal but acceptable conditions for a heat pump;

·         Completing the insulation of the building allows low temperature operation without replacing the heating terminals (and the distribution network) and the installation of a small heat pump.

Conclusion

The logical conclusion is that the correct order is:

·         first, insulating the houses;

·         then installing heat pumps.

This will allow an immediate reduction of fossil fuel use and reduce the overall cost of the renovation.

In the past, a lot has been done to improve the heating systems: ordinary boilers have been replaced by gas condensing boilers and room temperature control has been improved by e.g. TRVs.

The “easy” and economically advantageous upgrades have been done and are nearly over.

Now it is the turn of the tougher upgrades: building envelope insulation and shifting to heat pump technology. You have to do them in the right order.

·         first reduce the needs by insulating the building envelope. The side effects are low flow temperature even if you keep the existing heating terminals, a much lower size of the heat pump and a much higher comfort of the building (due to higher inner surface temperature of walls, meaning higher operative temperature and better stability of indoor temperature)

·         then install a small heat pump.

Doing the contrary, installing a heat pump in an un-insulated buildings causes a lot of issues:

·         need to replace the heating terminals or very poor performance (COP);

·         need of a large, expensive, heat pump and of a huge power supply connection;

·         no improvement of the indoor comfort;

·         risk of low indoor temperature in peak cold conditions;

·         low fossil fuel emission reduction;

·         high load on the grid.

The right sequence should be detailed for each building in its “Building Renovation Passport”

It is not surprising that the heat pump market had a pause. It started with a lot of incentives (in Italy the absurd and unsustainable level of 110% has been available for a couple of years) when a lot of “easy cases” were still available. Now there has been a general reduction in incentives, people start understanding that operating costs of heat pump are similar to that of boilers and the easy cases are less. It is the turn of the uninsulated buildings with radiators, that require building envelope insulation.

It is not unexpected that the decarbonisation process will slow down. The proposed pace of decarbonisation is overall not sustainable. Even assuming that 20% of buildings in Europe are already “zero emission grade”, it means that the deep renovation of existing building to bring them at the level of a new buildings should occur on 4% of the building stock per year, which is not realistic and sustainable. We miss the workforce, the materials, and the finance.

The easy solutions have been already implemented. It’s time of the expensive part (building envelope and heat pumps) and it will take time to absorb the shock.

There is no doubt on the direction but the schedule has to be revised and made realistic.