For many users of air conditioning systems, a heat pump is a reversible unit which can cool when it is warm and warm when it is cold. However, heat pumps have many other things and perhaps we need to introduce new names which will tell what the system is actually supposed to do. In some heat pump systems, we will see steam production using recovered heat as source for the heat pump. Some systems using water have been introduced in the market under the type name Mechanical Vapor Recompression (MVR) systems. It is basically also a heat pump but in an attempt to avoid confusion the MVR nomination has been attached to this type of system. A heat pump producing hot water at 90°C using waste process heat at 40°C. This is a heat pump cycle like any other heat pumps the main difference being that you use both the warm side and the cold side, so is this a process heat pump!
A traditional heat pump for domestic and small commercial applications can be seen hanging everywhere in countries with mild to medium warm countries.
What is often referred to as mini-splits are used in the millions, every year. In warm climates you see these systems hanging under the windows in high numbers. Figure 1 is from Beijing, but there are buildings with many more units hanging under the windows. Most of these units are reversible systems either cooling or heating the space in the
apartment. You can say when calculating the Coefficient of Performance (COP) that you are only using the energy that is removed from the occupied space or put in to the space. What if we could use both sides of the process at the
For cooling the COP calculation will look like this:
This means that the high cooling capacity you get for the same absorbed power, the better your system is working. The COP heating is with a good approximation the cooling COP+1. However, if we use both sides of the cycle the equation looks a little different:
This is the calculation you can do for an industrial and larger heat pump in a building where you can produce hot water while also cooling space, be it office or domestic space. In the commercial and industrial systems the heat pump technology has a great interest because you can become more competitive by reducing the production cost by managing more efficiently energy consumption. At the same time you liberate yourself from initiatives from the municipality targeting the air pollution, seen in many cities around the world. Also, the volatile prices for oil and gas will no longer be a concern for the production.
Heat pumps are expensive
When visiting potential heat pump buyers the sales person very often come across with the reservation that heat pumps are very expensive. Heat pumps are an investment which you have to add to the current park of equipment. But you can calculate the benefits in many ways. Here we will use a method shown in a German engineering handbook written by Cordin Arpagaus. The thoughts are published in papers presented at various conferences around the world and can be found on the Internet.
In Figure 2, some calculations are done showing that a heat pump can often be a good investment. However, in Germany the gas price has been very low and the tax on electricity high, which has made the investment a little more difficult, but in spite of this, a return on extra investment instead of a boiler of 3.3 years is still attractive. Larger boilers tend to be closer in price compared to larger heat pumps and small boilers tend to be much cheaper than a comparable size heat pump. In other countries with higher gas price and lower electricity price the business case is much more favorable. If the El/gas price ration comes over 3, it starts to become a little difficult sell, but there are other benefits with a heat pump over the dependence of the gas supply, such as you can optimize the use of electrically driven appliances and if you plan for it you can take advantage of low tariff hours. Also you can be operational much faster when you intend to start the production. Remember that the boiler is running relatively long before you see any hot water leaving the system, the bigger the longer. Heat pumps can be producing heat and hot water much faster depending of the size and water volumes.
In many modern societies, heat pumps are also used a mean for balancing the electric grid. By engaging more or less power you can help the local electricity production become more stable, but this can come to a favorable price. This is attractive to customers that can store hot or cold water for later use in their production. There are many ways to modernize and strengthen the energy systems.
One of the newer systems applied for district heating system in Denmark is using water on the first stage and ammonia on the second stage. Figure 3 shows the concept where an axial compressor lifts the temperature in the water vapor to about 35°C and the condensed water is pumped to the evaporator in the ammonia unit which then is the heat source for the heating of the district heating system. When the total system is up and running the capacity will be 12 MW. The hot water supply to the grid is 65°C. Both compressors are with variable speed drive.
The same concept can be used for producing air conditioning for an office building while producing warm water for the production of taper water or water for cleaning or many other uses. Water vapor compression systems is also being used experimentally for cooling production different machines with good results.
There is also a trend to produce hot water at temperatures over 100°C and we see new solutions, solving some problems. In future, heat pumps will be able to produce water that can be used for production of steam instead of oil or gas. This is a challenging target because many things have to be upgraded to the higher temperature level in the refrigeration system. It is not a problem on the secondary side, since they have been there for many years. Also, the pressures and vibrations in a heat pump higher than in most hot water systems, which is the reason for the challenges to be solved.
With higher temperatures on the secondary side, comes the new lesson to be learned by the heat pump service persons. Hot water is a danger to take very serious. It is not hot shower, steam can become an instant death if not addressed properly. There are many rules for hot water systems, which is a new area of legislation and standards to understand for most heat pump service persons and designers. It makes the everyday more complex and challenging but also an opportunity for the future.
Heat pumps are a new area of opportunities and challenges that can transform the way we perceive and use energy. In future, there will be much more focus on decarbonizing the way the industry produces and use energy. Part of the backbone of the future energy system is that the heat pumps will play a very important role. The change has started and the sooner the modern energy management understands this, the sooner they can start to benefit from the possibilities offered already today.
Emerging in the market are systems using water as the working fluid. These systems can be used both as chillers and as part of the heat pumps systems that will come to the market. Other solutions are also emerging but an analysis of the working fluids will show that hydrocarbons and water are the best solutions for producing high temperatures here meaning over 100°C.
Alexander Cohr Pachai,
Johnson Controls, Denmark