Fascinating seawater heat pumps in Esbjerg

That is why we find the seawater heat pumps in Esbjerg so fascinating

In the spring of 2023, two seawater heat pumps become the heart of a new, green energy supply. With their "Formula 1" muscles, the pumps can help repair the power balance and help secure the energy of the future.

As consulting engineers on the project, Added Values made use of complex calculations and new technologies from the very start. In our article, we focus on the seawater heat pumps and what makes them some real "powerhouses".



Left: FAT Test June 2022. Right: Site in Esbjerg 
With permission from DIN Forsyning June 2022

New technologies deployed

Five years ago, the Danish utility company DIN Forsyning in Esbjerg entered into an agreement with, among others, Added Values ​​to work on the detailed, technical design of a new district heating supply to replace Esbjergværket. This power station is scheduled to close in the spring of 2023 after 32 years of operation. Today, Esbjergværket produces approx. 460 MW district heating. However, the waste facility in Esbjerg, Energnist, continues generating heat for the city.

Our initial task in 2017 was to develop an optimization model to calculate which technologies should be deployed and the optimal size of the various new production capacities. Afterwards, these had to be designed in detail - right down to bolts and nuts. Calculations based on prices and security of supply showed that the new district heating supply should ideally be composed of several smaller and very different types of plants, such as primarily heat pumps (operated on seawater), woodchip boilers and electric boilers.

In addition, the calculations showed that in future DIN Forsyning's existing gas boilers and bio-oil boilers primarily function as peak and reserve loads for extra cold winters and in the event of unplanned outages.

By making a phased expansion of these production capacities, it was possible to retain the possibility of including surplus heat after the Esbjerg plant closes, e.g. from Power-to-X.


Layout of heat pump. Source: DIN Forsyning

Today, considering the global development, the choice of seawater-based heat pumps and wood-burning boilers – and the least possible use of gas boilers – was well chosen.

Two powerful heat pumps

We zoom right in on the large seawater-based heat pumps. From the spring of 2023, they are expected to generate around 60 MW of heat for approx. 6000 hours a year. This corresponds to the annual heat demand of up to approx. 16,000 households in Esbjerg. So, they are indeed powerful!

Right now, the heat pump building at the Port of Esbjerg is almost completed. And before long, the two huge heat pumps from MAN Energy Solutions in Zurich will be commissioned. At the same time as the installation of all equipment has begun, the commissioning of the seawater system itself, which supplies the "free energy" to the heat pump, is in progress. This system is thus equivalent to the "free" energy that comes from the air in a normal household heat pump known by must most people.

The seawater-based heat pump is based on the natural refrigerant CO2, which is circulated in a closed circuit in the heat pump. Using this refrigerant, it is among other things ensured that unwanted refrigerant substances cannot leak into the very sensitive "Natura 2000" area, which borders Esbjerg.

The seawater heat pump consists of two identical lines of approx. 30 MW.
They do impress even experienced district heating people who are used to large figures in the MEGAWATT class


Heat pump. Source: MAN Energy Solutions

About muscles and motors

An ordinary, small household heat pump “lives” on cooling an air stream that is sent through an outdoor module. In the same way, the heat pump in Esbjerg “lives” on the supply of 14000 m3 of seawater. An hour! The seawater is sucked in through specially designed filters, which sort out fish without damaging them.

From these filters, the water is pumped 600 meters to the actual heat pumps, where heat is "sucked" out of the seawater. Then the water is returned to the sea; now a few degrees colder. This happens through the approx. 2 x 1500-meter pipes with a diameter of approx. 1.2 meters.

Now the pumps flex their muscles. Because this small seawater temperature drop of approx. 2 ° C is converted by the heat pump to district heating produced at a temperature of 50–90 ° C depending on the demand. So, this water system is subject to enormous forces if, for example, a valve closes quickly. One thing is technique; something else – but equally important – is the design of methods to avoid biological fouling of pipes from mussels and various shellfish. You will get that story another time.


Biological fouling of pipe. Photo: Added Values

Back to the pumps: To achieve the desired district heating temperature, the circulating CO2 in the heat pump must be compressed. The compression process itself takes place in a very advanced turbocharger. These are powerful machines. Because at 100 % load, the compressors increase the CO2 pressure from approx. 35 bar to just over 100 bar. This happens at a flow of well over 100 kg CO2 per second. Each of the two compressors is powered by an electric motor of just over 10,000 kW (13,600 HP).

To put it into perspective: A Formula 1 engine provides approx. 1000 HP. Hence, each of the two electric motors delivers an output equivalent to approximately 13 Formula 1 engines!

The MAN heat pump

The detailed design of the MAN heat pump is, unfortunately, confidential, but the illustration below shows the heart of the system. The hermetically sealed turbocharger is connected to a turboexpander and the large electric motor of 13600 HP. The drive shaft floats; carried by magnets without any kind of oil lubrication. Since 2015, a similar compressor has been the heart of a plant located at a depth of 300 m on the seabed at the Åsgård gas field in the Norwegian Sea.


MAN compressor. Source: MAN Energy Solutions

Heat pumps ensure stable power supply

In Denmark, it makes sense to install more offshore wind turbines. The wind blows almost constantly. However, the challenge is this kind of power generation is not 100 % predictable. It fluctuates "as the wind blows". A heat pump can balance the fluctuations.

Let us look at an example: On the 400 MW wind farm Horns Rev 3, a wrong forecast at a local wind speed from Danish DMI of 1 m/s could easily lead to a generation deviation from the expected of, for example, 20 MW. If, based on a DMI forecast, a generation of 300 MW is expected, the turbine will only generate 280 MW, because the wind speed is 1 m/s lower than forecast.

In such a situation, it is imperative that a power station increases its load on the power grid literally momentarily. Alternatively, the situation can be repaired or balanced by a heat pump reducing the power consumption in a controlled manner.

Such a balancing of the power generation and consumption is becoming increasingly important as we expand with more volatile power generation such as wind turbines and solar cells.

Without complete control of the power balance, the power grid can ultimately collapse. Therefore, it is important to Energinet – the company responsible for the power balance in Denmark – that e.g. heat pumps can be regulated quickly up / down in load to repair a volatile power generation.

 Formula 1 and FAT test

And now we return to the heat pump plant in Esbjerg. This is precisely tailored to assist Energinet balancing the power: The heat pump can quickly switch from full load to minimum load and in fact also shortly after return to full load.

By braking up, each of the two electric motors may "throw" 5 MW power consumption and a minute later it may increase the load to 10 MW again. And if we jump back into the Formula 1 car: with its 1000 HP it has a formidable acceleration and braking ability; but so do the two frequency-controlled 13000 HP electric motors!

The heat pumps will be put into operation in the coming months. In May 2022, Added Values ​​visited MAN in Zurich to attend the factory test of engine / turbo compressor / turbo expander; a so-called Factory Acceptance Test (FAT). In addition to being fascinating to witness the test, it also confirmed the expectation that each heat pump can easily move 5 MW in power consumption within the requirements of what is called Frequency Containment Reserve (FCR) – that is, in less than 30 seconds.

Tricks or thermodynamics?

With soaring energy prices, heat pumps have become even more important. So, we look forward to the plant being put into operation. Seawater heat pumps convert the power from e.g. wind turbines to thrice as much heat by "stealing" a large amount of heat from the sea. The same applies even if the seawater temperature is only a few degrees above the freezing point. Tricks? No – it's just clever use of thermodynamics.

In addition to thermodynamics, we naturally use many other technical disciplines:

Optimal coupling of complex technical processes "paired" with advanced process control, process simulation and calculations, fluid mechanics and dynamics, knowledge of how materials corrode, fouling and heat transmission, and probably more…

Well, the green energy transition requires specialist proficiency, and the understanding of correlations of complex energy technologies. But this is also what makes it exciting and important to work with.



Torkild Christensen on site. Photo: Added Values