An underground energy reservoir

A brand new needle heat exchanger rests on the floor of the production hall of Retermia Oy in Heinola, ready to travel to Lintuvaara school in Esbo – a school that has undergone a massive indoor air renovation. Right next to it on the floor awaits transportation another piece of equipment. This one is designed for installation on the roof of McDonald’s, where it will recover heat energy from the fast food restaurant’s exhaust air. These devices have three sturdier cousins in Sweden: heat exchanger walls, that transfer heat from the world’s biggest energy reservoir to be used at Arlanda Airport.

Passengers with their bags and compagnions are swarming at Arlanda Sky City. Over twenty million passengers travel via Arlanda every year. They are catered for by 700 enterprises, five hotels and approximately 17500 employees. Around 220 000 take-offs and landings take place annually. The airport has a net surface area of a hundred football fields, about 450 000 square meters.

The energy consumption is also at the higher end of the spectrum. The airport used to consume as much bought energy as a city of 25 000 inhabitants. Then the situation was taken care of with Swedish dedication. And the end result could not be much more ecological.

Nearby Arlanda Airport, deep under ground, lies a post ice age underground lake with a volume of three million cubic meters – nature’s own thermos bottle. Ten wells were drilled in the ground: five for heat and five for cold intake. The principle of heat storage differs from the one in bore wells only in scale, and in that the lake is full of pure water, not heat transfer fluid.

70 similar but smaller energy reservoirs, aquifers, are utilized in Sweden. In other aquifers heat pumps are used for transferring heat and cold. Arlanda decided to use circulating water pumps and heat exchangers instead. In the summer two degree Celsius water is pumped from the lake to the district cooling system. As the water warms up, it returns to the other part of the lake, now twenty Celsius degrees warm.

This energy reservoir covers almost the whole cooling energy demand of the airport in the summer. During the winter the heat of the aquifer is utilized in warming the supply air and keeping the aircraft parking lots free of ice and snow.

The system is designed to store heat and cold energy from one season to another. The return water from the building’s cooling network is also recovered and used in preheating the supply air in wintertime. Therefore the system also transfers thermal energy within the building.

In between, a bit of Finnish technology is needed. Far from the hustle and bustle of Sky City, in an supply air chamber inside thick concrete walls, three walls of needle heat exchangers stand out.

HVAC consultant in leading role

The underground energy reservoir and associated systems were designed by Sweco Ab and the project was carried out by Swedavia Ab. Head consultant for the project, Lars Olof Mattson, gave the specs for the system.

The consultant was looking for coils with a high heat transfer capacity within a wide range of airflow. Minimal air side pressure loss as possible was also of great importance”, recalls MSc.Tech. Markus Castrén, managing director for Retermia Oy, the company that supplied the heat exchanger walls to Arlanda.

According to Castrén, Arlanda is a perfect example of an engineering project, where the HVAC consultant finds an optimum solution to ventilation heat exchange in co-operation with the equipment supplier.

As a whole, the aquifer system at Arlanda has reduced the airport’s total annual electricity consumption by approximately four gigawatthours (4 GWh) and district heat consumption by ten gigawatthours (10 GWh).

Case Arlanda was a really good project. It did require a lot of engineering that was time-consuming for us, but it was definitely worthwhile. It was exactly the kind of project, where product and system simulation truly pay off. Particularly when the piping diagram was such, that there was no rule of thumb for dimensioning the heat exchangers”, Castrén says.

Castrén is glad that similar projects where geothermal energy is utilized in passive heating and cooling of supply air getting more popular.

An engineer’s daydream

Originally it was the father of Markus Castrén, engineer Risto Castrén, who had the idea of developing applications for the needle heat exchange technology in the beginning of 1980s. As its name suggests, the surface of the heat exchanger consists of small needle-like aluminium finns. The needle pipe technology was the foundation for the company Retermia Oy launched in 1982. The company is still based in Heinola, but has grown over the years and the next generation has taken the reigns of the family business.

Initially Retermia focused on developing ventilation equipment for detached houses. At the same time the needle pipe machine and needle heat exchanger applications were further developed by skillfull, innovative teamwork. The spirit prevalent in Finland in the 1980s was very different from the currently prevalent eco-thinking.

My father is innovative and a bit of a visionary. He had the courage and determination necessary to get his ideas through at that time. Air side pressure loss or tightness of ventilation equipment were not given much thought back then. Air handling units were small and no one considered SFP values. And to think of recovering heat – that was just an engineer’s daydream”, Castrén says.

One of the first applications of Retermia’s indirect heat recovery from exhaust air was at Hotel Intercontinental in Helsinki in 1985. It was followed by Hotel Kalastajatorppa’s kitchen application. By the end of the decade run around heat recovering systems had become more common and Retermia still operates with them.


Over the years the needle heat exchanger has been subject to careful modifications. The heat transfer surface has been modified many times, the number of rows has been increased and the face surface altered.

If I have a mission, it is to develop a heat exchanger, in which the relationship of conductance to pressure loss is as high as possible within a large range of air flow rate. When intelligent ventilation systems are designed with these heat exchangers, a high annual thermal efficiency and COP are achieved. This is the kind of efficiency, that will lead to small E values. All along I have strived to accomplish this by developing new products and dynamic simulation programs, which make it possible to dimension and simulate different heat recovery systems and also to calculate annual energy consumption”, Castrén says. The associated software is developed to work in the IDA simulation environment.

Supporting products having to do with energy saving projects are also on the agenda. Retermia is developing a service consept for verifying and maximizing life cycle energy savings.

Unlike conventional heat exchangers, the needle heat exchanger has a linear air side and brine side heat transfer capacity as a function of air and brine velocity. The needle heat exchanger does not have a so-called laminar velocity, where the ability to transfer heat suddenly collapses on the air side nor on the brine side. Castrén particularly emphasizes the importance of a low air side pressure loss.

A crucial difference between the needle heat exchanger and other heat exchangers is that there is no filter before the needle heat exchanger. The needle heat exchanger itself protects the main filter from getting dirty and particularly from getting wet. According to Castrén, in conventional systems, on foggy weather, the unheated air meets the filter just as damp as the outdoor air. This means the relative humidity of the filter equals that of the outdoor air. In other words, the filter is wet and does not function as intended or according to the filter rating. The only way to prevent the filter from getting wet on foggy weather is to preheat the supply air.

Sweden as a forerunner

Retermia has had a representative in Sweden since 1994. Strengthening the Swedish representation is currently on the agenda.

The Swedes are not afraid of new innovative energy saving systems. That has been an advantage to us. A big part of our customer support is collaborating with HVAC consults, finding together the best solution for a particular case. ”

Castrén is delighted to note the recent developments in the HVAC branch. Regulations are stricter and the focus has shifted to the big picture, ie. the building as a whole.

I am thankful for the European Commission’s energy efficiency requirements, which have created a genuine opportunity for developing passive heating and cooling systems and other intelligent energy saving solutions.”

Retermia’s domestic sales have increased in recent years. Export covers 15-20 percent of turnover these days.

The more challenging the project the better for us. We’d rather be forerunners in a niche segment than the world’s largest manufacturer of bulk heat exchangers. We want to be involved in challenging projects like the Arlanda Airport and develop products and engineering service, that is in its own class. Naturally we have sales goals, but economic growth is not what matters most to us – it is the technology”, Castrén summarizes the philosophy of his company.

This story was written by Minna Kärkkäinen and published in Talotekniikka-magazine no 1/ 2015.

English translation by Nina Jaakkola.



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