Blowing hot and cold
11/30/2012 | Words: Katrin Hanger | Pictures: Robert Hack
Combined heat and power plant, Absorption chiller
Nordfrost, a food logistics service provider, operates cold storage facilities throughout Germany, keeping food imports from every region of the world in the best possible condition. Its cold storage centers are organized into different climate zones which are kept at just the right temperature by special chillers powered by the heat given off by cogeneration (CHP) modules from MTU Onsite Energy.
Pineapples are one of Germany’s favorite fruits. They can be bought in any supermarket all year round. But have you ever thought about how far they have traveled from the plantation to the supermarket shelf? And how they still manage to be so fresh? The secret lies in efficient logistics, proper storage and suitable transport things that Nordfrost, based in Schorten in Northern Germany, knows all about. Nordfrost is one of the leading service providers in Germany and Europe in the field of temperature-controlled food logistics. The newest of Nordfrost´s 40 locations is its seaport terminal in the Jade-Weser-Port, Wilhelmshaven, Germany’s only deep-water container port. The location began operations on August 1, 20012, and is currently building up to full capacity. In future, its 23,000m2 of space will be used for temporary storage of fresh fruit and vegetables, sausage, dairy products and general freight goods. Nordfrost will also be able to handle and process animal and vegetable products at the facility. For the fruit and vegetables there are cool stores in which the ideal conditions for storage are maintained. And that is a science in itself because not only is it important to keep the temperature between 1°C and 14°C, the air humidity is a critical factor too.
Energy supplied by modular CHP plants
For energy generation at the seaport terminal, Nordfrost relies on two natural-gas fueled combined heat and power (CHP) plants supplied by MTU Onsite Energy and driven by one 12-cylinder and one 20-cylinder Series 4000 engine. They produce a combined electrical output of roughly 3.1MW and a thermal output of around 3.5 MW; the overall energy efficiency is over 87%. As a result, the site is entirely self-sufficient in energy terms. The electricity generated supplies the power for electrical consumers such as lighting, cold store doors, offices and IT. Any surplus is fed into the public power grid. The heat recovered from the CHP modules is used for heating in winter, but more especially it is used in combination with absorption chillers to provide the refrigeration and cooling systems for the cold stores.
Ammonia as refrigerant
They operate within a sealed circulation system using two substances, a refrigerant and a solvent. The solvent is capable of absorbing the refrigerant, hence the name absorption chiller. In this case the two substances involved are ammonia and water. The recovered heat from the CHP modules provides the energy for the absorption chillers to perform their refrigeration/cooling function by means of a chemical process. The circulation process is divided into four essential stages. In the first stage, water and ammonia are separated from one another by being heated in a desorber. The CHP plants supply the necessary heat at a temperature of 100°C. The ammonia evaporates first because of its lower boiling point and is passed into a liquefier. There it is recooled and liquefied again in the second stage of the process. From there the ammonia passes into the evaporator. This third stage of the process is where the actual refrigeration effect is produced because in order to evaporate, the ammonia must absorb heat. That heat is provided by cold water at a temperature of –2°C (ammonia evaporates at low pressure and very low temperatures) running through coiled pipes. The evaporation process further cools the water to a temperature of –7°C so that it can be used for air conditioning the cold stores. The ammonia vapor then passes into the absorber where it is re-absorbed by the water in the final stage of the process. From there, the ammonia-and-water solution is returned to the desorber and the cycle starts again from the beginning.
Six sites in Germany equipped with CHP plants
Nordfrost employs the principle of combined heat, cooling and power generation at a total of six sites in Germany – using MTU Onsite Energy plants in each case. “Energy is getting more and more expensive – take the higher electricity prices due to come in next year as a result of the renewable energy legislation, for example,” explained Peter Wilke, Technical Manager at Nordfrost. “So we have clearly done the right thing by opting for CHP modules. And we are doing our bit towards preventing climate change by using CHP plants that save CO2”. Nordfrost means logistics solutions from a single source. So, the next time you are standing in front of the pineapple display in the supermarket, you will know what the secret of keeping them fresh is.