The Research Project in Detail

Generating local and renewable energy

‘Solar hybrid roof systems’ have been installed on the roof of Block H. These modules generate both solar power and solar heat. To ensure that the solar modules do not detract from the historical image of the Speicherstadt, methods were developed to make the modules deceptively imitate the existing slate shingles and copper sheets. From the street and the surrounding buildings, they are indistinguishable from the world-famous original roofing.

However, printing the surfaces also means that the modules can capture less solar energy. In order to be ecologically and economically successful, it is therefore necessary to weigh up exactly how heavily the modules need to be printed. True to the motto ‘as much as necessary, as little as possible’. Thanks to this ‘key technology’, it is possible to design the panels in such a way that the strict requirements of monument protection are met.

The first and also visible components are the aforementioned printed photovoltaic modules for generating electricity. The second component of the solar hybrid roof system generates thermal energy, i.e. heat. A heat transfer medium flows through pipes below the photovoltaic modules, which remains liquid until well below the freezing point. The medium is a mixture of water and glycol. It absorbs the ambient heat and feeds it to the heat pump located on the ground floor.

Heat generation with heat pumps

The heat generated on the roof surfaces is fed to the heat pump in the form of the heat transfer medium using a highly efficient circulation pump. The heat pump extracts so much heat from the heat transfer medium that the heat transfer medium cools down by a few Kelvin when returned to the roof. At outside temperatures just above freezing point, the heat transfer medium can reach a temperature of -5°C. The medium supplied to the roof is already heated at the uninsulated riser pipes leading up the roof as a result of the ambient temperature. However, significant warming occurs on the large thermally activated roof surfaces. In addition to the environmental heat gains, condensation also forms along the pipework and on the roof surfaces, which makes a significant contribution to heat recovery as a result of the associated latent heat release. The PV electricity generated on the roofs should be utilised in the best possible way to operate the heat pump. Accordingly, suitable electrical storage systems are the subject of further research in the storage block.

New storage technologies

The heat is stored in two ways - firstly as sensible heat in a new type of hybrid concrete storage tank and secondly as latent heat in a cascade of ice storage tanks. Firstly, the terminology of heat:

• Sensible (tangible) heat:
• If heat is added to a substance (e.g. water) and the temperature of the substance rises, this is referred to as sensible heat.
• This heat can be measured directly by a rise in temperature. The temperature change can be perceived with a thermometer
   
• Latent (hidden) heat:
• In contrast, latent heat leads to a phase change (e.g. melting, vaporisation) without the temperature of the substance increasing
• This heat cannot be measured by a change in temperature, as it is invested in the change in the aggregate state.

A new type of hybrid concrete storage tank was built in the basement of the building. A 6 cubic metre concrete block made of porous (coarse-pored) concrete. The cavities of the porous concrete are filled with water so that the heat stored in the hybrid concrete storage tank is stored in the form of sensible heat from the solid concrete and the water. The heat is stored via six heat exchanger levels arranged over the height of the concrete storage tank, through which the heat transfer medium is channelled in order to charge the storage tank with heat from the thermally activated roofs and discharge it again for later use to provide heating with the heat pump.

Outside temperatures of at least 4°C are required to charge the concrete storage tank. As soon as the temperature drops below this, the second storage system is accessed: the ice storage tanks.

The cascaded ice storage tanks contain our ‘heat reserves’ in the form of liquid water, which we access during the very cold weather periods when temperatures are below zero, as no environmental heat can be supplied to the heat pump via the solar-thermally activated roof surfaces during these periods. These are tanks that are filled with water. In addition to the water, each tank also contains a new type of heat exchanger system, which is characterised by a very large exchange surface. The heat transfer medium, which is exchanged with the heat pump, is channelled through the inner pipes of the heat exchanger. When the ice storage tank is in operation, the heat pump extracts heat from the water. Ice formation begins when the freezing point is reached. The phase change from a liquid to a solid state releases latent heat, which can be utilised by the heat pump to provide excellent heating. Once all the ice stores have frozen through, the heat supply is exhausted, which is why so many ice stores must be provided in the cellar rooms to ensure the heat supply during very cold weather periods. As soon as the outside temperature rises above freezing again, the ice stores can be defrosted again using the ambient heat available on the roof surfaces and thus regenerated for the next cold period.

At plus temperatures, the thermal energy of the ice store can be ‘harvested’ once or twice a week, which results in around 93 kilowatt hours per cubic metre of water in the store. That is the equivalent heat output of 9.3 litres of heating oil. How efficiently this works is uncharted scientific territory. The model test should now provide reliable data.

LowEx- heat transfer

LowEx, short for ‘Low Exergy’, is a concept in the building energy sector that focuses on the efficient use of energy at the lowest possible exergy level (brief explanation of the term: ‘energy’ always refers to the total amount of energy in a system, while ‘exergy’ is the usable energy).

In order to realise an energy-efficient heating system, the water-bearing radiators or underfloor heating systems are operated at the lowest possible temperature level, so that the flow of the heating system is kept at temperatures below 32 °C in order to ensure a high seasonal performance factor with the heat pump. Finally, the thermal energy generated on the roof during the heating period is at a low temperature level. Similarly, the heat obtained from the ice storage tanks during the very cold weather periods is at a temperature level around freezing point.

The water-based heating is supported by electric wall panel heating systems, which are also being trialled. Heating paint and wafer-thin textile heating mats provide pleasant radiant heat at a very efficient level. 

Insulating the building façade and controlling ventilation

The last and equally essential component in this system is the building envelope. It needs to be upgraded in order to minimise heat loss through the façade.

It's hard to imagine a warehouse district wrapped in a polystyrene thermal insulation composite system. We therefore utilise the possibility of internal insulation - and real space technology at that. A high-performance insulating plaster based on Aerogel is used. NASA uses it to insulate its spacecraft, we use it to plaster the interior walls.

A thin layer of just 3cm is enough to reduce heat transfer by 60%. The so-called U-value drops from 1.48 W/(m²K) to 0.58W/(m²K).

The roofing of the inner courtyard provides a further significant effect - the losses on this side of the façade are also reduced with a skilful design that is in keeping with the listed building.

Good building insulation in combination with panel heating already ensures a pleasant indoor climate. This is enhanced by intelligent ventilation control. Sensors measure the indoor air quality and add fresh air accordingly.

The digital building information model serves as the central data basis for integrating all use cases for a holistic assessment and refurbishment of the storage block.

More about BIM here.