Saving the gray energy

Germany’s office buildings are outdated. Almost 70 percent of these existing properties are at risk of obsolescence, as the industry magazine “Platow Immobilien” wrote in April 2023. For many owners, full refurbishment would appear to be a virtually incalculable risk, not least due to increasingly stringent energy norms and standards. This is why many opt for the seemingly more straightforward approach of demolition and new construction – a decision that is often made prematurely, as the latest research suggests.

This is because every demolition means wasting large quantities of still functional material and “gray energy”. This is stored in the building until it is demolished and accounts for an average of 50 percent of the energy used over its entire life cycle. The longer a building is therefore used, the better it is for the climate. A pioneering nationwide research project is aiming to find out whether or not, and how, this can be achieved for office properties, even subject to the more stringent conditions that apply to landmarked buildings. The collaborative project is called “Carbon-neutral World Heritage Site Speicherstadt Hamburg”.

Constructed in the period between 1885 and 1927 over an area that is 1.1 kilometres long, the Speicherstadt historical warehouse district is the largest contiguous storage block ensemble in the world. It has enjoyed landmarked site status since 1991 and has been a UNESCO World Heritage Site since 2015, meaning that it is subject to particularly stringent requirements for historical landmarks. But the Speicherstadt is far from just a museum. Commercial businesses, start-ups and restaurants are all vying for space in the world-famous district. Landlord HHLA Real Estate currently lets 300,000 square metres of the total gross floor space of 450,000 square metres.

In addition to its letting activities, however, HHLA Real Estate is also pursuing an ambitious climate target: the Speicherstadt is to be converted into an energy-efficient, climate-neutral district by 2040. Together with the Hamburg Ministry for the Environment and Energy, the companies that run the district are looking for ways to generate renewable energies locally and use them efficiently, all while respecting the landmarked building requirements and taking economic viability concerns into account.

On the roof the historical Warehouse Block H, two gable roof structures made of timber rafters were erected on a tubular steel frame and covered with “hybrid solar roof-mounted systems”. These modules generate both solar power and solar heat. The Speicherstadt district is subject to stringent landmarked building requirements. The roofs are traditionally covered with either sheet copper or slate shingles, typical features of the world-famous brick buildings. As a result, the hybrid solar roof-mounted systems were not allowed to distort this appearance with the light reflections and colour effects of conventional transparent solar cells.

This is why the modules on the two newly erected roof gable structures, each spanning an area of 70 square metres, were designed as completely novel imitations of slate shingles or sheet copper. From the street and the surrounding buildings, it is impossible to tell them apart from the original roof elements with the naked eye. ”In actual fact, they are not made of slate or copper, but of glass,” explains Professor Harald Garrecht from the University of Stuttgart. UV rays from the sun penetrate a transparent layer to generate both electricity and solar thermal energy.

Each module is connected to a system of copper pipes running underneath the module. A frost-proof mixture of water and glycol flows through these pipes and transports the heat to the block’s interior. At the same time, cold liquid is added to the circuit from below for heating. The emissions-free electricity produced by the hybrid roof-mounted modules is used to allow the climate-neutral operation of the entire test facility. In addition to the control electronics, the biggest consumer is a heat pump at the centre of the system. The first stage for the period running up to the end of the year will involve measuring and analysing the electricity and heat output in different weather conditions and seasons.

In the cellar of the warehouse blocks work two thermal energy storage units that operate based on completely different physical principles: an ice storage unit and a concrete storage unit. The hybrid concrete storage unit features a solid core through which water is circulated and which is very well insulated. Once the core has been heated up, it can store heat of up to 70 degrees generated on the roof in summer in the medium term in order to supply the offices in the warehouse unit with what is known as “sensitive” heat for weeks during the transitional period. But once the heat stored in the concrete block has been used up during a prolonged period of cold weather, it can no longer be regenerated there during a sunshine-poor winter. This is when the ice storage unit takes over and makes use of what is known as “latent heat”.

The phenomenon of “phase change” is utilised in a total of twelve cells of the specially developed and novel ice storage type: A heat exchanger extracts the energy from the water in the ice storage unit until it freezes. This changes the phase from liquid to solid, which triggers a latent heat energy boost. This energy boost can be utilised via the heat pump to supply the underfloor heating system. It is precisely in the coldest depths of winter that the use of heat pumps is most profitable.

In the meantime, the roof cells start to slowly reheat the medium flowing through the pipes between the roof and the ice storage unit. It is channelled into the cellar, used to thaw the ice block – and the cycle can begin anew. This method can be used once or twice a week to “harvest” the thermal energy from the ice storage unit, producing around 93 kilowatt hours per cubic metre of water in the storage unit. In terms of heat output, this is the equivalent of no less than 9.3 litres of heating oil. Investigating how efficiently this works means venturing into uncharted scientific territory. The model experiment should now supply reliable data.

The distribution point on the ground floor of the warehouse block is a complex network of pipes, control units, sensors and valves. Connected to the distribution point, and not far from it, is a box the size of a refrigerator: the central heat pump. This is where the pipes running from and to all components of the energy system come together. These include the two energy sources: solar power and solar thermal energy on the roof. Second, the heat flows from the two storage units in the basement that operate based on different approaches: the ice and hybrid concrete storage units. Third, the supply and return pipes in the underfloor heating system belonging to the test open-plan office.

The power lines, which are also supplied by the “hybrid solar roof-mounted system”, cannot be seen. They mainly supply the heat pump compressor via an intermediate storage battery. When the heat pump requests environmental heat from the roof, the liquid medium in the copper pipes is pumped into the roof modules by a circulating pump, where the winter sun and environmental heat heat it up to a few degrees Celsius. On the way back, it reaches the heat pump, which heats the medium to around 30 degrees with a high degree of efficiency and feeds it into the underfloor heating system.

Part of the research workshop is being used to simulate and measure how future tenants in climate-neutral, energy-refurbished Speicherstadt offices will experience their heat supply. This includes an underfloor heating system, although the latest generation of electric infrared heating elements is also being used to cover peak loads during cold winters. “Various discreet interior insulating plaster methods are being tested for thermal insulation that is consistent with the requirements that apply to landmarked buildings,” explains Peter Rosenzweig, project manager at HHLA Real Estate.

All in all, the researchers, HHLA Real Estate and the City of Hamburg are optimistic: all of them hope to be able to develop a “package of measures” that is easy to scale (see interview) for the large-scale climate-neutral thermal refurbishment of the entire Speicherstadt historical warehouse district. “While the test phase is still ongoing,” says Rosenzweig, “a roof like this one and the sort of storage technology we have in the basement could be enough to supply one warehouse block.” And the concept would be even easier to implement as a blueprint for older office properties in other locations that are not affected by the sort of planning restrictions that apply due to landmarked status. “These are prototypical developments and nobody knows what the outcome will be as yet,” says materials researcher Garrecht from the University of Stuttgart. “If they work, however, they could be applied to any normal existing building right away.”

Author: Oliver Driesen

Published 17.5.2024