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Themeninfos – A compact guide to energy research

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The technical centre “Gebäude G” at the Biberach University of Applied Sciences. Different types of TABS are built-in here. The building is used for research and teaching.
© Biberach University of Applied Sciences – Stefan Sättele
Thermo-active building systems and heat pumps
Themeninfo II/2016

Simple classification of exergy levels for energy sources and applications in buildings (Concept: IEA-Annex 49)
© Biberach University of Applied Sciences

Experimentally determined steady-state heating and cooling capacities of water-based TABS with a 5 K logarithmic temperature difference between the working medium and room, thickness of concrete slab: 28 cm.
© Biberach University of Applied Sciences

Provision of a non-residential building with concrete core thermal activation (CCTA) and edge strip elements (ESE) in the office spaces. Heating and cooling capacity [kWtherm] and supply temperatures (ST) [°C] for the transfer systems, shown for a week in heating and cooling operation.
© Biberach University of Applied Sciences

Steady-state heating and cooling capacity of a concrete core thermal activation system (total capacity upwards and downwards) with a 5 K logarithmic temperature difference between the heating/cooling water and the spaces above and below in accordance with DIN EN 15377 (heat resistance method)
© Biberach University of Applied Sciences

Heat provided in the building using concrete core thermal activation (supply with ground source heat pumps, temperature level 30/28 °C) and radiators (district heating, temperature level 75/55 °C). As a fast responsive system at a high temperature level, radiators in this case cover most of the heating load and heat consumption, and significantly limit the use of the CCTA.
© Biberach University of Applied Sciences

Thermal indoor comfort during occupancy in a demonstration building. Above: Hourly operative room temperature [°C] during occupancy, shown above the running daily mean for the ambient air temperature [°C] for two years of operation (comfort limits in grey). Below: Hourly operative room temperature [°C] for two selected offices and the ambient air temperature for a warm summer week. The periods of use are indicated with coloured markers.
© Biberach University of Applied Sciences
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Efficiently heating & cooling non-residential buildings

The use of environmental energy to cool or, in combination with heat pumps, to heat non-residential buildings via thermo-active building systems (TABS) has established itself in recent years. The objective of such building and system concepts is not only to consume as little energy as possible in quantitative terms (low energy), but also to achieve the most optimal energy conversion in thermodynamic terms that takes the quality of the energy used into account (low exergy, „LowEx“).

Many successful and well-functioning examples show that with such systems a high degree of thermal comfort in combination with a high energy efficiency can be achieved. Various heating and cooling supply systems for this purpose are available on the market and the most important building simulation programs now have libraries with LowEx components to design these systems. For the planning and operation, product-specific documents as well as standards and guidelines are available.

However, operational experience and the systematic scientific evaluation of a number of projects show that there are still possibilities within the planning, construction and operation phases for better exhausting the efficiency potential. What is often lacking is an optimal operational management of all sub-components as well as a critical analysis of the expended auxiliary energy. Furthermore, in practice this often raises questions as to the optimal control of the entire system in order to simultaneously ensure high efficiency and high workplace comfort.

In a research project (LowEx:Monitor), 25 non-residential buildings were measured, investigated and evaluated in detail over a period of several years of operation. This yielded in a comprehensive cross-analysis of the performance of individual components and systems as well as the thermal indoor comfort and the overall system. The research project was funded by the German Federal Ministry for Economic Affairs and Energy as part of the energy-optimised building (EnOB) research initiative. The aim of this Themeninfo brochure is to provide guidance for optimising the interaction of ground source heating/cooling and thermo-active building systems as LowEx transfer systems in rooms. In addition, benchmark parameters for hydraulic subsystems and the overall system are also provided, which can be used during design, dimensioning and operation and also for quality assurance purposes.

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The low-exergy concept

In the German Energy Saving Ordinance (EnEV), the energy utilisation in buildings is assessed in purely primary energy terms, i.e. quantitatively. Low-exergy concepts go further: the thermodynamic qualities of the provided and utilised energy are harmonised with one other. The more the temperature level of the heat source corresponds with the use, the lower the exergy utilisation.

Low-energy buildings with an energy-optimised overall concept encompassing the architecture, building physics and building services technology have a small heating and cooling energy demand. This can be achieved through a well-insulated and airtight building envelope, the consistently limited solar heat gain(for example, exterior solar shading systems), effective ventilation tailored to the hygienically required air change rates with heat recovery, sufficient thermal storage capacity of the building and limited internal loads (efficient office equipment, use of daylight). Such buildings can largely or even completely dispense with full air-conditioning and the use of chillers while still achieving a high degree of workplace comfort. They provide an ideal application for heating and cooling with thermo-active building systems (TABS), such as concrete core thermal activation or capillary tube mats, in combination with natural heat sources and sinks. The temperature difference between the indoor air and the heat sources for heating or natural heat sinks for cooling is lower than in conventional systems such as boilers with combustion processes. This therefore enables the exergy proportion of the supplied energy flow to be kept as low as possible: these are also known as LowEx systems.

Taking into account the energy quality

Exergy describes the proportion of the total energy of a system or material flow which can perform mechanical work when it is brought into thermodynamic (thermal, mechanical and chemical) equilibrium with the environment. This means, for example, that a certain amount of thermal energy that is present at a high temperature level is more valuable than the same energy content at a lower level. This is because work can only be obtained from the difference to the ambient temperature. The exergy-based approach reveals this difference, while the purely energybased approach evaluates both cases the same.

Currently, the evaluation of the energy utilisation in buildings is based on a consideration of the primary energy. Calculations of the primary energy requirement [EnEV 2016, DIN V 18599: 2013-05] are based on the application of energy balances taking into account all energy conversion steps and the resulting losses. However, this is a purely quantitative approach. Although different forms of energy are assessed differently based on the primary energy factors, there is no comprehensive consideration of the thermodynamic qualities of the required energy flows. This is where so-called low-exergy concepts come into play. The intention is not only to reduce the respective quantities for the demand and supply but also to harmonise the respectively deployed energy qualities with one another. It is only when the quality is taken into consideration that the use of adapted heat sources and sinks is able to take full effect.

While maintaining the necessary underlying conditions (e.g. thermal comfort), the exergy-based optimisation of supply concepts with the corresponding system components is aimed at minimising both the exergy destruction within a component or system as well as the external energy losses. This not only reduces the exergy requirement due to the lower demand for energy but also improves the use of the supplied exergy.

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BINE-Themeninfo II/2016
(PDF, 24 pages, 3,8 MB)

Authors

Dr.-Ing. Doreen Kalz
Fraunhofer ISE

Prof. Dr.-Ing. Roland Koenigsdorff
Hochschule Biberach, IGE

Links

EnOB research initiative
Projects, reports, news and analysis from the energy-optimised construction research field