Thermally Activated Building Systems Research Articles

Evaluation of adaptive thermal comfort models in moderate climates and their impact on energy use in office buildings

Abstract Thermally Activated Building Systems (TABS) are regarded as top-cooling systems rather than full air-conditioning systems. Therefore, adaptive thermal comfort models (ASHRAE55, ISSO74 or EN15251) are supposed to be applicable to TABS buildings, although the comfort model conditions are not necessarily satisfied. This paper investigates whether, for a moderate climate and with the heating and cooling set points chosen according to the adaptive models, the building’s energy use reduces. After all, applying adaptive models, if appropriate, is thought to lower energy use because higher maximum operative zone temperatures T op, max are allowed, compared to the conventional ISO7730 model. For purpose of generality, a building with an ideal heating and cooling system is considered. Analysis of moderate climate weather data reveals a low energy reducing potential for the ASHRAE55 and ISSO74 model, because high reference outdoor temperatures hardly occur. EN15251 on the other hand, allows very high T op, max and will lower the cooling need. A 2-zone building simulation demonstrates a higher cooling need for ASHRAE55 and ISSO74, compared to ISO7730. Because cooling is needed during the whole year, the lower winter T op, max of these adaptive models causes these unexpected results. With real data of warmer years or varying gains, this conclusion holds.

Thermally activated building systems (TABS): Energy efficiency as a function of control strategy, hydronic circuit topology and (cold) generation system

By integrating the building structure as thermal energy storage into the building services concept, thermally activated building systems (TABS) have proven to be economically viable for the heating and cooling of buildings. Having already developed an integrated design method and various control concepts in the past, in the present paper the impact of different aspects of TABS regarding the energetic performance of such systems is analyzed. Based on a simulation case study for a typical Central European office building, the following conclusions can be drawn. The energy efficiency of TABS is significantly influenced by the hydronic circuit topology used. With separate zone return pipes energy savings of approximately 15–25 kW h/m 2 a, or 20–30% of heating as well as cooling demand, can be achieved, compared to common zone return pipes, where energy losses occur due to mixing of return water. A strong impact on energy efficiency can also be observed for the control strategy. Thus, by intermittent operation of the system using pulse width modulation control (PWM), the electricity demand for the water circulation pumps can be reduced by more than 50% compared to continuous operation. Concerning cold generation for TABS, it is shown that free cooling with a wet cooling tower is most efficient, if the cold source is the outside air. Variants with mechanical chillers exhibit 30–50% higher electricity demands for cold generation and distribution, even though their runtimes are much shorter compared to the cooling tower runtimes. In conclusion, the results show that significant energy savings can be achieved using adapted system topologies and applying appropriate control solutions for TABS.

Reducing peak requirements for cooling by using thermally activated building systems

This paper presents general guidelines for the required cooling capacity of an entire office building using thermally activated building systems (TABS). By activating the thermal mass of the building using pipes embedded in the floor, peak loads can be reduced. On-site measurements were performed to obtain the required cooling power of an entire building as well as individual zones. Beside this, the internal climate conditions of rooms and surface temperatures of the TABS were measured. The measured data were used to analyze the predictive performance of a simulation model. In order to acquire general guidelines for the required cooling capacity of a standard office building, simulations of an entire building were used to determine the impact of variable internal heat gains and different sized windows. The required cooling capacity was compared to the cooling capacity of a system without energy buffering (e.g. chilled ceiling panels). It was found that reductions up to 50% of the cooling capacity for a chiller can be achieved using TABS. The presented results within this paper can be used as design guidelines in the first stage of a design process. The results focus on temperate climates and were derived using Dutch climate conditions.

Efficiently produced heat and cold is squandered by inappropriate control strategies: A case study

The large thermal time constant of thermally activated building systems (TABS) hampers communication between the system's production and emission. Therefore, conventional building control strategies, typically using room temperature feedback, are unadapted to control thermal comfort efficiently. In this paper, measurement data and simulation results reveal that unadapted TABS control has a dramatic impact on overall energy performance. Measurements in a TABS building with room temperature feedback show the HVAC system switching between heating and cooling in a very short time frame. A simplified, generic room model is used to simulate, understand and evaluate this behaviour. For room temperature feedback control, only 45% of the cold and 15% of the heat produced actually controls room temperature. The remainder is stored in the TABS and exchanged between the heating and cooling system. Enlarging the heating-to-cooling set point band improves this ratio, while maintaining thermal comfort. On the other hand, night time operation control of the circulation pump, adapted to the TABS thermal time constant, eliminates this ‘unused’ energy completely. In this case, however, even with perfect heat gain forecasts, it is difficult to avoid room temperatures dropping below thermal comfort limits during initial office hours. Hence, a supplementary air-conditioning system seems inevitable.

Control of thermally activated building systems (TABS) in intermittent operation with pulse width modulation

Thermally activated building systems (TABS) integrate the building structure as energy storage, and have proofed to be energy efficient and economic viable for the heating and cooling of buildings. Although TABS are increasingly used, in many cases control has remained an issue to be improved. In this paper, a method is outlined allowing for automated control of TABS in intermittent operation with pulse width modulation (PWM). This method represents one part of a TABS control solution with automatic switching between cooling and heating modes for variable comfort criteria which was published before. A first pulse width modulation control solution is derived based on a simple 1st order model of TABS. Then a second, even simpler solution is given that significantly reduces the tuning effort. Finally, the paper outlines a pulse width modulation control procedure and gives two application examples of the PWM control carried out in a laboratory test room.

Control of thermally-activated building systems (TABS)

Integrating the building structure to act as an energy-storage, thermally-activated building system (TABS) has proved to be energy efficient and economically viable for cooling and heating of buildings. However control has remained an issue to be improved. In this paper, a method is outlined allowing both for dimensioning and for automated control of TABS, with automatic switching between cooling/heating modes for variable comfort criteria. The method integrally considers both HVAC and building automation design aspects, as well as the fact that during design and operation heat-gains are unknown, but that bounds of them normally can be specified. This integral method is termed the Unknown-But-Bounded or UBB method. Applying the method guarantees that comfort can be maintained, as long as the actual heat-gains stay within the predefined range between the lower and upper bounds. The UBB method can also handle non-predictable day-to-day variations as well as room-to-room variations of the heat gains. The paper outlines the underlying thermal models and assumptions, and gives the procedure and an example for the application of the method.

Comparison of low-energy office buildings in summer using different thermal comfort criteria

Sustainable low-energy office buildings attempt to harness the buildings architecture and physics to provide a high quality working environment with the least possible primary energy consumption. A promising approach to condition those buildings in summer employs the utilization of the building's thermal storage activated by natural heat sinks (e.g., ambient air, ground water or soil) through night ventilation or thermally activated building systems (TABS). However, a certain room temperature cannot be guaranteed as occupants may influence the room energy balance by window opening, internal heat gains or sun shading control. Between 2001 and 2005, monitoring campaigns were carried out over 2 or 3 years in 12 low-energy office buildings which are located in three different summer climate zones in Germany. These climate zones are defined as summer-cool, moderate and summer-hot. The weather at the building site and the room temperatures in several office rooms were monitored by different scientific teams. The raw data are processed for data evaluation using a sophisticated method to remove errors and outliers from the database and to identify the time of occupancy. The comfort in all office rooms in each building is evaluated separately. For data presentation, these separate comfort votes per office room are averaged using the median instead of the arithmetic mean in order not to overestimate extremely cold or hot room temperatures. A comfort evaluation in these 12 low-energy office buildings indicates clearly, that buildings which use only natural heat sinks for cooling provide good thermal comfort during typical and warm summer periods in Germany. However, long heat waves such as during the extreme European summer of 2003 overstrain passively cooled buildings with air-driven cooling concepts in terms of thermal comfort.