Reducing peak requirements for cooling by using thermally activated building systems
Introduction
Thermally activated building systems (TABS) can be used for heating and cooling buildings. These systems are a part of the building structure. Therefore, the thermal mass of the building structure is a part of the building's HVAC installation. TABS have pipes to carry water for heating and cooling. The pipes are embedded into the building's concrete floor (see Fig. 1). The integrated functions of heating, cooling, as well as construction allows architects to obtain a much better ratio between the net and the gross volume of a building. TABS allow also to utilize the thermal buffer of the concrete mass.
The purpose of most control strategies for TABS is realizing good comfort together with reduced use of energy. Up to now, there was limited attention given to the benefits of a significant reduction of the required cooling capacity. Furthermore, most of the results are only representative for a single room [1].
The aim of this project is to obtain insight into the required cooling capacity for an entire ‘standard’ office building with and without TABS. We focus on buildings with TABS, where buildings without TABS are used as a reference in order to derive the quantitative reduction of the cooling capacity. We derive general guidelines using simulations. To analyze the predictive performance of the simulated cooling capacity comparisons between measurements and simulations were carried out. The corresponding control strategy has to realize a comfort which satisfies at least a standard level, apart from an active reduction of the cooling power (see Fig. 2).
We present the results in the following way. First we comment on the several strategies for reduction of the peak requirements for cooling (Section 2). Subsequently, the measurement method is presented as well as the methodology to compare the measurements with the (dynamic) computations (Section 3). Dynamic computations are essential to assess the peak requirements for cooling [2]. In the measurements, data were obtained for the cooling capacity of two zones, as well as the capacity of the entire office building. We elaborate on the computational method in the next section (Section 4). Subsequently, the measuring results were compared with the calculations (Section 5). As these results are valid for one specific example, we derive general design guidelines in the next section (Section 6).
The results are representative for a standard office building in temperate/mesothermal climates (group C, Köppen climate classification). The HVAC system comprises mechanical ventilation, including supply as well as exhaust of the air. The air-handling unit includes a heating and cooling battery. Heating and cooling energy is also delivered by the TABS. Several rooms, in combination with an air-handling unit with a cooling battery, were simulated simultaneously. Therefore, the results represent an entire building, instead of a single room. There is no specific cooling machine used for simulating the supply of cooling energy for the TABS. At the point of departure, the necessary cooling power is available.
Section snippets
Strategies for reduction peak requirements for cooling
There are several reasons for reducing the peak requirement for cooling. The main reason is reducing costs. With an optimised installation, the investment for the cooling equipment is lower as are the operational costs. Furthermore, smaller electrical supply power, the use of electricity at night, and higher performance of the cooling system gives lower contract costs.
The cooling peak depends on several variables. Outdoor temperature, internal heat loads, and solar radiation are the main
On-site measurements: methodology
On-site measurements were carried out in the office building “The Thermo-staete” (see Fig. 3) in Bodegraven/The Netherlands [12]. In this building, TABS were applied, as well as chilled ceiling panels. Energy for cooling is provided by an aquifer system. The main building parameters are given in Table 1. The building was in full operation during the measurements. The options for the system boundaries of the simulation model and for placing the measurement equipment into the building were
Computational method
There are several ways to calculate the reduced cooling load for TABS. An overview of these calculations, made for the European CEN 2005c standard (CEN prEN15377, Part 3) is given by Olesen et al. [7]. The methods vary from a ‘Rough sizing method’ to ‘Detailed simulation models’. For calculating the required cooling capacity for the current research project, the ‘Detailed simulation’ method was utilized. For these computations, the TRNSYS simulation package [13] was used.
In order to incorporate
Comparison computational and experimental results
The results of the measurements were compared to the results of the simulations to analyze the predictive performance of the simulation model. Following the approach of Fig. 4, we show the comparison on the zone level, followed by the results for the entire building.
General design guidelines
The results as found in office building ‘The Thermo-Staete’ are promising in terms of the feasibility of the control strategy as given in Fig. 2. To get an insight view into the required cooling capacity for several circumstances, general design guidelines must be drawn up. These guidelines were derived using TRNSYS simulations. We defined a standard office building, where the size of the windows and the internal heat gain were used as variables.
Conclusions
On-site measurements were conducted at an office building equipped with TABS. The cooling process of the TABS was only activated during the nights, when the air-handling unit was switched off. Therefore, the peak requirement for cooling could be reduced. The measured required cooling capacity for this building was 25 W/m2 floor area. The building had an internal heat gain of 22 W/m2 floor area and 30% glass in the façade. The rise of the operative temperature during the day was 2.5 K.
The results
Acknowledgements
The measurements were conducted in the office building “The Thermo-staete”. The authors gratefully acknowledge Klaas de Wit, architect and owner of this building.
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