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

Applied Energy 88 (2011) 180–191

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Thermally activated building systems (TABS): Energy efficiency as a function of control strategy, hydronic circuit topology and (cold) generation system B. Lehmann a,*, V. Dorer a, M. Gwerder b, F. Renggli b, J. Tödtli b,1 a b

Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Building Science and Technology, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland Siemens Switzerland Inc., Building Technologies Group, Gubelstrasse 22, CH-6301 Zug, Switzerland

a r t i c l e

i n f o

Article history: Received 22 April 2010 Received in revised form 8 June 2010 Accepted 11 August 2010 Available online 6 September 2010 Keywords: Thermally activated building systems, TABS Concrete core conditioning HVAC control Pulse width modulation control, PWM Hydronic circuit topology Energy efficiency of cold generation

a b s t r a c t 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/m2 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. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Mainly because of low investment costs and favorable comfort, the use of thermally activated building systems (TABS) for the heating and cooling of buildings in recent years has spread widely throughout Central and Northern Europe. With TABS the large thermal capacities of the building structure – such as massive floors and ceilings – is used as energy storage and is thereby integrated in the overall energy strategy of the building. By absorbing radiant and convective energy of heat gains or by release of stored energy, the slabs provide cooling or heating to the room. In contrast to radiant cooling by means of suspended ceiling panels, the dynamic thermal behavior of these elements is exploited. Through the intermediate storage of energy in the slabs, peaks in energy demand are flattened. In addition, there is no need to instantly supply the heating and cooling demand of the room to the slabs. Heat and cold can be transferred with time shift and at power levels which may differ from the actual demand. Moreover, * Corresponding author. Tel.: +41 44 823 47 84; fax: +41 44 823 40 09. 1

E-mail address: [email protected] (B. Lehmann). Present address: Hardeggstrasse 21, CH-8049 Zurich, Switzerland.

0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.08.010

the large areas of the thermo-active surfaces allow for substantial heat flows between room and structure, even for relatively low temperature differences. For these reasons TABS are predestined for the application of low temperature heating and high temperature cooling sources, such as near-surface geothermal energy, groundwater or outside air [1–3]. Within the framework of the research project ‘‘TABS-Control”, an integral approach for the design of TABS and of their control as well as different control strategies were developed [4]. A description of the underlying concepts and the base control strategy can also be found in [5] whereas in [6] an extension to the base control strategy for the intermittent operation of TABS applying pulse width modulation (PWM) is explained. Therefore the control aspects of TABS are quite well treated and different control solutions are available to choose from. Another aspect which has not been examined so far in the same depth is the energy efficiency of TABS. Although several studies on the energy consumption of individual buildings are available, little has been published on the implications of system design and control on the energy efficiency of TABS. A comprehensive analysis of primary energy consumption of TABS is given in [7], however, no hydronic system has been considered. Results of our studies clearly indicate a significant influence of the layout of the

B. Lehmann et al. / Applied Energy 88 (2011) 180–191

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Nomenclature BCS base control strategy C cooling cont. continuous (pump operation) cRt common return (pipe) CTmono cooling tower monovalent Eel,gen electricity demand for cold generation EER energy efficiency ratio H heating IT information technology MCfreeC mechanical cooling with free cooling option MCperm mechanical cooling permanent (without free cooling) N, E, S, W façade orientations north, east, south, west

hydronic circuit topology on energy demand, whose magnitude for real setups has never been shown. Concerning control, it is already mentioned in [8,9] that PWM operation promises substantial energy savings potential of the water circulation pumps. However, this potential has not been quantified so far. Furthermore, a question in dispute has always been which method is most efficient to recool the slabs in the cooling mode, i.e. how to extract and reject the heat from the slabs. The use of a wet cooling tower allows exploiting the free cooling potential of low outside temperatures during night time, whereas a mechanical chiller allows for higher power outputs and therefore shorter runtimes. Therefore, the aim of this paper is to analyze the energy efficiency and energy consumption of TABS in regard to three aspects: (i) what are the benefits of applying PWM control compared to continuous operation of TABS? (ii) What is the influence of the most commonly used hydronic schemes for TABS? (iii) Which cold generation systems, the wet cooling tower or the mechanical chiller, is more efficient to provide cooling for TABS (with outside air as the cold source)? In this paper, quantitative answers to these questions are given for typical office buildings, using combined and transient building and system simulations.

PWM QgenC QgenH QZoneC QZoneH RDN sRt SPF TABS #oa UBB

pulse width modulation cooling energy delivered by the generation systems heating energy delivered by the generation systems cooling energy transferred to zones heating energy transferred to zones recooling during night time separate return (pipes) seasonal performance factor thermally activated building system outside air temperature unknown-but-bounded (method, approach)

2.2. Pulse width modulation (PWM) The purpose of using pulse width modulation (PWM) is to increase the energy efficiency of TABS by intermittent operation of the zone water circulation pumps. In contrast to continuous operation, the heating and cooling energy required is transferred to the slabs only during a part of the time within a whole PWM period. To compensate for the off-periods, the supply water temperature is raised to higher values in the heating mode and to lower values in the cooling mode. Thereby the necessary on-periods for given supply temperatures are calculated using an energy balance calculation. A detailed description of the concept can be found in [6] and in [10], where the advantages and drawbacks of using PWM are listed. In chapter 0 the energy saving potential of using PWM control is analyzed. 2.3. Recooling during night time (RDN)

Within the framework of the research project TABS-Control [4], a new integrated design method for TABS and their control was developed. The basic idea behind this so called ‘‘Unknown-butbounded” or UBB-approach [5] is that the TABS is designed considering the following parameters: thermal building properties, profiles for expected upper and lower heat gain bounds, comfort set points for the room temperature and selected base control strategy (BCS). At the same time, commissioning parameter values for the control strategy are calculated.

The large thermal inertia of TABS allows the temporal decoupling of heat gain storage in the slabs from the energy discharge. Therefore – if the outside air is used as a cold source – recooling is performed favorably during night time, when cooling energy can be supplied most efficiently due to low outside temperatures. The control strategy RDN [4] corresponds to a PWM control with a period length of 24 h. Thereby recooling is started in the beginning of the night (e.g. at 10 pm) and maintained until the slabs are recooled to the necessary start-off temperature (the switch off criterion used depends on return water temperature). If there have been low heat gains during the day, the running hours are quite short and the system is switched off early. On the other hand, with high heat gains, it is possible that the recooling process is extended up to 24 h in the extreme case. In any case recooling is started only once a night.

2.1. Base control strategy (BCS)

3. Hydronic circuit topologies for TABS

As the base control strategy ‘‘outside temperature compensated supply water temperature control” is the most universal and commonly used, it is also applied within the case studies examined in this paper. To improve comfort and/or energy efficiency, it can be extended by optional control parts [6]. Two modules used are described in the following sections. The outputs of the BCS are the appropriate actual operating mode (one of Heating, Cooling, Heating or Cooling, Off) and the supply water temperature set points for heating and cooling. Except for periods without heating and cooling demand (operating mode Off), the system is operated continuously (zone water circulation pumps).

3.1. Systematic and commonly used topologies

2. Applied control strategies

According to Fig. 1 each TABS consists of the three main parts: generation, distribution and consumer loads (rooms, zones). To guarantee thermal comfort, the distribution system has to ensure that in each building part and in each individual pipe-loop the fluid’s appropriate mass flow rate and temperature level can be maintained. Two main results of the UBB-design process are the necessary number of zones as well as adequate heating and cooling curves for each zone. Due to differing thermal losses in common office buildings, at least two zones are usually necessary:

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Fig. 1. Variants for the overall hydronic circuit topology of TABS.

 Zone 1: normal offices or rooms with low window to façade fractions.  Zone 2: corner offices or rooms with high window to façade fractions. For two zones and the most widely used base circuitry ‘‘Distribution with pressure difference controlled pump” and ‘‘Zone control with injection circuit and primary valves‘‘, there exist five

topologies for the overall system (Fig. 1). The single variants differ in the number of distribution- and return pipes that are used:  For the variants 1 and 2 with two distribution pipes at the same time, only heating or cooling can be provided and therefore cannot be chosen for each zone independently.  For the variants 3–5 with three or four distribution pipes, heating and cooling can be provided at the same time.

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 With common zone- and/or distribution-return pipes (cRt) there exists a strong interaction between the zones. Depending on actual temperatures, this may lead to desirable mixing gains or to undesirable mixing losses (cf. Sections 3.2 and 3.3).  For the variants 2, 4, 5 with separate zone return pipes (sRt) there is no mixing of return water of different zones. Among other advantages this can be important to determine slab conditions using the so called purge operation in the case of PWM control. Variant 6 contains a very elementary topology for one zone with subzone valves. As opposed to the simplicity of this hydronic system, control is demanding. One possibility is to use a special kind of room temperature feedback control whose effect is a day-today correction of the room temperature. In combination with a time staggered PWM operation with specific time slots for heating and cooling, the different needs of the subzones can be met. This control strategy is not treated further here. Details can be found in [4]. Detailed advantages and disadvantages of the individual topologies as well as recommendations which topology is most appropriate in a concrete planning situation can be found in [4]. Generally, using a system with two distribution pipes, the requirements cannot be fulfilled (except for variant 6, cf. comment above) and for systems with four distribution pipes the installation effort is very high. This is the reason why the most commonly used topologies are the ones with three distribution pipes, either with common or with separate zone return pipes. Therefore, in the present paper the peculiarities and the energy efficiency of these two variants (3 and 4) are examined in detail.

3.2. Topology ‘‘three distribution pipes with common zone return” Fig. 2 shows general heating and cooling curves for a two zone TABS. In general, these curves and the corresponding supply water temperature setpoint bands differ in width and slope. Usually there also exists an outside air temperature range where the two setpoint bands overlap. Depending on the actual temperature situation of the two zones, different operational states are possible. For some of these operational states at first glance unexpected effects can occur. This evidence shall be explained by means of a typical stationary load situation in winter. Fig. 3 illustrates an example of such temperature conditions. The supply water temperature for zone 1 can be at most 23 °C and the one for zone 2 must be at least 26 °C. In both zones the water cools off by 0.5 K. In other words, heat is provided to all the rooms. Assuming an equal volumetric water flow rate through zones 1 and 2, the common return water

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temperature corresponds to the mean of the zone return water temperatures, i.e. 24 °C. Because the common return water is warmer than the setpoint for the supply water temperature for zone 1, the latter cannot be achieved by admixing common return water to the supply. Therefore, the return water must be cooled to 23 °C for zone 1 and heated to 26 °C for zone 2 to achieve the required supply water temperatures. Based on this example it is shown that:  although both zones exhibit heating demand, in addition to heating, cooling is also required;  if cold generation was blocked for this typical winter situation, the supply water temperature for zone 1 would be too high. As a result, the room temperatures would exceed the room temperature setpoint for cooling, i.e. thermal comfort could not be maintained. Based on a detailed examination of all other possible operation states in the heating/cooling curve chart (cf. [4]), the following generally valid statements for the topology ‘‘three distribution pipes – common zone return” can be formulated. If the common return temperature lies:  between the two zonal supply temperature setpoint bands, then mixing losses occur;  within both or outside both (not between) supply temperature setpoint bands, then so called ‘‘mixing gains” occur, where a desired heat exchange from zone to zone takes place. 3.3. Topology ‘‘three distribution pipes with separate zone return” Fig. 4 illustrates the temperature conditions for the topology with separate zone return pipes found for the same winter situation as in the last paragraph. The supply water temperatures must be at least 22 °C for zone 1 and 26 °C for zone 2. Water cools off by 0.5 K in both zones, i.e. heat is provided to all the rooms. The distribution supply water is heated to the highest demanded value of 26 °C (i.e. demand-controlled, zone 2). To achieve the setpoint temperature for zone 1, cooled off zone return water is admixed to the supply water. In contrast to the topology with common return pipe, here  only heating is required to cover the zone demands;  cold generation can be blocked for this typical winter situation. Based on a detailed examination of all other possible operational states in the heating/cooling curve chart (cf. [4]), for the topology ‘‘three distribution pipes – separate zone return” the following generally valid statements can be formulated. If the return temperature of heated zones is  colder than that of cooled zones, then mixing gains occur;  warmer than that of cooled zones, then mixing losses occur. 3.4. Discussion and conclusions

Fig. 2. Supply water temperature setpoint bands for a two zone plant, limited by heating and cooling curves.

Looking at the above examples, it can be seen that the operating conditions of a whole TABS plant can be heavily influenced by the hydronic circuit topology used. Thereby the energy demand depends on how long a building stays in situations with mixing gains or losses, which itself depends on the building configuration (zone ratio) and the zonal differences of the supply water setpoint bands. If, in addition, the output power of the generation system (e.g. an evaporative cooling tower) is limited, the achievable supply water temperatures also depend on the actual return water temperatures. Therefore the numerical assessment of the energy efficiency of TABS in a particular building is only feasible by combined and

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Fig. 3. Temperature conditions and valve settings for the topology with common zone return for a typical winter situation.

Fig. 4. Temperature conditions and valve settings for the topology with separate zone return for a typical winter situation.

transient building and system simulation, considering also the hydronic circuit topology. Hence this approach was selected in the present study. 4. Cold generation systems In practice different generation systems for the recooling of TABS are applied. The most common energy sources are ground water, ground heat exchangers, outside air and conventional chillers. In Switzerland, with its distinct continental climate, a large number of plants use the relatively low wet bulb temperatures of the outside air for free cooling with evaporative cooling towers, but mechanical chillers can also be found. As the maximum power of cooling towers is lowest during high temperature periods (where much cooling is needed), the question is how significant the assumed advantages of this generation system in terms of energy performance really are in comparison to mechanical chillers. This is evaluated here in the case studies, where the following three variants of cold generation systems are compared (Fig. 5):  Wet cooling tower (evaporative cooler with straight-tube heat exchanger).  Mechanical chiller with intermediate cold storage, during winter time and transitional periods operated in free cooling mode.

Fig. 5. Schemata of cold generation system variants: wet cooling tower monovalent (upper) and mechanical chiller optionally with free cooling bypass (lower).

 Mechanical chiller with intermediate cold storage, operated permanently (without free cooling). 5. Method and fundamentals for the case studies 5.1. Method In order to get meaningful results on the energy performance of TABS, it is necessary to consider whole buildings, including build-

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ing characteristics, system layout, hydronic circuit topology and occupancy patterns. As it is difficult to get data for various operating modes with the appropriate accuracy and time resolution needed for a real building, a case study based on combined building and system simulations with TRNSYS [11] is used. The building model of TRNSYS has been tested extensively. Moreover, the newly developed control strategies for TABS have been validated in laboratory tests [6]. As a consequence this simulation environment permits realistic predictions of the thermal as well as the energetic behavior of the buildings and systems modeled and therefore its use is suitable for the intended case studies. 5.2. Method for evaluation of energy efficiency

Fig. 6. Configuration and orientations of the reference building. Table 2 Main parameters and indicators of the reference building.

The main focus of the case studies is to compare different system setups and control solutions with each other. As conventional generation systems are usually used for heating, the evaluation for the heating mode is performed on a useful energy basis only (output energy of the generation system). In the cooling mode, where with the use of cooling towers and mechanical chillers renewable energy – stemming from the outside air – is involved, the evaluation is also performed on a delivered energy basis, i.e. electricity consumption. Assuming that the energy consumption for other purposes like ventilation or general electricity demand is the same for all variants, these parts are excluded from the comparison.

Building

5.3. Definition of simulation experiments Energy consumption and energy efficiency are analyzed for six different variants. Identifiers and description of the variants are listed in Table 1. The simulations were conducted for the location of Zurich, Switzerland, using the Design Reference Year weather data set of the meteorological station of Zürich airport. As time frame for the simulations a whole year was chosen. 5.4. Reference building and occupancy setup The analysis of the energy performance of TABS is carried out using one representative reference building shown in Fig. 6. The configuration of this reference building is chosen according to the widespread characteristics of office buildings using TABS: two main orientations (north, south) with ‘‘normal offices” along the main façades and ‘‘corner offices” with glazing on two sides at the front faces. The main parameters and indicators of the building are summarized in Table 2. In order to get a realistic setup for the generation systems (cooling), five identical floors are chosen, each of which consists of four corner offices and 12 intermediate offices. With a net area of 36 m2 per room, the total floor area of the building amounts to 2880 m2. As office usage is assumed, the occupancy of the building is defined correspondingly. The internal gains of persons, equipment and lighting are chosen based on valTable 1 Definition of simulation variants. Identifier

Variant

CTmono_cRt CTmono_sRt MCfreeC_cRt

Cooling tower monovalent, common zone return pipe Cooling tower monovalent, separate zone return pipes Mechanical chiller with free cooling option, common zone return pipe Mechanical chiller with free cooling option, separate zone return pipes Mechanical chiller permanent (no free cooling), common zone return pipe Mechanical chiller permanent (no free cooling), separate zone return pipes

MCfreeC_sRt MCperm_cRt MCperm_sRt

a b

Location Dimensions single room module L  W  H Mean U-value of the façades

Zürich 6m6m3m 0.65 W/(m2 K)

Glazing fractiona Normal office Corner office

21% 42%

Solar heat gain coefficient for Glazing alone Glazing with sun-shading

0.41 0.08

TABS configuration Thickness concrete slab Pipe spacing Specific mass flow rateb TABS coverage fraction of floor area Floor/ceiling covering

0.25 m 0.20 m 15 kg/(h m2) 80% Carpet/none

Ratio between glazing area and net floor area. With reference to TABS pipe register area.

ues for group offices stated in the Swiss standard [12]. With respective specific design values of 7, 10 and 10 W/m2, the occupancy defined corresponds to an upper medium level for office buildings. With a blind control engaging the external blinds in the case of direct radiation on a façade, the solar gains are limited. Typical diurnal sequences (profiles) of upper and lower bound heat gains are shown in Fig. 7. Layout and tuning of the TABS are executed according to the UBB method described in [4,5]. As a result of this process, it has been found that all intermediate offices (N, S) as well as all corner offices (NE, SE, SW, NW) can be connected to one of the corresponding hydronic zones 1 and 2. Fig. 8 shows the resulting heating and cooling curves for both zones. Assuming that the thermal losses of hydronic parts, such as pipes and valves, are small in comparison to the total energy transferred to the slabs, the hydronic system is ideally modeled. Pressure drop of these elements is considered implicitly in the dimensioning of the electricity demand of the corresponding pumps and auxiliary drives. 5.5. Parameterization of control strategies Table 3 contains the control strategies applied in the case studies, and in Table 4 the main settings chosen for PWM control are listed. To be comparable with RDN, in mode PWM Cooling, a period length of 24 h is intentionally used. Starting time for both, RDN and PWM Cooling, is fixed at 10 pm. Note that either of the control strategies 1 and 2 are examined with both hydronic circuit variants (cRt and sRt). 5.6. Dimensioning and operation of the generation systems For a fair comparison all cold generation systems are designed in such a way that they are able to meet the comfort requirements

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Fig. 7. Typical profiles of upper (left) and lower (right) bound heat gains for weekdays (corner office south-west).

power of the reference building is 11.9 W/m2/34.3 kW with 24-h operation and 35.7 W/m2/102.8 kW with 8-h operation. To prevent from freezing, the cooling towers in all variants are operated in dry mode during the period from October to April. In the remaining summer period, wet (sprayed) operation is applied. In the heating mode it is assumed that the generation system is able to provide the necessary power as well as the required supply water temperature set points at any time.

Fig. 8. Effective supply water temperature setpoint bands for the two zones of the reference building as used in the case studies. Set point bands valid for weekdays. For zone 2 the setpoint band is reduced to almost 0 K, factually resulting in one combined heating/cooling curve.

Table 3 Applied control strategies. Operating mode

Control strategy Number 1: case study efficiency hydronic circuit topology (chapter 6)

Number 2: case study efficiency PWM (chapter 7)

Heating Heating or Cooling Cooling

BCS, cont. operation BCS, cont. operation

PWM Heating PWM Undefined

RDN

PWM Cooling

Table 4 Main settings chosen for the PWM control module. Control parameter

Symbol

Value

Period length mode PWM Heating Period length mode PWM Undefined Period length mode PWM Cooling Start time PWM period (24 h) Minimal purge timea Minimal switch-on time Minimal shut-off time

DtH DtU DtC T1,24h Dt1,Purge,Min Dt1,Min Dt0,Min

5h 5h 24 h 10 pm 24 min 0 min 0 min

a At the beginning of each PWM period an initial purge phase is needed to determine the actual condition of the slabs. This is decisive for the selection of the appropriate PWM mode.

without additional heating and cooling systems. Thereby the ventilation system is assumed to be operated in a thermally neutral way (no additional heating/cooling input). The design cooling

5.6.1. Cooling tower For the variants CTmono_xx, a plain tube evaporative cooling tower is used. The characteristics in the design point are: dimensioning wet bulb temperature 17 °C, design power 32 kW, operation 24 h per day, recooling efficiency wet operation 0.30, recooling efficiency dry operation 0.06, and efficiency of the heat exchanger used for system separation 0.4. The adaptation of the output power is realized by speed control of the recoolingventilators. 5.6.2. Chiller To minimize system running hours, the dimensioning of the chiller is based on an operation time of 8 h. Following target efficiency values for cooling production systems stated in the Swiss standard [13], the energy efficiency ratio (EER) of the chiller in the design point is chosen to be 6.0 [–]. With 102.8 kW design cooling power, the electrical and mechanical capacities of the compressor are 17.2 kW and 15.0 kW respectively. According to typical values found in practice for this size of chiller, the Carnot cycle efficiency factor – as a quality grade measure of the chiller – is chosen to be 0.5 [–]. In the case of mechanical cooling, the chiller is intermittently operated at zero and full power, thereby charging the cold storage. Because the supply of the zones is effected from the storage, it is independent of the runtime of the chiller. Recooling is provided using a wet cooling tower. In free cooling mode (variants MCfreeC_xx), the same cooling tower takes over the cold production, thereby bypassing the chiller via a heat exchanger. In this case the power output is also controlled via speed regulation of the recooling-ventilator. From dimensioning it has been found that from September to mid May the cooling demand can be provided with free cooling. Therefore mechanical cooling in this case takes place only from mid may until the end of August. 5.7. Dimensioning of auxiliary drives and circulation pumps The dimensioning of the auxiliary drives is based on the chosen equipment, e.g. for the cooling towers. For the dimensioning of TABS circulation pumps, experience values of real plants are used.

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The seasonal performance factor is an efficiency measure of the respective cooling system. It is defined as the ratio between the generated recooling energy and the electricity demand for pumps, ventilators and other auxiliary drives. The resulting, comparatively high numbers show that in terms of energy efficiency an optimal system design was chosen, which might be above average practice.

Table 5 Dimensioning of auxiliary drives and circulation pumps. Auxiliary drive/pump

Variant cooling tower monovalent Pump TABS-distribution Pump TABS-zone normal offices Pump TABS-zone corner offices Ventilator cooling tower Spray water pump cooling tower Water-side circulation pump cooling tower Total at full load Variants mechanical cooling Pump TABS-distribution (incl. allowance for pressure drop cold storage) Pump TABS-zone normal offices Pump TABS-zone corner offices Ventilator cooling tower Spray water pump cooling tower Water-side circulation pump cooling tower Electric motor compressor chiller Total at full load, mode free cooling Total at full load, mode mechanical cooling a b

Spec. power at full load (W/m2)

Power control at part load

0.85 1.4 0.5 1.7 0.75

0.30 0.63 0.63 0.59 0.26

Yes No No Yes No

0.5

0.17

No

5.7

1.95

1.05

0.36

Yes

1.4 0.5 1.5 0.75

0.63 0.63 0.52 0.26

No No Yesa No

0.62

0.22

No

17.24

5.98

Nob

5.82

1.99

23.06

7.97

Power at full load (kW)

6.1. Thermal energy demand heating/cooling The energy demand of the zones and the energy delivered by the generation systems are shown in Fig. 9. For all variants with separate return pipes, the heating and cooling energy delivered by the generation systems (QgenH, QgenC) is nearly identical to the energy transferred to the zones (QZoneH, QZoneC). For variants with common return pipes, the energy delivered from generation in the heating mode are around 20% higher, in the cooling mode about 17% higher than transferred to the zones. This fact can be attributed to mixing losses, which mainly occur during winter time and transitional periods. In these situations rooms of zone 1 are unnecessarily heated by the high common return water temperature. As a consequence, room temperature in said rooms often rises near to the upper comfort limit. Together with the occurrence of heat gains, there soon results cooling demand, where beforehand heating energy supplied has to be cooled away. The comparison between the same variants with different hydronic system shows that by avoiding these mixing losses, the thermal generation energy demand with separate return pipes is approximately 30% lower in the heating mode and around 20% lower in the cooling mode.

Operating mode free cooling only. PWM operation.

6.2. Electricity consumption for cold generation Considered design values for electricity consumption and part load operating modes of auxiliary drives and circulation pumps can be found in Table 5.

With 2.8 kWh/m2 a electricity consumption for cold generation, the variant CTmono_sRt performs best, followed by the two chiller variants with and without free cooling. Because the lower cooling capacity of the chiller variant with free cooling has to be compensated by much longer recooling runtimes, the difference between the last two variants is relatively small. Also here (electricity demand), substantial savings can be achieved with separate return pipes compared to common return pipes (CTmono_sRt 38%, MCfreeC_sRt 33%, MCperm_sRt 31%). By using separate return

6. Energy efficiency as a function of hydronic circuit topology and cold generation systems In Table 6 the resulting heating and cooling demands as well as the electricity consumption for cold generation are summarized. Table 6 Results compilation of energy and electricity demand. Resulta

Variant CTmono_cRt

Heating energy transferred to zones, QZoneH (kWh/m2 a) Cooling energyb transferred to zones, QZoneC (kWh/m2 a) Heating energy delivered generation, QgenH (kWh/m2 a) Cooling energyb delivered generation, QgenC (kWh/m2 a)

a

c d e f

MCperm_cRt

CTmono_sRt

MCfreeC_sRt

MCperm_sRt

15.9 19.8 18.9 23.0

16.8 21.4 20.6 25.1

13.6 18.2 13.7 18.4

13.6 18.1 13.6 18.2

13.7 18.6 13.7 18.6

Electricity demand cold generation (kWh/m2 a) Generation, free cooling Generation, mechanical cooling Distribution pumps, cooling Zone pumps, cooling

2.0 – 0.8 1.7

1.6 2.0 0.9 1.6

– 4.8 0.2 1.2

1.6 – 0.4 0.8

1.0 2.0 0.4 0.7

– 3.6 0.2 0.5

Total Eel,tot (kWh/m2 a)

4.5

6.1

6.2

2.8

4.1

4.3

16.1 11.9 9.2 5.3

6.7 6.4 5.5 3.8

5.4 5.2 4.7 4.0

14.2 11.8 8.2 6.7

6.4 6.1 5.2 4.4

5.4 5.2 4.6 4.4

Mean seasonal performance factors (SPF) cooling (–) Generationc, cooling period onlye, SPFgen,CP Generationc, total operating hoursf, SPFgen,tot Systemd, cooling period onlye, SPFsys,CP Systemd, total operating hoursf, SPFsys,tot b

MCfreeC_cRt

16.0 20.0 19.2 23.4

All specific values are related to net floor area (not TABS area). Cooling energy displayed as negative values. Consideration limit: auxiliary drives and pumps generation system only. Consideration limit: whole system including generation and distribution. Cooling period with mainly cooling demand lasts from May to mid September. Total cooling time including during winter and transitional periods.

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contrast – with common return pipes – already in winter there is a need for cooling, and the heating demand is also higher during this period. The reasons for the unexpected cooling demand and the elevated heating demand in winter are a consequence of the effects described in Section 3.2. 6.4. Influence on thermal comfort

Fig. 9. Energy demand for heating (positive axis) and cooling (negative axis) and electrical consumption for cold generation.

pipes, the percentage of electricity savings is even higher for cooling than for heating because cooling operations during winter and transitional periods (permanent operation) at low part load and thus low efficiency can largely be avoided. The subdivision of electricity demand on pumps and auxiliary drives of the generation systems is shown in Fig. 10. It can be seen that the additional power demand for mechanical cooling in comparison to the variants with cooling towers is larger than the achievable savings for the distribution and zone pumps and is therefore responsible for the lower performance of the variants with mechanical cooling. As RDN is already a special case of PWM (applied during operating mode Cooling), no substantial reduction potential can be expected for the zone and distribution pumps through application of PWM during the remaining cooling times. The influence of PWM application on electricity consumption in the heating mode is treated in Section 7.1. 6.3. Power output heating and cooling generation systems The reason for the differences in thermal and electricity demand between variants with different hydronic circuit topologies are qualitatively well visible from the temporal devolution of the power output of the generation systems, as shown in Fig. 11 for the example CTmono. With separate return pipes there is less heating demand; and cooling during winter can largely be avoided. In

Fig. 10. Subdivision of the electrical consumption into generation system and auxiliary drives.

Fig. 12 shows the influence of the hydronic circuit topology on thermal comfort. Hourly room temperatures during occupancy times for the normal offices south and north are displayed. For the topology with common return, in winter and transitional periods elevated temperatures up to the upper comfort bound occur. This is due to the indirect heating of rooms of the zone normal offices by the overly warm common return water. In contrast, the variant with separate return pipes shows the expected behavior with temperatures near the lower comfort bound in winter and near the upper bound in summer. 7. Energy saving potential using PWM control The energy saving potential of PWM is analyzed for the same reference case as described in chapter 5. As the main focus is on control influence, only the hydronic circuit topology with separate return is examined. This allows independent purge phases for each zone, which are needed to determine the appropriate PWM mode at the beginning of each PWM period. As generation systems, the variants CTmono, MCfreeC and MCperm are again considered. 7.1. Electricity consumption with PWM control Table 7 contains the evaluation of the energy consumption for heating and cooling with PWM (last three columns). For comparability the values of the variants with BCS are also evaluated and repeated (first three columns, cf. chapter 6). The graphical representation can be found in Fig. 13. 7.1.1. Comparison PWM Heating vs. Heating in permanent operation Because for all investigations an identical heating system was assumed, the electricity demand for heat distribution is nearly identical for all variants. As can be seen from Table 7 and Fig. 13, with PWM the electricity demand for the zone pumps can be reduced by 80%. On the other hand, there is an additional share for the purge phases in intermittent operation. Together with the reduction of the distribution pump share, the overall electricity saving potential for heat distribution using PWM is 55%. 7.1.2. Comparison PWM Cooling vs. RDN As RDN is a special case of PWM, two PWM methods are in fact compared here. The main difference between these two methods is the shut-off criterion they use for the termination of an active cooling phase. With RDN a return water temperature based shut-off criterion is calculated, whereas with PWM Cooling a model based calculation of the cycle runtime is used. Because the return water temperature implicitly contains information on the heat gains of the last day, with RDN a more precise shut-off is possible than with the model based criterion, in which maximum gains always have to be assumed. Besides that, in the case of PWM, there is an additional demand for the purge phases. Consequently, PWM Cooling shows a 10–15% higher overall electricity demand for cold generation and distribution than RDN. As in all cases practically the same level of comfort is reached – and therefore the thermal energy transferred to the slabs is almost identical – the share of electricity consumption for cold generation alone is nearly independent of the chosen PWM method (PWM Cooling or RDN).

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Fig. 11. Power output of heating and cooling generation systems for the variant CTmono. Topologies with common (top) and separate (bottom) return pipes.

Fig. 12. Influence of hydronic circuit topology on comfort in rooms of zone 1 normal offices. Topologies with common (left) and separate (right) return pipes.

Table 7 Result compilation of electricity demand. Comparison continuous operation/RDN vs. PWM. Control strategy and operating modea Heating: BCS cont. Heating/cooling: BCS cont. Cooling: RDN Variant CTmono_sRt

a

Heating: PWM H Heating/cooling: PWM U Cooling: PWM C

MCfreeC_sRt

MCperm_sRt

CTmono_sRt

MCfreeC_sRt

MCperm_sRt

Electricity demand cooling incl. generation (kWh/m2 a)b Generation, free cooling 1.6 Generation, mechanical cooling – Distribution pumps, active cooling 0.4 Zone pumps, active cooling 0.8 Zone pumps, purge op., cooling sharec –

1.0 2.0 0.4 0.7 –

– 3.6 0.2 0.5 –

1.5 – 0.5 1.0 0.3

1.0 2.0 0.5 0.9 0.3

– 3.6 0.2 0.4 0.3

Total cooling operation

4.1

4.3

3.3

4.8

4.6

Electricity demand heating excl. generation (kWh/m2 a)b Distribution pumps, active heating 0.4 Zone pumps, active heating 2.8 Zone pumps, purge op., heating sharec –

2.8

0.5 2.8 –

0.5 2.7 –

0.2 0.5 0.7

0.2 0.5 0.7

0.2 0.5 0.6

Total heating operation

3.3

3.2

1.3

1.3

1.4

3.2

BCS cont.: base control strategy with continuous operation, RDN: recooling during night time, PWM H, U, C: PWM Heating, Undefined, Cooling. Reference area: net floor area reference building. c The split-up of the electricity demand for purge phases on heating and cooling modes is assumed to be proportional to the total runtimes with active heating and cooling respectively. b

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free cooling with cooling towers and by mechanical chillers (with and without free cooling operation), and (iii) different control strategies with continuous and intermittent system operation. The results and conclusions can be summarized as follows:

Fig. 13. Subdivision of electricity demand for cooling (including generation, displayed as positive values) and heating (without generation part, negative values).

7.2. General remarks PWM control and RDN  PWM control is also applicable for hydronic circuit topologies with common return pipes. But in order to determine the slab conditions separately for each zone, a sequential, i.e. temporally staggered, purge operation has to be applied.  With PWM control strategy a considerable share of the electricity consumption is allotted to the purge phases. Depending on the concrete circumstances, different possibilities exist to reduce this share: (1) adaptation of the purge time to the minimally necessary value (considering pipe lengths), (2) perform purge operation every other time or once a day (only for short PWM periods like in the examples) or (3) the application of purging for operating mode PWM Undefined only, and omit it for the other operating modes with clearly distinguishable heating or cooling demands. The latter variant shall be deployed only in combination with room temperature feedback control (cf. following statement).  Owing to the missing feedback and the reduced self-regulation in the case of PWM operation, there is a latent danger of sub-cooling or overheating of the slabs, mainly for longer PWM periods (e.g. 24-h periods). Remedy can be found by applying a special room temperature feedback control [4].  RDN is in principle not restricted to the cooling and can also be adapted to heating. The effects of using RDN in Heating mode can be estimated by comparing the results for Heating with continuous operation against those for Heating with PWM operation (cf. Fig. 13).  As RDN technically corresponds to a return water temperature control, it is not generally suitable in any case. With common return pipes for instance, the return temperature is a mixture of all zones and therefore individual needs of the zones in the case of differing heat gains cannot be identified. Because of inappropriate recooling termination, this can lead to discomfort.  A further drawback of RDN is the high number of parameters of the shut-off criterion, whose adjustment in practice can be difficult.

8. Summary and conclusions In this article, the impact of configuration and operation mode on the energy efficiency of thermally activated building systems (TABS) is analyzed. Compared are (i) hydronic circuit topologies with common and separate return pipes, (ii) cold generation by

(1) Independent of the control strategy applied, the selection of an appropriate hydronic circuit topology is crucial for the energy efficiency of TABS. Owing to the mutual interference of the zones, for the topology with common zone return pipes (cRt), mixing gains and losses occur, of which in typical buildings the latter overbalance. Therefore the topology with separate return pipes (sRt) is the favorable choice, not only in terms of energy consumption, but also concerning comfort performance. For the reference building considered with cRt, the thermal energy delivered by the generation systems in the heating mode is 15–20 kWh/m2 a and 20– 25 kWh/m2 a in the cooling mode. The savings with sRt amount to 30% (heating) and 20% (cooling). Therefore, if considered already in the planning phase, the additional costs of the topology with sRt are negligible and worth investing in. (2) With 2.8 kWh/m2 a electricity demand for cold generation and distribution the variant CTmono_sRt is the most efficient system, followed by the variants with mechanical cooling (MCfreeC_sRt, MCperm_sRt), both with around 4 kWh/m2 a. Also in the case of electricity, with sRt all system variants exhibit 30–40% lower demand for cold generation than with cRt. However, in comparison to the general electricity demand in buildings for IT, lighting, ventilation, etc., all variants exhibit very low consumption. Therefore the decision for one or another cooling system is more a trade-off between very low consumption (cooling tower) and unrestrained power availability (mechanical cooling). In addition, also the building site with its climatic potential for free cooling may play a role [14]. (3) Compared to continuous operation, with PWM control the electricity use for distribution and zone pumps can be reduced by more than 50% (valid for heating as well as cooling). The major part of these savings can be realized under part load conditions where PWM uses available power reserves to reduce the necessary cycle runtimes. Furthermore, with PWM the operating hours can be shifted to favorable times to a certain extent, e.g. to night time or times with low energy prices. This can be realized using special PWM control strategies or configurations like RDN or PWM with 24-h periods. (4) Strictly speaking, the results described in this paper are valid only for the reference case used and the controller settings applied. However, it has been shown that the behavior of zones with sRt is quite independent of the building configuration and that with cRt mainly the situation of normal offices is affected. As the number of normal offices in common office buildings (as in the reference case) significantly exceeds the number of corner offices, it can be expected that for similar building configurations and boundary conditions no major differences for single results, such as the saving potential would occur. Therefore, also the fundamental conclusions remain applicable to any ‘‘general use” office building with a two zone TABS. (5) For cases which considerably deviate from the reference building configuration, occupancy or boundary conditions, there exists a simplified method to determine the energy performance of TABS, depending on the hydronic circuit topology. Prerequisite to using this method is that the generation systems are able to meet the required power demands at any time. In this case the generation energy demand can be evaluated by post processing of the results of single

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zone-(room-) simulations. The formulas for the calculation for a two zone TABS with supply water temperature control can be found in [4]. (6) As already shown in [4] the optimal adjustment of the heating and cooling limits in cases with common return pipe topologies can significantly increase the energy efficiency of TABS. Therefore, carefully conducted optimizing operations after the commissioning of TABS are essential to minimize periods with simultaneous heating and cooling demands and to adapt the control parameters to the effective situation in the building. Methods to adjust and optimize the control of TABS have been developed and can be found in [4]. (7) With an electrical design power of roughly 2 W/m2 for the generation system cooling tower monovalent, a 75% peak power reduction compared to the variants with mechanical chillers (8 W/m2) is possible. Depending on the conditions of sale of the electricity provider, this fact may play a role for the selection of an appropriate cooling system. (8) Compared to the overall energy consumption of today’s buildings, the disclosed electricity demands and the related absolute savings potential seem to be rather small. However, given the necessity to drastically reduce the energy use of both the building stock and of new buildings, TABS, as a proven and available technology, can contribute to this change in the building sector. With the publication of the design and commissioning handbook on the control of TABS [4], one major knowledge gap concerning these systems has been bridged: several automated control solutions for different application fields and configurations are now available and are increasingly deployed in real buildings as, e.g. is shown in [15]. The results in this paper show that there is potential for significant energy savings using adapted system topologies and applying appropriate control solutions. Whereas system topologies have to be considered when designing new buildings, new control strategies can also help to improve the efficiency of existing TABS plants. Therefore, the objectives of future work are to demonstrate these control solutions in real buildings and to disseminate such concepts to operational plants.

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Acknowledgements The partial funding of the project by the Swiss Confederation’s innovation promotion agency, CTI and the input of the project steering committee are gratefully acknowledged. References [1] Koschenz M, Dorer V. Interaction of an air system with concrete core conditioning. Energy Build 1999;30(2):139–45. [2] Lehmann B, Dorer V, Koschenz M. Application range of thermally activated building systems TABS. Energy Build 2007;39(5):593–8. [3] Koschenz M, Lehmann B. Thermoaktive bauteilsysteme TABS. Duebendorf, Switzerland: Empa; 2000. ISBN: 3-905594-19-6 [in German]. [4] Tödtli J, Gwerder M, Lehman B, Renggli F, Dorer V. TABS-control: Steuerung und regelung von thermoaktiven bauteilsystemen. Faktor Verlag Zurich, Switzerland; 2009. ISBN: 978-3-905711-05-9 [in German]. [5] Gwerder M, Lehman B, Tödtli J, Dorer V, Renggli F. Control of thermally activated building systems TABS. Appl Energy 2008;85(7):565–81. [6] Gwerder M, Tödtli J, Lehman B, Dorer V, Güntensperger W, Renggli F. Control of thermally activated building systems (TABS) in intermittent operation with pulse width modulation. Appl Energy 2009;86(9):1606–16. [7] Henze PG, Felsmann C, Kalz ED, Herkel S. Primary energy and comfort performance of ventilation assisted thermo-active building systems in continental climates. Energy Build 2008;40(2):99–111. [8] Meierhans R. Room air conditioning by means of overnight cooling of the concrete ceiling. ASHRAE Trans 1996;102(1):693–7. [9] Deecke H, Günther M, Olesen BW. Velta contec: die betonkernaktivierung. Norderstedt, Germany: Wirsbo-VELTA GmbH and Co.; 2003. [10] Gwerder M, Tödtli J, Lehman B, Renggli F, Dorer V. Control of thermally activated building systems. In: Proceedings: 9th REHVA world congress clima; 2007, Helsinki, Finland. [11] TRNSYS 16.1. Transient system simulation program. SEL, University of Wisconsin, USA; TRANSSOLAR, Stuttgart, Germany; 2006. [12] Technical bulletin SIA Merkblatt 2024. Standard-Nutzungsbedingungen für die Energie- und Gebäudetechnik. SIA, Swiss Society of Engineers and Architects, Zurich, Switzerland; 2006. [13] Standard SIA 382/1. Lüftungs- und Klimaanlagen – Allgemeine Grundlagen und Anforderungen, SIA, Swiss Society of Engineers and Architects, Zurich, Switzerland; 2007. [14] Artmann N, Manz H, Heiselberg P. Climatic potential for passive cooling of buildings by night-time ventilation in Europe. Appl Energy 2007;84(2): 187–201. [15] Gerber Media. Erstes Minergie-P-Industriegebäude der Schweiz, Special Edition. Nachhaltig Bauen, Zurich, Switzerland; 2010.