Passive Cooling with Low-temperature Heating Systems

Transcription

1 September 2004 Page 1 of 6 Passive Cooling with Low-temperature Heating Systems Pieter de Wilde 1 and Hans van Wolferen 2 1 TNO Building and Construction Research, Delft, Netherlands 2 TNO Environment, Energy and Process Innovation, Apeldoorn, Netherlands ABSTRACT: In the Netherlands people are increasingly getting used to active cooling systems; for instance most new cars and office buildings are equipped with air conditioning units. This trend also impacts expectations regarding houses. However, an increased use of active cooling systems in houses increases energy demands, and therefore is undesirable from a sustainability point of view. Passive cooling is the use of systems that prevent overheating and that try to remove unwanted heat by natural means, for instance through the use of shading, thermal mass, nocturnal ventilation etc. An interesting passive cooling option for the Netherlands is the use of thermal storage in the soil or aquifers. Thermal storage in the soil is already used for heating purposes; in that case systems consist of a heat exchanger in the soil, a heat pump and a low-temperature heating system. This paper explores the feasibility of using such systems for cooling purposes. Conference Topic: 5 Materials and building techniques Keywords: cooling, thermal storage systems, housing 1. INTRODUCTION In the Netherlands the cooling of houses is an issue that is rapidly gaining momentum. Traditionally houses are built without cooling systems. Yet measures and features that are introduced in new houses in order to make these houses more energyefficient (like for instance the use of additional thermal insulation materials, high-performance glazing and tightening of building shells) result in a risk of overheating in summer. Furthermore, occupants seem to expect increased control over indoor air temperatures, as exemplified by the presence of air conditioning units in most new cars and office buildings. In houses this also results in an increased use of air conditioning units, especially simple add-on systems as available in home improvement centers. All over Europe, a doubling of cooling power per inhabitant from 360 to 720 Watt per person is expected for the timeframe (Adnot, 2000). changes seem to increase the number of warm summers and reinforce these developments. However, an increased use of such active cooling systems increases energy demands, and therefore is undesirable from a sustainability point of view. Passive cooling is the use of systems that prevent overheating and that try to remove unwanted heat by natural means, for instance through the use of shading devices, thermal mass, nocturnal ventilation etc. Spiekman and van Dijk (2001) have studied the prospects of different passive cooling strategies for houses in the Netherlands; they recommended the use of thermal storage in the soil or aquifers as an interesting passive cooling option that allows for a substantial reduction of overheating. Thermal storage in the soil is already used for heating purposes. In that case systems consist of a heat exchanger in the soil (or an aquifer), a heat pump and a low-temperature (floor or wall) heating system. This combination already has a high user acceptance. It would be advantageous to use such existing systems for cooling purposes as well, averting the need to install additional systems. However, so far this idea has only be applied in few projects and mostly remains at an experimental stage. This paper presents the results of computational research that has been carried out by TNO in order to study the feasibility of using systems consisting of a heat exchanger in the soil, a heat pump and a lowtemperature (floor) heating system for cooling (de Wilde et al., 2003). 2. OBJECTIVES A number of aspects might hinder the use of thermal storage in the soil, heat pumps and lowtemperature heating systems for cooling purposes: 1. the capacity of a thermal storage system dimensioned for heating purposes might be insufficient for cooling purposes; 2. the use of low-temperature heating systems for cooling might result in problems in the field of

2 September 2004 Page 2 of 6 heat transfer and thermal comfort due to stratification, radiation asymmetry etc; 3. other practical problems, like condensation risk. In order to study the problems related to each of these aspects the following a set of research questions has been developed to drive the research efforts: 1. what capacity is required for a thermal storage system in the soil, when this is used for cooling purposes? 2. what are the effects of using a low-temperature heating system for cooling on: heat exchange between the system and the air in the rooms? stratification effects in the rooms? thermal comfort in the rooms? 3. Is there condensation risk, and if so, where does this risk occur and what can be done to reduce this risk? 4. Are there any other obvious barriers that can be identified and remedied by the computational study? 3. APPROACH In order to answer these questions a computational study has been carried out. The findings of the study have been compared with an experimental project in real practice: the project Hofstad in the city of Houten in the Netherlands. See Figure 1. Project Hofstad, Houten, NL 5. assessment of condensation risks, and possible solutions; 6. analysis of other practical aspects that might hinder the use of thermal storage in the soil, heat pumps and low-temperature heating systems for cooling purposes. 3.1 Heating and cooling load Heating and cooling loads have been obtained for a reference house which is based on the design of the houses of the project Hofstad, in order to enable the comparison of computational results with findings from actual practice. The loads have been calculated by means of hourly simulation, using the transient building simulation tool VA114 (VABI, 1993) For normal climate conditions use has been made of the climate reference for the Netherlands of debilt.try. For warm climate conditions the data of the actual climate of de Bilt in the 1995 have been used. The reference house is assumed to be positioned in the middle of a row. The house is well-insulated (0.100 m of mineral wool in a cavity wall, glazing with an U-value of 1.700). It is modelled as consisting of three zones/spaces: a living room on the ground floor, with two spaces containing bedrooms on top. These spaces will be referred to in the remainder of this paper to as zone 1, 2 and Dimensioning of the thermal storage system Using the heat and cooling load as calculated with VA114 as the required capacity, the tool E.E.D. [Earth Energy Designer] (Knoblich & Partner, 1997) has been used to find the required dimensions for the thermal storage system in the soil. In the Netherlands, three different types of soil exist: clay, sand and silt. In most cases there is a combination of these types, which are present in different layers. Therefore the computational study has worked with average soil properties (which coincide with those of silt). Figure 1: Project Hofstad, Houten, the Netherlands The computational study consists of the following steps: 1. calculation of the heat and cooling loads of one house; 2. dimensioning of the thermal storage system in the soil; 3. assessment of the efficacy of cooling through a low-temperature heating system; 4. assessment of the impact of cooling through a low-temperature heating system on thermal comfort; 3.3 Efficacy of cooling through a low-temperature heating system and impact on thermal comfort The efficacy and impact on thermal comfort of the use of a low-temperature heating system for cooling rooms requires yet another simulation approach, incorporating air flow, radiative heat exchange, ventilation, transmission, and heating and cooling on the scale of individual rooms. This has been realized by using the CFD simulation engine WISH3D that calculates air flow and temperature distributions, heat and cooling loads, and which generates thermal comfort indices as well.

3 September 2004 Page 3 of 6 The simulations with WISH3D have been limited to a simple rectangular room, assuming symmetry in both length and breadth. Furthermore it has been assumed that the floor, walls and ceiling have a constant temperature. While this limits the capability to analyse radiative heat exchange, this is necessary to allow for manageable air flow calculations. For the cooled floor or wall a surface temperature of 16 o C has been assumed, other walls and ceiling have been set to 22 o C. It is expected that for these relatively extreme values convectional heat exchange will be maximal. 3.4 Condensation risk Condensation risks have been assessed through a comparison of cooling loads and dew point temperatures in the building. Dew point temperatures have been obtained from the relative humidity of the outdoor air as provided in the climate reference debilt.try, assuming an internal humidity production of 2 gr/m3 (related to inhabitants, plants etc). The cooling loads have been obtained from VA Practical aspects In order to obtain an overview of other relevant aspects that might be important in this cooling approach, feedback from inhabitants of the project Hofstad in the city of Houten in the Netherlands has been obtained through the power company REMU that is responsible for the energy supply of these houses. 4. RESULTS 4.1 Heating and cooling load The heating and cooling load of the reference house have been calculated for both an average and a warm. Moreover, the impact of simple passive cooling measures (application of sun shading when there is a risk of overheating, night-ventilation regime) has been studied as well. The results are summarized in Table I. Note that a number of other aspects might influence the calculated loads: the cooling set point, other ventilation regimes, parameters of occupancy etc. For brevity these are not included in this table. Table I: Simple passive measures heating and cooling loads Heating load (kwh/yr) Cooling load (kwh/yr) 1 no average no warm yes average yes warm Depending on actual outdoor climate (average or warm ), ventilation regime, use of sun shading devices etc the cooling load of the reference house varies between 300 and 2400 kwh/yr. The heating load for the same house is in the range of 4500 to 5000 kwh/yr. The results show that the impact of a warm is overwhelming, and can multiply the cooling load with a factor 7. Use of simple passive cooling measures like shading helps to reduce the cooling loads. In warm s of course the proportional impact of these measures is less noticeable. 4.2 Dimensioning of the thermal storage system The heating and cooling loads as calculated with VA114 in the previous paragraph have been used to calculate the required dimensions of the thermal storage system in the soil by means of the tool E.E.D. Results are based on a number of assumptions that correspond to common practice regarding the design of such systems for heating purposes in the Netherlands, for instance: type: double U-tube, 3 in a row, at 5 m distance; transport medium: water with 25% monopropylene glycol. Results are presented in the following tables: Table II presents the dimensions needed for the boreholes and heat exchanger (total length) for heating purposes only; Table III presents the dimensions needed for cooling purposes, however without coverage of peak cooling loads; Table IV presents the dimensions needed for cooling purposes, including overage of peak cooling loads. These have been assumed to be as follows: peak load of each 2.5 kw of 4 hours in June, 6 hours in July, 4 hours in August. See tables. Table II: Dimensions for heating purposes Depth of borehole* Heat exchanger* 1 average warm * The total length of the heat exchanger is 3 times the length of one borehole Table III: Dimensions for cooling, no peak load Simple passive measures Depth of borehole Heat exchanger 1 no average no warm yes average yes warm

4 September 2004 Page 4 of 6 Table IV: Dimensions for cooling, incl. peak loads Simple passive measures Depth of borehole Heat exchanger 1 no average no warm yes average yes warm Table V: Convective heat flows (W/m2) Case A Case B Case C floor facade fr facade bck wall (l) wall (r) ceiling sunspot For an average without peak cooling loads the capacity of a thermal storage system in the soil as designed for heating in the winter period is adequate for cooling during summer as well. In this case the thermal storage system can consist of a borehole of 20 meters containing a heat exchanger in the form of a double U-loop. In a warm or in a with peak cooling loads a higher cooling capacity is required. In this case boreholes of up to 56 meters might be required, assuming the same heat exchanger. Exemplary temperature field 3.3 Efficacy of cooling through a low-temperature heating system Using the CFD-tool WISH3D, the convective heat flow through the surfaces enclosing the room has been calculated for three cases: a. floor cooling; b. wall cooling; c. floor cooling with a sunspot (resulting in mixing). Results are presented in Table V; in this table negative values represent cooling (heat flow directed into the walls/floors), positive values heating (heat flow into the indoor air volume). For all cases air flow velocity field and temperature fields have been obtained as well; Figure 2 shows an example of results for case B. From these results it is clear that using a floor with a low-temperature heating system for cooling purposes results in only low convective heat exchange between this floor and the indoor air. Due to stratification the cooling effect of the floor is reduced. As can be expected, this effect is less troublesome when using a low-temperature heating system that is embedded in a wall. Radiative heat exchange between floor, walls and ceiling however results in a significant indirect cooling effect. Addition of a sun spot results more air flow in the space, resulting in better mixing. I this case all enclosing surfaces contribute to the convective cooling of the space; however, the cooled floor contributes less than the uncooled walls. Figure 2: Exemplary temperature field as simulated with WISH3D 3.4 Impact on thermal comfort The risk of overheating of the house under different circumstances is represented in Table VI. Overheating hours for a situation that is cooled with a low-temperature heating system is not given: if properly seized and operated (idealized situation), this results in zero hours for all zones. Table VI: Overheating (hours exceeding comfort temperature) 1: No measures, average : No measures, warm

5 September 2004 Page 5 of 6 3: Simple measures only, average introduce a condensation risk. The critical point depends on the cooling load: supplying high cooling loads requires low surface temperatures, which in turn results in a higher condensation risk. In Figure 4 cooling loads are plotted against dew point temperatures. 4: Simple measures only, warm The impact of cooling with a low-temperature heating systems on the thermal comfort inside the spaces has been assessed trough different parameters: by means of PPD values, operative temperatures and draught risk, as calculated for fields inside the space by WISH3D. An example on the operative temperatures for case B is given in Figure 3. Results from the calculations show that in all cases: -10% < PPD < 10% 22.5 o C < T operative < 24.5 o C DR < 15% Exemplary operative temperature field Figure 3: Exemplary operative temperature field as simulated with WISH3D In general, while the convectional heat exchange between a cooled floor and the indoor air is low, the low temperature of the floor has a positive impact on the radiative temperature. This results in an operative temperature (or comfort temperature) that is substantially lower than that of a house without a cooled floor. Of course, radiative asymmetry must be avoided, as well as temperature differences between floor and indoor air that result in unacceptable stratification effects. Draught effects are not likely to cause problems. 3.5 Condensation risk Differences between indoor air temperature and the surface temperature of cooled floors or walls Constante T-regeling A B Dauwpuntsregeling C1 Figure 4: Cooling load versus dew point temperature In order to prevent condensation two control settings can be applied to the system. The optimal solution is to use dew point regulation; in this case the temperature of the cooling water is limited in such a way as just to prevent condensation. This is represented by the diagonal line in Figure 4. Only in a limited number of instances (area marked C and D) does this lead to insufficient cooling. A more simple control mechanism is to limit the cooling water temperature to a constant minimal value (constant temperature regulation, CTR). If the cooling load requires lower temperatures the reduction is stopped at the horizontal line in Figure 4 (area marked B and C). According to general calculation procedures and construction methods for floor heating in the Netherlands (ISSO 49, 2003) this limits the cooling capacity of the reference house to ca 600W per floor (equivalent to 12 W/m 2 ). In a small number of cases (area marked D) there remains a condensation risk. Note that the limitation of the cooling capacity has an important consequence: there is no longer a need to dimension the heat exchangers in the soil according to maximum peak loads. The impact of using an anti condensation control (and consequently limited cooling capacity) on thermal comfort during warm s inside the house as equipped with the passive cooling system and using sun shading is presented in Table VII. Comparison with Table VI shows the improvements obtained with the use of this passive cooling system. C2 C C3 C4 Overall it can be concluded that the use of lowtemperature heating systems for cooling indeed introduces a condensation risk. Condensation can be avoided through the application of an anticondensation setting in the systems controls. This will only have a small impact on the cooling capacity D

6 September 2004 Page 6 of 6 (order of magnitude of 10%) and therefore is strongly advisable. Table VII: Overheating with anti condensation control in warm s 4: Limited cooling capacity, simple passive measures, warm Practical aspects Feedback from the occupants of the houses in the Hofstad project in the city of Houten provides the following information: Contrary to user expectations, passive cooling by means of thermal storage in the soil, heat pumps and low-temperature heating systems is not equivalent to an air conditioning system. It is important that occupants of houses that are equipped with this systems are informed about the peculiarities of their passive cooling system in order to prevent wrong expectations. Metering data from Hofstad suggest that there might be situations in spring and autumn where the system both provides heating and cooling in the same day, counteracting on it's own previous action. This ought to be prevented. Occupants prefer not to have heating in their bedrooms. This results in systems being closed during the winter months, that remain out of operation when the cooling season starts. This could be prevented by automated settings. 5. CONCLUSIONS AND REMARKS The research described in this paper allows to make the following conclusions about passive cooling through low-temperature heating systems: 1. The cooling load of houses in the Netherlands is strongly dependent on actual climate, sun shading, ventilation regime etc. Depending on circumstances it varies between 5 and 50% of the heating load. 2. For an average (not a warm, no peaks in cooling demand) and for a modern house like the reference case studied here the capacity of a thermal storage system in the soil as dimensioned for heating is sufficient for cooling purposes as well. To cover completely for warm s or s with peak cooling loads this capacity is insufficient and needs to be tripled. 3. Passive cooling with low-temperature heating systems is only successful when used in combination with traditional passive cooling measures like sun shading etc. 4. The working of passive cooling with lowtemperature heating systems is based on radiative heat exchange between (cooled) floors, walls and ceilings, rather than on convective heat exchange with the indoor air. This convective heat exchange is limited due to stratification effects. Similarly the effect of this cooling system on thermal comfort is based on its impact on the radiative temperature, and not on a reduction of the mean indoor air temperature 5. The use of low-temperature heating systems for cooling results in condensation risk. However, this can be prevented by the use of an anticondensation setting in the systems controls; this has only a small (10%) impact on the cooling capacity. This still significantly reduces the overheating risk, while allowing the use of heat exchangers as dimensioned for heating purposes. 6. It is important to inform the occupants of houses equipped with passive cooling with lowtemperature heating systems about the peculiarities of their passive cooling system in order to prevent wrong expectations. Overall this leads to the conclusion that passive cooling with low-temperature heating systems is indeed feasible in the Netherlands. Since systems for thermal storage in the soil are already widely used for heating purposes and have a high user acceptance this step is easy to make and only require a few simple additional measures (modified capacity, anticondensation controls, occupant information) as indicated in this paper. ACKNOWLEDGEMENT The research described in this paper was made possible by a research grant by Novem, the Netherlands Agency for Energy and the Environment, contract no REFERENCES [1] Adnot, Air conditioning equipment in use in Europe: present situation and forecast. White paper, available from Pfd/4208.pfd [2] Spiekman, M., H. van Dijk, Bevordering passieve koeling in woningen. Delft: TNO-report GI- R088 [3] de Wilde, P., J. van Wolferen, L. de Wit, Passief koelen met lage-temperatuur verwarmingsystemen. Delft: TNO report 2003-DEG-R025 [4] VABI, Gebouwsimulatie-programma VA114. Delft: Vereniging voor Automatisering in de Bouw en Installatietechniek [5] Knoblich & Partner GmbH. E.E.D. Earth Energy Designer Anwendungsbeschreibung, version 1, 2. Wetzlar, 1997 [6] ISSO, Publication ISSO no 49. Vloer- / wandverwarming en vloer- / wandkoeling