A simulation of the night time cooling system was developed in TRNSYS, using the standard components within the library. The model was based on a typical multistorey commercial building with a total floor area of 8000 m2. Rather than simulating the building load, an empirical equation was developed applying standard design loads as provided by the Australian Institute of Refrigeration, Air conditioning and Heating [7] and an assumed load profile, defined by the dry bulb temperature. This approach will produce an overestimation of the load as the solar load is assumed to be maximum for all days.

At the design temperature of 38 oC, the design load is 120 W/m2 or 960 kW. The average load, assumed as half the design load, was assumed to occur at the average summer 3pm temperature. It was assumed that at 23 oC, no cooling was required by the system, below which an economy cycle would deliver cooling. Based on these points, Eqn. (1) was developed for the load of the building.

Q =-3.474 Tdb2 + 276 Tdb — 4509 (1)

where Q is the load in kW and Tdb is the dry bulb temperature in oC.

The system was designed based on average night time conditions during the summer and the design load. Since the specified cooling tower must be capable of operating as a conventional cooling tower, the condenser load was determined from the design load and assuming a COP for a chiller of 3.5.

Based on this load, a cooling tower was sized from the specifications of a local cooling tower manufacturer [8]. This cooling tower had a flow rate of 60.5 L/s of water, 106150 m3/hr air flow,

7.5 kW fan power and 6 m3 sump volume.

The mean summer 9 am wet bulb temperature is 15.3 oC which was assumed as the average night time wet bulb temperature. Applying an average approach of 2 oC, this gives the average tank temperature in the morning of 17.3 oC. The return temperature from the chilled beam is generally fixed at approximately 21 oC as shown from measured results [1]. Based on a night time operation of 9 hours at the specified flow rate, this equates to a total potential storage capacity of 8517 kWhrs. Assuming a day time operation of 11 hours, this equates to an average cooling capacity of 774 kW which is 81% of the design load. This value represents a satisfactory capacity factor and suggests that the design has the potential to meet a significant portion of the cooling load. Therefore the night time operation of the cooling tower requires a 2000 m3 tank.

The key component within the TRNSYS model is the cooling tower defined by TYPE 51b. The model is based on determining mass transfer number of transfer units [9]. This parameter is defined by

empirical coefficients which vary widely. In order to validate the cooling tower model, performance data was obtained from the manufacture [8]. Data at low dry and wet bulb conditions, are not readily available, as a result, the cooling tower model was validated against a data set based on standard daytime conditions. Overall, the difference in outlet water temperature was less than 2%.

The TRNSYS model is presented in Fig. 1. A stratified tank (TYPE 4a) is used for thermal storage. The tank height is fixed at 12 m. Section 1 of Fig. 1 represents the simulation of the night time cooling method and section 2 determines the energy balance of the tank. For the simulations, the overall energy balance was less than 1% of the total energy transferred. The control strategy adopted was based on a day time (0700 — 1800) and night time (2100 — 0600) operation. During the night, if the temperature of the top of the tank was greater than the wet bulb temperature, water from this point would be directed to the cooling tower and returned to the bottom of the tank. As specified from the manufacturers, cooling towers are best operated at a constant water mass flow rate to prevent scaling. Therefore the cooling tower was operational for the entire night period at the specified flow rate of

60.5

Section 2: J Energy balance

kg/s. For the designed tank size of 2000 m3, the average approach from the cooling tower was 2 oC. Based on the standard deviation most approach values ranged from 0.6 to 3.4 oC. These values reflect the approaches presented in [3] as well as that presented by the manufacturer’s data [8].

During the day, chilled water was taken from the bottom of the tank only when this temperature was less than 18 oC. Chilled beam systems are controlled through variable flow [1]. Therefore under different load conditions, the mass flow rate was determined based on the load from Eqn. (1). The return temperature from the chilled beam was fixed at 21 oC as specified in [1].

A second TRNSYS model (Fig. 2) was developed in which the cooling tower was used to meet the condensing load from a chiller system during the day. With this arrangement the load was determined using equation 1 and modified by the COP of the chiller. The mass flow rate was constant at 60.5 kg/s and therefore the outlet temperature varied with load.

Fig 2. TRNSYS model of a conventional cooling tower system.

2. Results

The TRNSYS simulations were run over the entire year with a time step of 1 hour and tolerances maintained at 0.001. The system was modelled with different sized tanks. Fig. 3 shows the proportion of the cooling load achieved using the cooling tower in combination with night time thermal storage for each tank volume. At the minimum volume of 250 m3 a meaningful contribution of 36% of the load was met, at 2000 m3 the contribution was 84% while for a 5000 m3 tank, 97% of the load was provided by the system. This result shows that for many days when cooling is needed the preceding night provides an adequate heat sink for the system. Increasing the tank size reflects storage of many days, with diminishing benefits at larger tank volumes. The contribution values at large tank volumes reflect the impact of a few nights each year where the wet bulb at night is high and insufficient cooling can be achieved for the following day. Overall the results show that a 5000 m3, tank 12 m high which equates to a diameter 23 m, can provide essentially the entire sensible cooling demand of the building.

Due to the lower driving potential, a cooling tower operating at night may need to operate for longer, requiring more fan power and use more water. Fig. 4 shows the annual energy based COP of the cooling tower for each tank volume and Fig. 6 shows the corresponding annual water usage of the cooling tower operating at night. The COP was determined by the ratio of the total cooling delivered and the total fan energy used over the year. The additional pumping power required for the night time operation was ignored. From the simulation of the conventional system with the cooling tower operating during the day with a chiller, and ignoring the heat of compression from the chiller system, the COP of the cooling tower was found to be 61 and the annual water consumption was 875 m3.

Overall the COP of the cooling tower shown in Fig. 1 reduced significantly compared to a conventional cooling tower COP, ranging from 12 to 24. However, relative to the COP of a chiller, which ranges from 3.5 to 10, a significant saving can be achieved. Fig. 4 shows the saving which can be achieved with respect to tank volume. To calculate the saving, the cooling demand which could not be met by the proposed system was assumed to be met by a backup chiller with a COP of 3.5. This value was then compared to a conventional chiller system with an annual energy COP of the chiller of 6 as well as 3.5. A lower COP for the unmet load of the proposed system was applied as this load is more likely to occur when the chiller is fully loaded. For most tank volumes a significant energy saving was obtained. For the specified 2000 m3 tank, the saving ranged from 50% to 71%, and savings occurring at a tank volume greater than 500 m3, based on a chiller COP of 6. The energy reductions achievable, particularly at the low tank volumes, are strongly dependant on the COP of the comparative chiller.

Tank volume, m3 Fig. 3. The energy contribution of the night time cooling system to the total sensible cooling load.

A conventional cooling tower removes both the heat from the cooling load and the heat of compression, which is a function of the COP of the chiller. Compared to the water consumption not including the heat of compression of 875 m3, this heat can increase the water consumption by up to 18%, at the lowest chiller COP. Therefore, potential exists for a water saving.

Tank volume, m3

Fig. 5. Water consumption of a conventional cooling tower with various chiller COPs.

Fig. 6 shows the water saving of the proposed cooling tower system. To determine the total water usage of the proposed system, the water usage of a backup chiller to provide the unmet load was calculated from the data set, based on a chiller COP of 3.5. The water saving, was then calculated against a conventional system operating with a chiller COP of 3.5 and 6. In most cases the water consumption of the proposed system is lower than the water consumption of the conventionally operated cooling tower. The water saving reduces with tank volume and at the low tank volumes, an overall increase in water consumption is observed. However, this is strongly dependant on the COP of the chiller. For the 2000 m3 tank the saving ranges from 12% to 17% with savings achieved at a tank volume greater than 1000 m3, based on a chiller COP of 6. Chiller COP

A TRNSYS simulation was conducted on a cooling tower driven chilled ceiling system with thermal storage. Overall, the system can readily meet the majority of the load. At the specified tank size of 2000 m3, 84% of the load was met while at 5000 m3, 97% of the load was met. Due to the lower driving potential, the cooling tower was found to operate at a significantly lower COP then when operating conventionally. However, significant energy savings were still achieved with this arrangement. Furthermore, water savings are also achievable. The performance of the system is strongly dependant on the COP of the chiller and the tank volume. For a building with 8000 m2 floor space, savings in energy and water were achieved with a tank volume of 1000 m3. For the specified tank volume of 2000 m3, the saving in energy was at least 50% and the saving of water was at least 12%. For a 5000 m3 the savings of energy and water were at least 69% and 16%, respectively. Consequently, the proposed system has the potential to provide the vast majority of sensible cooling and achieve significant energy and water savings.

References

[1] M. Virta, D. Butler, J. Graslund, J. Hogeling, E. Kristiansen, M. Reinikainen and G. Svensson (2005). Chilled Beam Application Guidebook, Federation of European HVAC Engineers (REHVA).

[2] F. Alamdari, D. Butler, P. Grigg and M. Shaw, Renewable Energy, 15 (1998) 300-305.

[3] B. Costelloe and D. Finn, Energy and Buildings, 13 (2007) 1235-1243.

[4] F. Bruno (2006). Centralised PCM Systems for Shifting Cooling Loads During Peak Demands in Buildings, Sup. Tech. Paper, Melbourne City Council, Australia.

[5] S. Esmore, Innovative design — Batiso and night sky cooling, Ecolibrium — Journal of the Australian Institute for Refrigeration Air conditioning and Heating (AIRAH), September 2005 (2005) 30-35.

[6] C. Broadbent, Legionella in hot water systems, Ecolibrium — Journal of the Australian Institute for Refrigeration Air conditioning and Heating (AIRAH), April 2003 (2003) 24-29.

[7] AIRAH (2005). AIRAH Handbook, Australian Institute of Refrigeration, Air-conditioning and Heating Inc., Melbourne, Australia.

[8] Aqua — Cool Towers Pty Ltd (accessed 2008), MSS Cooling Tower Specifications, URL://www. aquacooltowers. com. au/.

[9] J. Braun (1988). Methodologies for the Design and Control of Chilled Water Systems, Ph. D. Thesis, University of Wisconsin — Madison.