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Modeling and energy simulation of the variable refrigerant flow air conditioning system with water-cooled condenser under cooling conditions
Energy and Buildings 41 (2009) 949–957
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Modeling and energy simulation of the variable refrigerant flow air conditioning system with water-cooled condenser under cooling conditions Yueming Li a, Jingyi Wu a,*, Sumio Shiochi b a b
Shanghai Jiao Tong University, Institute of Refrigeration and Cryogenics, China Daikin Industries Ltd., Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 November 2008 Received in revised form 31 March 2009 Accepted 11 April 2009
As a new system, variable refrigerant flow system with water-cooled condenser (water-cooled VRF) can offer several interesting characteristics for potential users. However, at present, its dynamic simulation simultaneously in association with building and other equipments is not yet included in the energy simulation programs. Based on the EnergyPlus’s codes, and using manufacturer’s performance parameters and data, the special simulation module for water-cooled VRF is developed and embedded in the software of EnergyPlus. After modeling and testing the new module, on the basis of a typical office building in Shanghai with water-cooled VRF system, the monthly and seasonal cooling energy consumption and the breakdown of the total power consumption are analyzed. The simulation results show that, during the whole cooling period, the fan-coil plus fresh air (FPFA) system consumes about 20% more power than the water-cooled VRF system does. The power comparison between the water-cooled VRF system and the air-cooled VRF system is performed too. All of these can provide designers some ideas to analyze the energy features of this new system and then to determine a better scheme of the air conditioning system. ß 2009 Published by Elsevier B.V.
Keywords: Energy simulation Office building HVAC system Variable refrigerant flow air conditioning system with water-cooled condenser Cooling power
1. Introduction The increasing emphasis on using energy efficiently motivates a lot of researchers to study and analyze the performance and operational strategies of HVAC systems in buildings. In China, HVAC systems in commercial, industrial, and residential buildings will consume 35% of the total energy in these buildings [1]. In order to save energy, the information about the energy evaluation of different HVAC schemes must be studied, which can help designers or users choose the best one of them. Many conventional central air conditioning systems can be simulated by various commercial programs [2–7,17]. But for some new systems, the dynamic simulation has not been developed, which becomes a subject matter not only for researchers, but also for governments who must consider the sustainable development of efficient energy use. As a new system, water-cooled VRF system can offer several interesting characteristics for potential users, such as higher energy efficiency, reduced noise levels, well-appointed products and so on. It is getting popular in the Europe market, and also beginning to be used in other lands [8,9]. However, at present, the dynamic simulation of the system simultaneously in association
* Corresponding author. Tel.: +86 21 34206776; fax: +86 21 34206309. E-mail address: [email protected] (J. Wu). 0378-7788/$ – see front matter ß 2009 Published by Elsevier B.V. doi:10.1016/j.enbuild.2009.04.002
with building and other equipments is not yet included in the energy simulation programs. Some researchers in China calculated the energy consumption of water-cooled VRF. They calculated the building load first, and then used the performance curves of part load ratio (PLR) based on the load obtained from the first step to get the output of power [8]. To a certain degree, this method is feasible, and the results about the energy evaluation of the system in China are useful for designers. But this method does not consider further about the influence of other equipment and many other factors except for the part load ratio. In fact, as a dynamic process, cooling power of the system is affected by many factors which include indoor and outdoor conditions, the performance under part load conditions, parameters of cooling water, combination ratio (the ratio of the total rated capacity of indoor units to the rated capacity of outdoor unit) and so on [10–16,18,19,22]. As the process operates, each factor in the operating time acts upon the others and is consequently changed in the process. As one component of the whole system, each piece of the equipment is not independent, thus the process of the energy consumption is a complicated interactional process. On the basis of the EnergyPlus’s codes, and using the manufacturer’s performance parameters and data, a special simulation module for water-cooled VRF is developed and embedded in the software of EnergyPlus. The module is linked with the cooling tower, condenser’s cooling water pump and fan
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modules, and thus dynamic simulation of all main equipments in the whole system can be performed simultaneously. The first part of this paper aims to explain the mathematical models and the programming process of the new model embedded in EnergyPlus. The second part focuses on the verification of the model, which is accomplished by comparing simulation results with common thermal theory and data provided by manufacturer. The third part is to simulate a typical office building in Shanghai with water-cooled VRF system. The monthly and seasonal cooling energy and the breakdown of the total cooling power are analyzed. Meanwhile, the power of other popular systems based on the same building is also simulated and analyzed. 2. Modeling and programming 2.1. Simple introduction of water-cooled VRF system The normal VRF system is cooled by air (air-cooled VRF), while the water-cooled VRF system is cooled by water. So the structures of the condenser for these two systems are different. Fig. 1 shows the schematic diagram for the whole loop of water-cooled VRF system. Similar to air-cooled VRF, one outdoor unit of water-cooled VRF can connect multiple indoor units at the same time. Different from the air-cooled VRF system, the outdoor unit of water-cooled VRF is linked with the cooling tower, and can be installed indoors. There is no restriction on the length of water pipe for water-cooled VRF, and plate heat exchangers provide the link between refrigerant circuits and the water loop. 2.2. Basic mathematical model of water-cooled VRF system Like the developed simulation model of air-cooled VRF system [10,12], the mathematical model of water-cooled VRF system is also based on the object of existing DX (direct expansion) coil in EnergyPlus. There are multiple indoor units linked with one outdoor unit in one system. Some assumptions and appropriate conditions for the model must be built before doing the coding work, and the model is under ideal conditions. One DX coil is assumed to be one indoor unit. The total cooling capacity of all the indoor units connected to one outdoor unit is the summation of all the DX coils’ cooling capacity: Q realtotal ¼
n X ðQ DXreal Þi
(1)
i¼1
where Qrealtotal is the real total cooling capacity of all indoor units connected to one outdoor unit, W, QDXreal is the real cooling capacity of each DX coil, W, i is the index of indoor units, and n is the total number of indoor units.
The performance curve-oriented way is utilized in EnergyPlus to calculate the energy performance of HVAC systems. There are five performance curves describing the change in total cooling capacity and the energy efficiency at part load conditions for the cooling DX coil [12,24]. On the basis of the real total cooling capacity, the real power consumption of the compressor can be obtained by using the energy performance curves. The main equations are as follows: Power ¼ ðQ realtotal ÞðEIRcool ÞðPFcool Þ
(2)
EIRcool ¼ ðEIRTempModFacÞcool ðEIRFlowModFacÞcool
(3)
where power is the power of outdoor unit, W, EIRcool is the energy input ratio which is the reverse of COP (coefficient of performance) curve-fitted from catalogue data, PFcool is the modifier for the operating performance at part load condition based on the total cooling capacity, (EIRTempModFac)cool is the energy input ratio (EIR) temperature modifier, and (EIRFlowModFac)cool is the energy input ratio (EIR) flow modifier. (EIRTempModFac)cool is a bi-quadratic curve with two independent variables: the wet-bulb temperature of the air entering the cooling coil, and the temperature of the water entering the water-cooled condenser. Thus ðEIRTempModFacÞcool ¼ a þ bðT wb;i Þ þ cðT wb;i Þ2 þ dðT w;i Þ þ eðT w;i Þ2 þ f ðT wb;i ÞðT w;i Þ
(4)
where Twb,i is the wet-bulb temperature of the air entering the cooling coil, 8C, Tw,i is the temperature of the water entering the water-cooled condenser, and a, b, c, d, e, and f are the fitted coefficients. (EIRFlowModFac)cool is a quadratic curve with one independent variable which is the ratio of the actual water flow rate across the condenser to the rated one. 0
ðEIRFlowModFacÞcool ¼ a0 þ b ðffÞ þ c0 ðffÞ
2
(5)
where ff is the flow ratio, and a0 , b0 , and c0 are the fitted coefficients. PFcool is the modifier of PLR, and thus is the function of PLR [12,24]. The above dominating mathematical equations can determine the energy features of water-cooled VRF system. Some of the data for the COP of a water-cooled VRF unit are listed in Fig. 2 along with predictions from Eqs. (4) and (5). The percentage difference in this case can be as high as 2%, but overall the agreement between manufacturer’s data [23] and predictions from Eqs. (4) and (5) is good. The COP in Fig. 2 accounts only for the power of compressor. Powers for the indoor fan, cooling tower and pump are not included in this figure.
Fig. 1. Schematic diagram of the whole loop of water-cooled VRF system.
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2.3. Related simulation models Water-cooled VRF system includes two additional pieces of equipment: cooling tower and pump. EnergyPlus uses a loop based HVAC system formulation. There are three main loops: air loop, plant loop, and condenser loop. The cooling water cycle of the water-cooled VRF system corresponds to a condenser loop. The overall structure of the loop is defined with branch and connector objects. The detail is filled with components and their inlet and outlet nodes. Based on this method, the outdoor unit can be linked with the cooling tower and pump’s models already existing in EnergyPlus. Some related simulation models are simply explained as follows.
Fig. 2. Curve fitting results.
2.3.1. Cooling tower model The cooling tower model in EnergyPlus is used to simulate the cooling waterside of the water-cooled VRF system. The EnergyPlus cooling tower model uses effectiveness–NTU relationships for counterflow heat exchangers. The objective of the cooling tower model is to predict the exiting water temperature and the fan
Fig. 3. Schematic flowchart of core subroutine.
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power required to meet the exiting water set point temperature. Since only the inlet air and inlet water temperatures are known at any simulation time step, an iterative procedure is required to determine the exiting fluid temperatures.
of the condenser is equal to inlet or outlet water temperature of cooling tower respectively.
2.3.2. Pump model The existing pump models in EnergyPlus can be chosen to simulate the condenser’s cooling water pump for the water-cooled VRF system. The water pump is quite simply the component that drives the flow. It can add heat to loop and make the outlet water temperature increase. Pump power can also be calculated by this model.
Based on the mathematical model, the coding work for calculation of the energy consumption of the water-cooled VRF system is established at the right place in the EnergyPlus source code and in the EnergyPlus coding style. The new specific input parameters of the system also need to be defined in the input deck, including name of outdoor unit, nominal capacity, product type, name of indoor unit, performance curves, all the information about the condenser loop, and so on. The new energy simulation module is embedded in the so-called ManageHVAC module which is the top manage module of the calling tree about the HVAC system’s energy simulation in EnergyPlus. The process of the modeling is similar to the one of air-cooled VRF system. Fig. 3 shows the schematic flowchart of the energy computation program based on all the aforementioned modeling considerations. First, the information about building and system is inputted. When the problem description is finished, a subroutine is called to read all the necessary inputs for the model. During every time-step of the simulation, when there is a need for cooling in the conditioned zones, the cooling model subroutine is called. The cooling requirement is passed on to the cooling model subroutine. After getting the cooling load, electricity consumption of the whole system can be determined for the time step based on the simultaneous communication among all the components existing in the system. After the simulation is finished successfully, results in CSV format can be obtained.
2.3.3. Indoor unit of water-cooled VRF Same as air-cooled VRF model, DX coil unit is assumed to be the indoor unit of water-cooled VRF. The existing fan model linked with DX coil in EnergyPlus is assumed as the indoor fan for the indoor unit. The volume of outside air can be determined according to the real condition. Normally, there is no fresh air for the indoor unit. 2.3.4. Condenser model of water-cooled VRF The condenser of water-cooled VRF is like the one of chiller model, and is connected with cooling tower and pump by water loop. So this model focuses on the calculation of water temperature. The water temperature can be calculated by the following equation: T w;o ¼ T w;i þ
Q cond m˙ cond C p;cond
(6)
where Tw,o is the water temperature leaving the condenser, 8C, m˙ cond is the mass flow rate through the condenser, kg/s, Cp,cond is the specific heat of water entering the condenser at Tw,i, J/kg 8C, and Qcond is the cooling capacity of the condenser, W. When the heat loss or gain from other components such as pump is not taken into account, outlet or inlet water temperature
2.4. Water-cooled VRF system modeling in EnergyPlus
3. Analysis and validation 3.1. Building and system description As shown in Fig. 4, a three-storey multi-zone building located in Shanghai (weather data file is Shanghai CTYW.EPW) is used to
Fig. 4. Image of the building model.
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Table 1 Information of water-cooled VRF system. HVAC systems
Description
Outdoor unit Indoor units (two sets) Cooling tower Pump
Nominal cooling capacity: 21 kW Nominal cooling capacity: each is 10.5 kW Rated power: 1000 W Rated power: 750 W
testify the simulation effect of the new energy-calculation module embedded in EnergyPlus. One water-cooled VRF system composed of one outdoor unit and two indoor units is assumed to be located in the core area of the second floor, with two identical zones (room A and room B) in simulation. All the neighboring conditioned zones are cooled by the purchased air conditioning systems and the indoor-temperature set points of all the conditioned zones are the same. The system is sized by the building load on summer design day. The detailed parameters of cooling tower and pump are determined according to the example files in EnergyPlus. The main information of the system is shown in Table 1. The detailed information about the building model can be seen in Ref. [12]. 3.2. Simulation results 3.2.1. Analysis and validation of energy simulation The real cooling capacity is mainly determined by the indoor and outdoor conditions if the building model is fixed. The power input of the system tracks the change of the cooling capacity. The day simulated in Figs. 5 and 6 is a summer design day in Shanghai. Fig. 5 shows that the cooling power increases as the indoor-temperature set point decreases. In cooling condition, the heat exchanger of indoor side acts as the evaporator unit for the whole cooling loop, thus the indoor temperature influences the evaporating temperature directly. The simulation results in Fig. 5 show that the power input of the compressor decreases when the evaporating temperature increases, while the condensing temperature does not change at the same time. This is in accordance with the thermal theory. Because the outdoor unit is cooled by water, the condensing temperature is directly related to the operating parameters of the cooling water which can influence the energy consumption of the system. It is known that, within a normal range, if the flow rate gets higher, or the temperature of cooling water gets lower, the condensing temperature will be lower. This will usually result in a
Fig. 5. Hourly power input of compressor at different indoor-temperature set points.
Fig. 6. Hourly power input of compressor at different inlet water temperatures of the condenser.
less power input of compressor. Fig. 6 shows that the higher inlet water temperature of the condenser will get the higher power because of the higher condensing temperature. These simulation results of the new module are in accordance with the common thermal theory. 3.2.2. Coefficient of performance and part load ratio The coefficient of performance (COP) for the system is defined as the ratio of the total cooling capacity to the power of compressor. Fig. 7 shows the hourly data reflecting the relationship between COP and PLR in summer design day. In this figure, the change tendency of COP is opposite to that of PLR, and when the value of PLR is more than 0.55, this tendency becomes more obvious. The simulation result is consistent with that in Refs. [10,11]. The above results also show that the water-cooled VRF system has superior performance at part load conditions, and this is an important reason for the energy saving of this system. 4. Comparison between water-cooled VRF system and other systems 4.1. Building and parameters of systems A comparison for energy consumption between the watercooled VRF system and other two air conditioning systems, i.e., the
Fig. 7. Correlations between COP and PLR in summer design day.
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954 Table 2 Construction thermal properties. Layers (outer to inner)
Thickness (m)
Conductivity (W/(m K))
Density (kg/m3)
Specific heat (J/(kg K))
Exterior wall Cast concrete XPS extruded polystyrene Air Plasterboard
0.15 0.02 0.01 0.015
1.13 0.034 0.025 0.25
2000 35 1.23 2800
1000 1400 1008 896
Interior wall Plaster (lightweight) Cast concrete Plaster (lightweight)
0.013 0.1 0.013
0.16 1.13 0.16
600 2000 600
1000 1000 1000
Ground floor Cultivated clay soil XPS extruded polystyrene Cast concrete
0.5 0.124 0.15
1.59 0.034 1.13
2000 35 2000
1550 1400 1000
Roof Cast concrete Air gap Plasterboard Slate tiles Air gap Ceiling tiles
0.15 Thermal resistance R = 0.18 m2 K/m2 0.01 0.01 Thermal resistance R = 0.16 m2 K/m2 0.01
1.13
2000
1000
0.25 2
2800 2700
896 753
380
1000
1.13
2000
1000
0.25 2
2800 2700
896 753
Ceiling/floor (the order of floor materials is the reverse of that of ceiling materials) Cast concrete 0.15 Air gap Thermal resistance R = 0.18 m2 K/m2 Plasterboard 0.01 Slate tiles 0.01
air-cooled VRF system and the FPFA (fan-coil plus fresh air) system, is made in this section. In Ref. [10], the energy comparison between air-cooled VRF and other two popular central systems, VAV (variable air volume) and FPFA, was presented, and the results showed that the VAV system consumes the most power while the air-cooled VRF system consumes the least. Further in the present study, analysis about the energy comparison between air-cooled VRF and water-cooled VRF is done first. As one of the popular central air conditioning systems, the FPFA system, which consumes less power than the VAV system, is chosen to be compared with the water-cooled VRF system which is expected to be an energy saving system. With the reasonable results obtained from these comparisons, the new simulation model can be checked further, and the simulation can also give some useful information for designers to make a preferable system plan. According to the main features of commercial buildings in China, a simplified typical middle-scale office building is set up to do the power comparison. It is a ten-storey building with a square foot-print, and each floor is sub-divided into six thermal zones. Construction material properties are listed in Table 2. The ventilation unit without any heat recovery is used in the watercooled/air-cooled VRF systems to get the same volume of fresh air as the central system. All the other detailed information about the building such as floor plan and internal loads can be seen in Ref. [10]. All the systems are simulated only for the cooling season, and the performance parameters are presented in Table 3. The system
0.056
components are mainly sized by the software, and most of the parameters are determined according to the example files in EnergyPlus. In order to be different from Ref. [10], product types with different performance curves for chiller and other main components are chosen in this paper. All the system components have the same or similarly rated parameters. 4.2. Analysis and comparison between air-cooled VRF and watercooled VRF In fact, the compressor of the water-cooled VRF system is the same as or is similar to that of the air-cooled VRF system with the same capacity if these two systems are installed by the same manufacturer. So the performance difference between compressors of the two systems is mainly caused by the different working conditions. Among so many factors, the influence of condensing temperature is very important [20,21]. For air-cooled system, the condensing temperature is mainly determined by the dry-bulb temperature of outdoor air. But for water-cooled system, the condensing temperature is mainly determined by the wet-bulb temperature of outdoor air. Normally, the outlet water temperature of cooling tower is about 2–5 8C higher than outdoor wet-bulb temperature, and if the performance of cooling tower is better, the temperature difference will get smaller. In Fig. 8 the hourly comparison of compressor power between the two systems is shown. Now the simulation model cannot output the compressor power of air-cooled VRF directly. Assuming
Table 3 Performance parameters of HVAC system. Items
Description
HVAC systems (with same rated capacity) Chiller type Space design temperature Cooling source rated COP Weather data Run period
Fan-coil plus fresh air (FPFA); air-cooled VRF; water-cooled VRF Water-cooled screw chiller (FPFA) 26 8C Water-cooled VRF: 4.3; air-cooled VRF (compressor and outdoor fan): 3.38; screw chillers for FPFA: 4.7 Shanghai CTYW.EPW June 1–August 31; 8:00–18:00; ON during weekdays
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Fig. 10. Monthly energy consumption percentage indexes. Fig. 8. Comparison of compressor power between water-cooled VRF and air-cooled VRF.
that the outdoor fan has constant power consumption (the rated power), the compressor power of air-cooled VRF can be calculated from the power of outdoor unit which mainly includes compressor and outdoor fan. For water-cooled VRF, the power of outdoor unit is equal to the power of compressor. For most of the hours shown in Fig. 8, compressor power of aircooled VRF is higher than that of water-cooled VRF mainly because of the higher condensing temperature of the air-cooled VRF. When there is a big difference between the dry-bulb temperature and the wet-bulb one, the power difference is normally big. When the temperature difference between the dry-bulb and the wet-bulb is lower than 2–5 8C, the compressor power of the water-cooled system is close to or even a little higher than the air-cooled system, which is mainly due to the fact that the outlet temperature of cooling tower is close to or even a little higher than outdoor drybulb temperature. The compressor performance is influenced by many factors such as water flow rate, part load performance and so on, and only the influence of the outdoor temperature is considered in Fig. 8. However, higher compressor power does not always result in higher total power of the system. Fig. 9 presents the comparison for the total power of the two systems. Because they use the same indoor fan model in the simulation, the power of the indoor fan is not taken into account in Fig. 9. The rated power of pump and cooling tower in the water-cooled VRF system is the same as that in the central system which is sized
by the software. So, this rated condition may be safer than the real condition. The power consumed by pump and cooling tower is larger than that consumed by outdoor fan which can result in even higher power input of the total system. As shown in Fig. 9, in many hours the total power input of water-cooled VRF is higher than that of air-cooled one, and the overall power difference between the two systems is not very big. 4.3. Monthly energy features of different systems Only the cooling power of the air conditioning system is considered, and power of lighting and other devices in the building are not taken into account. The monthly power comparison is made first to make clear the energy feature of the three systems under different weather conditions. To be more convenient, a coefficient, monthly energy efficiency ratio (MEER), which is defined as the ratio of the monthly total cooling capacity to the monthly compressor power, is used to evaluate the energy efficiency of compressor on a monthly basis. In Fig. 10 the monthly power comparison among the different air conditioning systems is shown. An index system is composed and included in the figure with the monthly electricity used in the FPFA system as the base of 100%. Fig. 10 is a diagrammatic representation of the monthly electricity index. The power difference between the water-cooled VRF and the air-cooled one is small and does not change much in different months. Because of the large power for condenser’s cooling loop, the water-cooled system consumes a little more power than the air-cooled system does. The energy consumption gap between the water-cooled/aircooled VRF system and the FPFA system is different in different months, and reaches its maximum in June. This result can also be confirmed by the comparison of MEER. Table 4 gives the MEER of the water-cooled VRF system and the FPFA system. In June, the difference of MEER between the two systems also becomes the greatest. If all the other conditions such as the building’s structure and indoor temperature are fixed, the cooling load is mainly determined by the outdoor weather conditions. Overall, the weather in June is similar to the one of transitional season, i.e., it is not as hot as in July or August. Consequently, most of PLR in Table 4 MEER of different systems.
Fig. 9. Comparison of total power between water-cooled VRF and air-cooled VRF.
System
June
July
August
FPFA Water-cooled VRF
4.81 7.37
5.14 5.96
5.15 5.88
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the refrigerant and the air in conditioned zone. All of these increase the power consumed by VAV or FPFA system. 5. Conclusions
Fig. 11. Energy comparisons between different systems.
June ranges from 0.35 to 0.6, while in July or August it is mainly in the range of 0.5–0.9. The working performance under part load conditions is different between different systems. For the FPFA system, the performance of chiller changes a little with the change of PLR, and this can be confirmed by the values of MEER shown in Table 4. But for the water-cooled VRF system, MEER in June is much higher than in the other two months, which implies that the operating performance of compressor of the system can get better if the PLR gets lower in a normal range. Overall, MEER of the watercooled VRF system is higher than that of the FPFA system and thus the compressor performance of the former is better. 4.4. Breakdown of cooling energy consumption Fig. 11 shows the power percentage index of the three systems with the total cooling power used in the FPFA system as the base of 100%, and in this figure, the total electrical energy consumption is sub-divided into various parts including system fan, chiller, pump, cooling tower, and outdoor unit of water-cooled/air-cooled VRF system. On the airside of a direct expansion system, indoor air exchanges heat with refrigerant directly. The specific enthalpy of refrigerant is higher than that of water or air, and the phase change in an evaporator also improves the heat exchange efficiency, which results in less fan power consumption of water-cooled/air-cooled VRF system. Furthermore, because there is no chilled water pump in the water-cooled VRF system, the system consumes less pump power than the FPFA system. The power of cooling tower for the two systems is almost equal, which is mainly attributed to the same cooling tower used in the two systems. It is observed in Fig. 11 that, during the whole cooling period, the FPFA system consumes about 20% more power than the watercooled VRF system does. The power comparison between the two kinds of VRF systems is analyzed hereinbefore. The water-cooled VRF system consumes only about 4% more power than the aircooled one. According to Ref. [10], the VAV system consumed more power than the FPFA system. So under the same working conditions, the water-cooled VRF system saves more energy than the two popular central systems. For the two VRF systems, the refrigerant is distributed into the conditioned zone and cools the air directly. But for most VAV systems, chilled water from chiller is supplied to cooling coils by pump for the first heat exchange between water and air, then the chilled air goes into air pipes and finally reaches conditioned zone. Similarly, for FPFA systems, chilled water from chiller is supplied to the water pipes by pump and then goes into fan-coil in each conditioned zone. So, on the one hand, there are secondary heattransfer processes and transport power of chilled water in the latter two systems, and on the other hand, the evaporating temperature of chiller in the two systems, which needs secondary heat-transfer between the refrigerant and the air in conditioned zone, may be lower than that of indoor unit in water-cooled/aircooled VRF system which can make direct heat exchange between
In order to assess the energy performance of a new system, the water-cooled VRF system, a new simulation module embedded in the software of EnergyPlus is developed. The module is linked with cooling tower, condenser’s cooling water pump and fan modules, and thus the simultaneous dynamic simulation of all the main equipment in the whole system can be done. Some conclusions can be drawn from the simulation results, which are listed as following: The compressor’s power input is reduced when the evaporating temperature increases, while the condensing temperature does not change. The higher inlet water temperature of the condenser will get the higher power because of the higher condensing temperature. The above two simulation results are in accordance with the common thermal theory. The change tendency of COP is opposite to that of PLR when the value of PLR is in a normal range. The energy consumption gap between the water-cooled/aircooled VRF system and the FPFA system is different in different months, and reaches its maximum in June. For the water-cooled VRF system, MEER in June is much higher than that in July or August, and the difference can reach 1.49. But for the FPFA system, MEER in three months is close to each other. The compressor power of the air-cooled VRF system can be usually higher than that of the water-cooled one when the indoor and outdoor conditions are the same. But the total power of the former is even a little lower than that of the latter because of the large power of pump and cooling tower of the latter. During the whole cooling period, the FPFA system consumes about 20% more power than the water-cooled VRF system. Besides the good part load performance, water-cooled VRF system does not have large heat exchange and transport loss, and thus can have higher evaporating temperature than VAV or FPFA system. All of these can provide designers some ideas to analyze the energy features of this new system and help them to determine a better scheme of the HVAC system. References [1] Y.X. Zhu, B.R. Lin, Sustainable housing and urban construction in China, Energy and Buildings 36 (12) (2004) 1287–1297. [2] W.Z. Huang, M. Zaheeruddin, S.H. Cho, Dynamic simulation of energy management control functions for HVAC systems in buildings, Energy Conversion and Management 47 (7–8) (2006) 926–943. [3] S.C. Sekhar, A critical evaluation of variable air volume system in hot and humid climates, Energy and Buildings 26 (2) (1997) 223–232. [4] J. Neymark, R. Judkoff, G. Knabe, H.-T. Le, et al., Applying the building energy simulation test (BESTEST) diagnostic method to verification of space conditioning equipment models used in whole-building energy simulation programs, Energy and Buildings 34 (9) (2002) 917–931. [5] P. Cui, H. Yang, J.D. Spitler, Z. Fang, Simulation of hybrid ground-coupled heat pump with domestic hot water heating systems using HVACSIM+, Energy and Buildings 40 (9) (2008) 1731–1736. [6] M.S. Al-Homoud, Computer-aided building energy analysis techniques, Building and Environment 36 (4) (2001) 421–433. [7] M.M. Ardehali, T.F. Smith, Evaluation of HVAC system operational strategies for commercial buildings, Energy Conversion and Management 38 (3) (1997) 225– 236. [8] Z Zhiqiang, Analysis of energy use in water source VRF system and evaluation for its use in China, Master thesis, School of Municipal & Environmental Engineering, Harbin Institute of Technology, 2006 (in Chinese). [9] Z. Zhiqiang, J. Yiqiang, Y. Yang, M. Zuiliang, Water loop VRF air conditioning system and its project application, Refrigeration Air Conditioning and Electric Power Machinery 27 (110) (2006) 59–61 (in Chinese).
Y. Li et al. / Energy and Buildings 41 (2009) 949–957 [10] Y.P. Zhou, J.Y. Wu, R.Z. Wang, et al., Energy simulation in the variable refrigerant flow air conditioning system under cooling conditions, Energy and Buildings 39 (2) (2007) 212–220. [11] W.H. Xue, P.L. Chen, C.J. Liu, The experimental study of the operating characteristic and energy consumption of the VRV air conditioning system, Energy Conservation Technology 21 (119) (2003) 3–5 (in Chinese). [12] Y.P. Zhou, J.Y. Wu, R.Z. Wang, S. Shiochi, Y.M. Li, Simulation and experimental validation of the variable-refrigerant-volume (VRV) air conditioning system in EnergyPlus, Energy and Buildings 40 (6) (2008) 1041–1047. [13] S.-C. Hu, R.-H. Yang, Development and testing of a multi-type air conditioner without using AC inverters, Energy Conversion and Management 46 (3) (2005) 373–383. [14] Z. Lian, S. Park, H. Qi, Analysis on energy consumption of water-loop heat pump system in China, Applied Thermal Engineering 25 (1) (2005) 73–85. [15] L. Xinguo, Thermal performance and energy saving effect of water-loop heat pump systems with geothermal, Energy Conversion and Management 39 (3–4) (1998) 295–301. [16] R.N.N. Koury, L. Machado, K.A.R. Ismail, Numerical simulation of a variable speed refrigeration system, International Journal of Refrigeration 24 (2) (2001) 192– 200.
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[17] P.G. Loutzenhiser, G.M. Maxwell, H. Manz, An empirical validation of the day lighting algorithms and associated interactions in building energy simulation programs using various shading devices and windows, Energy 32 (10) (2007) 1855–1870. [18] F.W. Yu, K.T. Chan, Improved energy performance of air cooled centrifugal chillers with variable chilled water flow, Energy Conversion and Management 49 (6) (2008) 1595–1611. [19] L. Yang, J.C. Lam, J. Liu, C.L. Tsang, Building energy simulation using multi-years and typical meteorological years in different climates, Energy Conversion and Management 49 (1) (2008) 113–124. [20] W. Shujun, Power consumption comparison of air cooled and water-cooled chiller units, HV and AC 27 (3) (1997) 24–25 (in Chinese). [21] L. Nanxi, L. Shi, H. Lizhong, Z. Mingshan, Moderately high temperature water source heat-pumps using a near-azeotropic refrigerant mixture, Applied Energy 80 (4) (2005) 435–447. [22] C. Chen, F. Sun, L. Feng, M. Liu, Underground water-source loop heat-pump airconditioning system applied in a residential building in Beijing, Applied Energy 82 (4) (2005) 331–344. [23] DAKIN Technical Document,VRV-WII, DAKIN Industries Ltd., 2007. [24] EnergyPlus Documentation, Engineering reference, DOE, 2002.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Modeling and energy simulation of the variable refrigerant flow air conditioning system with water-cooled condenser under cooling conditions Yueming Li a, Jingyi Wu a,*, Sumio Shiochi b a b
Shanghai Jiao Tong University, Institute of Refrigeration and Cryogenics, China Daikin Industries Ltd., Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 November 2008 Received in revised form 31 March 2009 Accepted 11 April 2009
As a new system, variable refrigerant flow system with water-cooled condenser (water-cooled VRF) can offer several interesting characteristics for potential users. However, at present, its dynamic simulation simultaneously in association with building and other equipments is not yet included in the energy simulation programs. Based on the EnergyPlus’s codes, and using manufacturer’s performance parameters and data, the special simulation module for water-cooled VRF is developed and embedded in the software of EnergyPlus. After modeling and testing the new module, on the basis of a typical office building in Shanghai with water-cooled VRF system, the monthly and seasonal cooling energy consumption and the breakdown of the total power consumption are analyzed. The simulation results show that, during the whole cooling period, the fan-coil plus fresh air (FPFA) system consumes about 20% more power than the water-cooled VRF system does. The power comparison between the water-cooled VRF system and the air-cooled VRF system is performed too. All of these can provide designers some ideas to analyze the energy features of this new system and then to determine a better scheme of the air conditioning system. ß 2009 Published by Elsevier B.V.
Keywords: Energy simulation Office building HVAC system Variable refrigerant flow air conditioning system with water-cooled condenser Cooling power
1. Introduction The increasing emphasis on using energy efficiently motivates a lot of researchers to study and analyze the performance and operational strategies of HVAC systems in buildings. In China, HVAC systems in commercial, industrial, and residential buildings will consume 35% of the total energy in these buildings [1]. In order to save energy, the information about the energy evaluation of different HVAC schemes must be studied, which can help designers or users choose the best one of them. Many conventional central air conditioning systems can be simulated by various commercial programs [2–7,17]. But for some new systems, the dynamic simulation has not been developed, which becomes a subject matter not only for researchers, but also for governments who must consider the sustainable development of efficient energy use. As a new system, water-cooled VRF system can offer several interesting characteristics for potential users, such as higher energy efficiency, reduced noise levels, well-appointed products and so on. It is getting popular in the Europe market, and also beginning to be used in other lands [8,9]. However, at present, the dynamic simulation of the system simultaneously in association
* Corresponding author. Tel.: +86 21 34206776; fax: +86 21 34206309. E-mail address: [email protected] (J. Wu). 0378-7788/$ – see front matter ß 2009 Published by Elsevier B.V. doi:10.1016/j.enbuild.2009.04.002
with building and other equipments is not yet included in the energy simulation programs. Some researchers in China calculated the energy consumption of water-cooled VRF. They calculated the building load first, and then used the performance curves of part load ratio (PLR) based on the load obtained from the first step to get the output of power [8]. To a certain degree, this method is feasible, and the results about the energy evaluation of the system in China are useful for designers. But this method does not consider further about the influence of other equipment and many other factors except for the part load ratio. In fact, as a dynamic process, cooling power of the system is affected by many factors which include indoor and outdoor conditions, the performance under part load conditions, parameters of cooling water, combination ratio (the ratio of the total rated capacity of indoor units to the rated capacity of outdoor unit) and so on [10–16,18,19,22]. As the process operates, each factor in the operating time acts upon the others and is consequently changed in the process. As one component of the whole system, each piece of the equipment is not independent, thus the process of the energy consumption is a complicated interactional process. On the basis of the EnergyPlus’s codes, and using the manufacturer’s performance parameters and data, a special simulation module for water-cooled VRF is developed and embedded in the software of EnergyPlus. The module is linked with the cooling tower, condenser’s cooling water pump and fan
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modules, and thus dynamic simulation of all main equipments in the whole system can be performed simultaneously. The first part of this paper aims to explain the mathematical models and the programming process of the new model embedded in EnergyPlus. The second part focuses on the verification of the model, which is accomplished by comparing simulation results with common thermal theory and data provided by manufacturer. The third part is to simulate a typical office building in Shanghai with water-cooled VRF system. The monthly and seasonal cooling energy and the breakdown of the total cooling power are analyzed. Meanwhile, the power of other popular systems based on the same building is also simulated and analyzed. 2. Modeling and programming 2.1. Simple introduction of water-cooled VRF system The normal VRF system is cooled by air (air-cooled VRF), while the water-cooled VRF system is cooled by water. So the structures of the condenser for these two systems are different. Fig. 1 shows the schematic diagram for the whole loop of water-cooled VRF system. Similar to air-cooled VRF, one outdoor unit of water-cooled VRF can connect multiple indoor units at the same time. Different from the air-cooled VRF system, the outdoor unit of water-cooled VRF is linked with the cooling tower, and can be installed indoors. There is no restriction on the length of water pipe for water-cooled VRF, and plate heat exchangers provide the link between refrigerant circuits and the water loop. 2.2. Basic mathematical model of water-cooled VRF system Like the developed simulation model of air-cooled VRF system [10,12], the mathematical model of water-cooled VRF system is also based on the object of existing DX (direct expansion) coil in EnergyPlus. There are multiple indoor units linked with one outdoor unit in one system. Some assumptions and appropriate conditions for the model must be built before doing the coding work, and the model is under ideal conditions. One DX coil is assumed to be one indoor unit. The total cooling capacity of all the indoor units connected to one outdoor unit is the summation of all the DX coils’ cooling capacity: Q realtotal ¼
n X ðQ DXreal Þi
(1)
i¼1
where Qrealtotal is the real total cooling capacity of all indoor units connected to one outdoor unit, W, QDXreal is the real cooling capacity of each DX coil, W, i is the index of indoor units, and n is the total number of indoor units.
The performance curve-oriented way is utilized in EnergyPlus to calculate the energy performance of HVAC systems. There are five performance curves describing the change in total cooling capacity and the energy efficiency at part load conditions for the cooling DX coil [12,24]. On the basis of the real total cooling capacity, the real power consumption of the compressor can be obtained by using the energy performance curves. The main equations are as follows: Power ¼ ðQ realtotal ÞðEIRcool ÞðPFcool Þ
(2)
EIRcool ¼ ðEIRTempModFacÞcool ðEIRFlowModFacÞcool
(3)
where power is the power of outdoor unit, W, EIRcool is the energy input ratio which is the reverse of COP (coefficient of performance) curve-fitted from catalogue data, PFcool is the modifier for the operating performance at part load condition based on the total cooling capacity, (EIRTempModFac)cool is the energy input ratio (EIR) temperature modifier, and (EIRFlowModFac)cool is the energy input ratio (EIR) flow modifier. (EIRTempModFac)cool is a bi-quadratic curve with two independent variables: the wet-bulb temperature of the air entering the cooling coil, and the temperature of the water entering the water-cooled condenser. Thus ðEIRTempModFacÞcool ¼ a þ bðT wb;i Þ þ cðT wb;i Þ2 þ dðT w;i Þ þ eðT w;i Þ2 þ f ðT wb;i ÞðT w;i Þ
(4)
where Twb,i is the wet-bulb temperature of the air entering the cooling coil, 8C, Tw,i is the temperature of the water entering the water-cooled condenser, and a, b, c, d, e, and f are the fitted coefficients. (EIRFlowModFac)cool is a quadratic curve with one independent variable which is the ratio of the actual water flow rate across the condenser to the rated one. 0
ðEIRFlowModFacÞcool ¼ a0 þ b ðffÞ þ c0 ðffÞ
2
(5)
where ff is the flow ratio, and a0 , b0 , and c0 are the fitted coefficients. PFcool is the modifier of PLR, and thus is the function of PLR [12,24]. The above dominating mathematical equations can determine the energy features of water-cooled VRF system. Some of the data for the COP of a water-cooled VRF unit are listed in Fig. 2 along with predictions from Eqs. (4) and (5). The percentage difference in this case can be as high as 2%, but overall the agreement between manufacturer’s data [23] and predictions from Eqs. (4) and (5) is good. The COP in Fig. 2 accounts only for the power of compressor. Powers for the indoor fan, cooling tower and pump are not included in this figure.
Fig. 1. Schematic diagram of the whole loop of water-cooled VRF system.
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2.3. Related simulation models Water-cooled VRF system includes two additional pieces of equipment: cooling tower and pump. EnergyPlus uses a loop based HVAC system formulation. There are three main loops: air loop, plant loop, and condenser loop. The cooling water cycle of the water-cooled VRF system corresponds to a condenser loop. The overall structure of the loop is defined with branch and connector objects. The detail is filled with components and their inlet and outlet nodes. Based on this method, the outdoor unit can be linked with the cooling tower and pump’s models already existing in EnergyPlus. Some related simulation models are simply explained as follows.
Fig. 2. Curve fitting results.
2.3.1. Cooling tower model The cooling tower model in EnergyPlus is used to simulate the cooling waterside of the water-cooled VRF system. The EnergyPlus cooling tower model uses effectiveness–NTU relationships for counterflow heat exchangers. The objective of the cooling tower model is to predict the exiting water temperature and the fan
Fig. 3. Schematic flowchart of core subroutine.
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power required to meet the exiting water set point temperature. Since only the inlet air and inlet water temperatures are known at any simulation time step, an iterative procedure is required to determine the exiting fluid temperatures.
of the condenser is equal to inlet or outlet water temperature of cooling tower respectively.
2.3.2. Pump model The existing pump models in EnergyPlus can be chosen to simulate the condenser’s cooling water pump for the water-cooled VRF system. The water pump is quite simply the component that drives the flow. It can add heat to loop and make the outlet water temperature increase. Pump power can also be calculated by this model.
Based on the mathematical model, the coding work for calculation of the energy consumption of the water-cooled VRF system is established at the right place in the EnergyPlus source code and in the EnergyPlus coding style. The new specific input parameters of the system also need to be defined in the input deck, including name of outdoor unit, nominal capacity, product type, name of indoor unit, performance curves, all the information about the condenser loop, and so on. The new energy simulation module is embedded in the so-called ManageHVAC module which is the top manage module of the calling tree about the HVAC system’s energy simulation in EnergyPlus. The process of the modeling is similar to the one of air-cooled VRF system. Fig. 3 shows the schematic flowchart of the energy computation program based on all the aforementioned modeling considerations. First, the information about building and system is inputted. When the problem description is finished, a subroutine is called to read all the necessary inputs for the model. During every time-step of the simulation, when there is a need for cooling in the conditioned zones, the cooling model subroutine is called. The cooling requirement is passed on to the cooling model subroutine. After getting the cooling load, electricity consumption of the whole system can be determined for the time step based on the simultaneous communication among all the components existing in the system. After the simulation is finished successfully, results in CSV format can be obtained.
2.3.3. Indoor unit of water-cooled VRF Same as air-cooled VRF model, DX coil unit is assumed to be the indoor unit of water-cooled VRF. The existing fan model linked with DX coil in EnergyPlus is assumed as the indoor fan for the indoor unit. The volume of outside air can be determined according to the real condition. Normally, there is no fresh air for the indoor unit. 2.3.4. Condenser model of water-cooled VRF The condenser of water-cooled VRF is like the one of chiller model, and is connected with cooling tower and pump by water loop. So this model focuses on the calculation of water temperature. The water temperature can be calculated by the following equation: T w;o ¼ T w;i þ
Q cond m˙ cond C p;cond
(6)
where Tw,o is the water temperature leaving the condenser, 8C, m˙ cond is the mass flow rate through the condenser, kg/s, Cp,cond is the specific heat of water entering the condenser at Tw,i, J/kg 8C, and Qcond is the cooling capacity of the condenser, W. When the heat loss or gain from other components such as pump is not taken into account, outlet or inlet water temperature
2.4. Water-cooled VRF system modeling in EnergyPlus
3. Analysis and validation 3.1. Building and system description As shown in Fig. 4, a three-storey multi-zone building located in Shanghai (weather data file is Shanghai CTYW.EPW) is used to
Fig. 4. Image of the building model.
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Table 1 Information of water-cooled VRF system. HVAC systems
Description
Outdoor unit Indoor units (two sets) Cooling tower Pump
Nominal cooling capacity: 21 kW Nominal cooling capacity: each is 10.5 kW Rated power: 1000 W Rated power: 750 W
testify the simulation effect of the new energy-calculation module embedded in EnergyPlus. One water-cooled VRF system composed of one outdoor unit and two indoor units is assumed to be located in the core area of the second floor, with two identical zones (room A and room B) in simulation. All the neighboring conditioned zones are cooled by the purchased air conditioning systems and the indoor-temperature set points of all the conditioned zones are the same. The system is sized by the building load on summer design day. The detailed parameters of cooling tower and pump are determined according to the example files in EnergyPlus. The main information of the system is shown in Table 1. The detailed information about the building model can be seen in Ref. [12]. 3.2. Simulation results 3.2.1. Analysis and validation of energy simulation The real cooling capacity is mainly determined by the indoor and outdoor conditions if the building model is fixed. The power input of the system tracks the change of the cooling capacity. The day simulated in Figs. 5 and 6 is a summer design day in Shanghai. Fig. 5 shows that the cooling power increases as the indoor-temperature set point decreases. In cooling condition, the heat exchanger of indoor side acts as the evaporator unit for the whole cooling loop, thus the indoor temperature influences the evaporating temperature directly. The simulation results in Fig. 5 show that the power input of the compressor decreases when the evaporating temperature increases, while the condensing temperature does not change at the same time. This is in accordance with the thermal theory. Because the outdoor unit is cooled by water, the condensing temperature is directly related to the operating parameters of the cooling water which can influence the energy consumption of the system. It is known that, within a normal range, if the flow rate gets higher, or the temperature of cooling water gets lower, the condensing temperature will be lower. This will usually result in a
Fig. 5. Hourly power input of compressor at different indoor-temperature set points.
Fig. 6. Hourly power input of compressor at different inlet water temperatures of the condenser.
less power input of compressor. Fig. 6 shows that the higher inlet water temperature of the condenser will get the higher power because of the higher condensing temperature. These simulation results of the new module are in accordance with the common thermal theory. 3.2.2. Coefficient of performance and part load ratio The coefficient of performance (COP) for the system is defined as the ratio of the total cooling capacity to the power of compressor. Fig. 7 shows the hourly data reflecting the relationship between COP and PLR in summer design day. In this figure, the change tendency of COP is opposite to that of PLR, and when the value of PLR is more than 0.55, this tendency becomes more obvious. The simulation result is consistent with that in Refs. [10,11]. The above results also show that the water-cooled VRF system has superior performance at part load conditions, and this is an important reason for the energy saving of this system. 4. Comparison between water-cooled VRF system and other systems 4.1. Building and parameters of systems A comparison for energy consumption between the watercooled VRF system and other two air conditioning systems, i.e., the
Fig. 7. Correlations between COP and PLR in summer design day.
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954 Table 2 Construction thermal properties. Layers (outer to inner)
Thickness (m)
Conductivity (W/(m K))
Density (kg/m3)
Specific heat (J/(kg K))
Exterior wall Cast concrete XPS extruded polystyrene Air Plasterboard
0.15 0.02 0.01 0.015
1.13 0.034 0.025 0.25
2000 35 1.23 2800
1000 1400 1008 896
Interior wall Plaster (lightweight) Cast concrete Plaster (lightweight)
0.013 0.1 0.013
0.16 1.13 0.16
600 2000 600
1000 1000 1000
Ground floor Cultivated clay soil XPS extruded polystyrene Cast concrete
0.5 0.124 0.15
1.59 0.034 1.13
2000 35 2000
1550 1400 1000
Roof Cast concrete Air gap Plasterboard Slate tiles Air gap Ceiling tiles
0.15 Thermal resistance R = 0.18 m2 K/m2 0.01 0.01 Thermal resistance R = 0.16 m2 K/m2 0.01
1.13
2000
1000
0.25 2
2800 2700
896 753
380
1000
1.13
2000
1000
0.25 2
2800 2700
896 753
Ceiling/floor (the order of floor materials is the reverse of that of ceiling materials) Cast concrete 0.15 Air gap Thermal resistance R = 0.18 m2 K/m2 Plasterboard 0.01 Slate tiles 0.01
air-cooled VRF system and the FPFA (fan-coil plus fresh air) system, is made in this section. In Ref. [10], the energy comparison between air-cooled VRF and other two popular central systems, VAV (variable air volume) and FPFA, was presented, and the results showed that the VAV system consumes the most power while the air-cooled VRF system consumes the least. Further in the present study, analysis about the energy comparison between air-cooled VRF and water-cooled VRF is done first. As one of the popular central air conditioning systems, the FPFA system, which consumes less power than the VAV system, is chosen to be compared with the water-cooled VRF system which is expected to be an energy saving system. With the reasonable results obtained from these comparisons, the new simulation model can be checked further, and the simulation can also give some useful information for designers to make a preferable system plan. According to the main features of commercial buildings in China, a simplified typical middle-scale office building is set up to do the power comparison. It is a ten-storey building with a square foot-print, and each floor is sub-divided into six thermal zones. Construction material properties are listed in Table 2. The ventilation unit without any heat recovery is used in the watercooled/air-cooled VRF systems to get the same volume of fresh air as the central system. All the other detailed information about the building such as floor plan and internal loads can be seen in Ref. [10]. All the systems are simulated only for the cooling season, and the performance parameters are presented in Table 3. The system
0.056
components are mainly sized by the software, and most of the parameters are determined according to the example files in EnergyPlus. In order to be different from Ref. [10], product types with different performance curves for chiller and other main components are chosen in this paper. All the system components have the same or similarly rated parameters. 4.2. Analysis and comparison between air-cooled VRF and watercooled VRF In fact, the compressor of the water-cooled VRF system is the same as or is similar to that of the air-cooled VRF system with the same capacity if these two systems are installed by the same manufacturer. So the performance difference between compressors of the two systems is mainly caused by the different working conditions. Among so many factors, the influence of condensing temperature is very important [20,21]. For air-cooled system, the condensing temperature is mainly determined by the dry-bulb temperature of outdoor air. But for water-cooled system, the condensing temperature is mainly determined by the wet-bulb temperature of outdoor air. Normally, the outlet water temperature of cooling tower is about 2–5 8C higher than outdoor wet-bulb temperature, and if the performance of cooling tower is better, the temperature difference will get smaller. In Fig. 8 the hourly comparison of compressor power between the two systems is shown. Now the simulation model cannot output the compressor power of air-cooled VRF directly. Assuming
Table 3 Performance parameters of HVAC system. Items
Description
HVAC systems (with same rated capacity) Chiller type Space design temperature Cooling source rated COP Weather data Run period
Fan-coil plus fresh air (FPFA); air-cooled VRF; water-cooled VRF Water-cooled screw chiller (FPFA) 26 8C Water-cooled VRF: 4.3; air-cooled VRF (compressor and outdoor fan): 3.38; screw chillers for FPFA: 4.7 Shanghai CTYW.EPW June 1–August 31; 8:00–18:00; ON during weekdays
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Fig. 10. Monthly energy consumption percentage indexes. Fig. 8. Comparison of compressor power between water-cooled VRF and air-cooled VRF.
that the outdoor fan has constant power consumption (the rated power), the compressor power of air-cooled VRF can be calculated from the power of outdoor unit which mainly includes compressor and outdoor fan. For water-cooled VRF, the power of outdoor unit is equal to the power of compressor. For most of the hours shown in Fig. 8, compressor power of aircooled VRF is higher than that of water-cooled VRF mainly because of the higher condensing temperature of the air-cooled VRF. When there is a big difference between the dry-bulb temperature and the wet-bulb one, the power difference is normally big. When the temperature difference between the dry-bulb and the wet-bulb is lower than 2–5 8C, the compressor power of the water-cooled system is close to or even a little higher than the air-cooled system, which is mainly due to the fact that the outlet temperature of cooling tower is close to or even a little higher than outdoor drybulb temperature. The compressor performance is influenced by many factors such as water flow rate, part load performance and so on, and only the influence of the outdoor temperature is considered in Fig. 8. However, higher compressor power does not always result in higher total power of the system. Fig. 9 presents the comparison for the total power of the two systems. Because they use the same indoor fan model in the simulation, the power of the indoor fan is not taken into account in Fig. 9. The rated power of pump and cooling tower in the water-cooled VRF system is the same as that in the central system which is sized
by the software. So, this rated condition may be safer than the real condition. The power consumed by pump and cooling tower is larger than that consumed by outdoor fan which can result in even higher power input of the total system. As shown in Fig. 9, in many hours the total power input of water-cooled VRF is higher than that of air-cooled one, and the overall power difference between the two systems is not very big. 4.3. Monthly energy features of different systems Only the cooling power of the air conditioning system is considered, and power of lighting and other devices in the building are not taken into account. The monthly power comparison is made first to make clear the energy feature of the three systems under different weather conditions. To be more convenient, a coefficient, monthly energy efficiency ratio (MEER), which is defined as the ratio of the monthly total cooling capacity to the monthly compressor power, is used to evaluate the energy efficiency of compressor on a monthly basis. In Fig. 10 the monthly power comparison among the different air conditioning systems is shown. An index system is composed and included in the figure with the monthly electricity used in the FPFA system as the base of 100%. Fig. 10 is a diagrammatic representation of the monthly electricity index. The power difference between the water-cooled VRF and the air-cooled one is small and does not change much in different months. Because of the large power for condenser’s cooling loop, the water-cooled system consumes a little more power than the air-cooled system does. The energy consumption gap between the water-cooled/aircooled VRF system and the FPFA system is different in different months, and reaches its maximum in June. This result can also be confirmed by the comparison of MEER. Table 4 gives the MEER of the water-cooled VRF system and the FPFA system. In June, the difference of MEER between the two systems also becomes the greatest. If all the other conditions such as the building’s structure and indoor temperature are fixed, the cooling load is mainly determined by the outdoor weather conditions. Overall, the weather in June is similar to the one of transitional season, i.e., it is not as hot as in July or August. Consequently, most of PLR in Table 4 MEER of different systems.
Fig. 9. Comparison of total power between water-cooled VRF and air-cooled VRF.
System
June
July
August
FPFA Water-cooled VRF
4.81 7.37
5.14 5.96
5.15 5.88
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the refrigerant and the air in conditioned zone. All of these increase the power consumed by VAV or FPFA system. 5. Conclusions
Fig. 11. Energy comparisons between different systems.
June ranges from 0.35 to 0.6, while in July or August it is mainly in the range of 0.5–0.9. The working performance under part load conditions is different between different systems. For the FPFA system, the performance of chiller changes a little with the change of PLR, and this can be confirmed by the values of MEER shown in Table 4. But for the water-cooled VRF system, MEER in June is much higher than in the other two months, which implies that the operating performance of compressor of the system can get better if the PLR gets lower in a normal range. Overall, MEER of the watercooled VRF system is higher than that of the FPFA system and thus the compressor performance of the former is better. 4.4. Breakdown of cooling energy consumption Fig. 11 shows the power percentage index of the three systems with the total cooling power used in the FPFA system as the base of 100%, and in this figure, the total electrical energy consumption is sub-divided into various parts including system fan, chiller, pump, cooling tower, and outdoor unit of water-cooled/air-cooled VRF system. On the airside of a direct expansion system, indoor air exchanges heat with refrigerant directly. The specific enthalpy of refrigerant is higher than that of water or air, and the phase change in an evaporator also improves the heat exchange efficiency, which results in less fan power consumption of water-cooled/air-cooled VRF system. Furthermore, because there is no chilled water pump in the water-cooled VRF system, the system consumes less pump power than the FPFA system. The power of cooling tower for the two systems is almost equal, which is mainly attributed to the same cooling tower used in the two systems. It is observed in Fig. 11 that, during the whole cooling period, the FPFA system consumes about 20% more power than the watercooled VRF system does. The power comparison between the two kinds of VRF systems is analyzed hereinbefore. The water-cooled VRF system consumes only about 4% more power than the aircooled one. According to Ref. [10], the VAV system consumed more power than the FPFA system. So under the same working conditions, the water-cooled VRF system saves more energy than the two popular central systems. For the two VRF systems, the refrigerant is distributed into the conditioned zone and cools the air directly. But for most VAV systems, chilled water from chiller is supplied to cooling coils by pump for the first heat exchange between water and air, then the chilled air goes into air pipes and finally reaches conditioned zone. Similarly, for FPFA systems, chilled water from chiller is supplied to the water pipes by pump and then goes into fan-coil in each conditioned zone. So, on the one hand, there are secondary heattransfer processes and transport power of chilled water in the latter two systems, and on the other hand, the evaporating temperature of chiller in the two systems, which needs secondary heat-transfer between the refrigerant and the air in conditioned zone, may be lower than that of indoor unit in water-cooled/aircooled VRF system which can make direct heat exchange between
In order to assess the energy performance of a new system, the water-cooled VRF system, a new simulation module embedded in the software of EnergyPlus is developed. The module is linked with cooling tower, condenser’s cooling water pump and fan modules, and thus the simultaneous dynamic simulation of all the main equipment in the whole system can be done. Some conclusions can be drawn from the simulation results, which are listed as following: The compressor’s power input is reduced when the evaporating temperature increases, while the condensing temperature does not change. The higher inlet water temperature of the condenser will get the higher power because of the higher condensing temperature. The above two simulation results are in accordance with the common thermal theory. The change tendency of COP is opposite to that of PLR when the value of PLR is in a normal range. The energy consumption gap between the water-cooled/aircooled VRF system and the FPFA system is different in different months, and reaches its maximum in June. For the water-cooled VRF system, MEER in June is much higher than that in July or August, and the difference can reach 1.49. But for the FPFA system, MEER in three months is close to each other. The compressor power of the air-cooled VRF system can be usually higher than that of the water-cooled one when the indoor and outdoor conditions are the same. But the total power of the former is even a little lower than that of the latter because of the large power of pump and cooling tower of the latter. During the whole cooling period, the FPFA system consumes about 20% more power than the water-cooled VRF system. Besides the good part load performance, water-cooled VRF system does not have large heat exchange and transport loss, and thus can have higher evaporating temperature than VAV or FPFA system. All of these can provide designers some ideas to analyze the energy features of this new system and help them to determine a better scheme of the HVAC system. References [1] Y.X. Zhu, B.R. Lin, Sustainable housing and urban construction in China, Energy and Buildings 36 (12) (2004) 1287–1297. [2] W.Z. Huang, M. Zaheeruddin, S.H. Cho, Dynamic simulation of energy management control functions for HVAC systems in buildings, Energy Conversion and Management 47 (7–8) (2006) 926–943. [3] S.C. Sekhar, A critical evaluation of variable air volume system in hot and humid climates, Energy and Buildings 26 (2) (1997) 223–232. [4] J. Neymark, R. Judkoff, G. Knabe, H.-T. Le, et al., Applying the building energy simulation test (BESTEST) diagnostic method to verification of space conditioning equipment models used in whole-building energy simulation programs, Energy and Buildings 34 (9) (2002) 917–931. [5] P. Cui, H. Yang, J.D. Spitler, Z. Fang, Simulation of hybrid ground-coupled heat pump with domestic hot water heating systems using HVACSIM+, Energy and Buildings 40 (9) (2008) 1731–1736. [6] M.S. Al-Homoud, Computer-aided building energy analysis techniques, Building and Environment 36 (4) (2001) 421–433. [7] M.M. Ardehali, T.F. Smith, Evaluation of HVAC system operational strategies for commercial buildings, Energy Conversion and Management 38 (3) (1997) 225– 236. [8] Z Zhiqiang, Analysis of energy use in water source VRF system and evaluation for its use in China, Master thesis, School of Municipal & Environmental Engineering, Harbin Institute of Technology, 2006 (in Chinese). [9] Z. Zhiqiang, J. Yiqiang, Y. Yang, M. Zuiliang, Water loop VRF air conditioning system and its project application, Refrigeration Air Conditioning and Electric Power Machinery 27 (110) (2006) 59–61 (in Chinese).
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