In this paper, medium-temperature chiller with induction air-conditioning system is introduced. Several onsite measurements were carried out on temperature and relative humidity distribution and energy consumption in summer 2020 in order to study the potential performance when applied in hot summer cold winter zone. The performance test results show that (1) the indoor temperature and humidity both comply with the range of design parameters; (2) the corresponding indoor air dew point temperature is much lower than the supply air temperature of air inductive unit, which implies no condensed water or dew on surface of air inductive unit; (3) the performance of the circulation pump, extensive type air handling unit fan; and medium-temperature chiller has exceeded the requirements of national standards.
Recent years has witnessed a large number of buildings being built in China; energy consumption in building sector has increased sharply. Of all consumed amount, heating, ventilation and air-conditioning (HVAC) system occupies approximately 40% of the total building energy consumption (rests are of electricity, hot water etc.), becoming one of the most principal portion [1, 2]. Seeking an improved air-conditioning mode with both high efficiency and thermal comfort is one of the most primary concerns to mitigate the problem of high building energy consumption. In order to condense moisture out, the popular air handling process is to firstly cool air below the dew point by the chiller (supply and return water temperature of 7 and 12°C, respectively) in the current vapor compression HVAC systems. However, this method will result in evaporating temperature being too low and a low chiller coefficient of performance (COP) value; thus, energy consumption of the system increases [3]. A cooling system having high outlet temperature integrates liquid/solid desiccant devices into the desiccant hybrid system; the former is used to remove the sensible portion of the cooling load while the latter is occupied to deal with the latent cooling load. It can avoid the previously mentioned problems and become a promising alternative to existing conventional systems [4]. High outlet temperature chiller is an important component of the desiccant hybrid system, which can remove indoor sensible heat and supply chilled water of 16~18°C with higher COP. Zhu et al. [5] investigated that the average energy efficient ratio of high-temperature refrigerator (owing to high-evaporating temperature) for independent temperature-humidity control air-conditioning system is 4.38. A residential air conditioner with temperature-humidity separate control has been developed by Han et al. [6]. Water cooling evaporator for the radiation panel of medium evaporating temperature cools the water to 16–18°C. Compared to the traditional room air conditioner, ~15.6% of the cooling energy consumption is saved. At present, some manufactures have developed high-evaporating temperature chiller successfully for practical utility [5, 6].
Air inductive unit is a type of air-conditioning terminal designed for hotels, hospital patient rooms, apartments, office buildings and others. Induce air circulation uses outdoor fresh air to supply to air inductive unit. Most air inductive units are wall mounted; ceiling mounting is also a type of installing form [7]. A large amount of power is consumed by a fan-based energy transmission system and water pump at the same time, and variable frequency control is considered as an effective energy saving method [8, 9]. Low-temperature air supply system of high economic becomes the focus of the literature [10].
In this paper, medium-temperature chiller system, the extensive type air handling unit (AHU), and the air induction unit with higher energy efficiency and better thermal comfort are proposed. The system contains the techniques for high evaporating chiller, fan and pump variable frequency control, low-temperature air supply and air inductive unit. These techniques were applied to an office building. Field tests of this system were carried out to analyze its actual system operating performance during the summer of 2020.
The tested office building located in Binjiang District, Hangzhou City, China. As shown in Figure 1, the office building with building area of 2032 m2 covers an area of 1206 m2. The first floor consists of a 331-m2 meeting room and other offices; the second floor is mainly made up of offices. medium-temperature chiller with induction air-conditioning system is installed on the first and second floor in 2011.
As shown in Figure 2, the medium- temperature chiller with induction air-conditioning system consists of three parts: a medium-temperature chiller with 10°C supply water and 20°C return water, which has higher temperature difference of 10°C. From the psychrometric chart in Figure 3, the air handling process of the system is as follows: indoor return air (state point 2) and fresh air (state point 1) are mixed (state point 3) and put into the extensive type AHU. The air supply (state point 4) temperature is 13°C, instead of the normal 16°C by subcooling and dehumidifying, then the air enters to the induction air-conditioning unit. The 19°C air (state point 5) is obtained by reheat the mixed indoor air (state point 4), then it is supplied to the air conditioning room.
Figure 2
Open in new tabDownload slidePrinciple of medium-temperature chiller with induction air-conditioning system.
Figure 3
Open in new tabDownload slideAir handling process in psychrometric chart.
As the key component of the system, fin oval tube heat exchanger is introduced to not only promote the tube side heat transfer effect but also decrease the pressure drop of the fin due to better aerodynamic characteristics [11, 12]. Eight rows of fin oval tube applications in extensive type AHU with the standard velocity of 1.5 m/s can pre-cool the outdoor warm air and return air at the 20°C chilled water outlet. In the meantime, outdoor warm air and return air is cooled down to 13°C at the 10°C chilled water inlet, as shown in Figure 4.
Figure 4
Open in new tabDownload slideFin oval tube heat exchanger with fluid flow direction.
Figure 5 presents the schematic diagram of ceiling air inductive unit. The proportion of the primary air supply to inductive is about 6:4. The static working pressure ranges from 17 to 30 Pa, and air supply temperature is at the primary speed of 0.2~0.8 m/s.
The system can reheat the supply air by inducing the indoor air. The air sensible heat is removed by induction mixing after dehumidification cooling. It can avoid condensation and reduce the cold air downdraft of conventional large-temperature difference with low-temperature supply air. It puts the energy-saving target of large-temperature difference–low-temperature air supply into practice.
$$ \begin{equation} Ws=N/L, \end{equation}$$
(1)
According to Chinese national standard JGJ/T 177–2009 Standard for Energy Efficiency Test of Public Buildings, fan power consumption of unit air volume is defined as:
where Ws is fan power consumption of unit air volume, N is actual fan input power and L is actual fan air volume.
$$ \begin{equation} ER=0.002342H/\Big(\varDelta T\bullet n\Big) \end{equation}$$
(2)
According to Chinese national standard GB 50189–2005 Design Standard for Energy Efficiency Of Public Buildings, the air conditioning water system transmission energy efficiency ratio can be calculated as below:
where H is design water pump head, △T is temperature difference of supply and return water and η is water pump efficiency in the design working point. The two standards are to decide the energy saving of air-conditioning system.
Cooling load of two floors is 253.6 kW based on calculation method of cooling load coefficient [7, 13]. Air-conditioning design parameters for summer season of Hangzhou and cooling load calculation characteristics of the office building are listed in Table 1.
Table 1
Item.
Unit.
Values.
Remarks.
Outdoor dry bulb temperature.
Unit.
Values.
Remarks.
Outdoor dry bulb temperatureTable 1
Item.
Unit.
Values.
Remarks.
Outdoor dry bulb temperature.
Unit.
Values.
Remarks.
Outdoor dry bulb temperatureThe onsite test system is composed of five components: the medium-temperature chiller, extensive type AHU, circulation pump, air inductive unit and data acquisition system.
According to the calculated cooling load, two medium-temperature chillers are installed on the roof of the office building. Chillers are two air-to-water units of scroll hermetic compressors with refrigerant 46 kg R134a. The power input and cooling capacity of the chiller are 41.67 and 130 kW under conditions of chilled water supply temperature 10°C and return temperature 20°C, respectively.
Four extensive type, AHUs are installed in the air handling room. Heat exchanger of the AHUs is of oval copper tube (equivalently to Ф10 round tube) bunched aluminum fins. The air volume, cooling capacity, rated power and outside excess pressure are 9500m3/h, 66 kW, 55 kW and 300 Pa, respectively.
Chilled water loop for the test is water circulating loop. A dual-use and an emergency circulating pump system with 12-m3/h rated flow and 20-mH2O head are chosen for chilled water circulating loops. For the purpose of changing the flow rate of circulating water, two identical frequency converters are adapted to control dual-use variable-speed pumps. The emergency circulating pump is constant frequency controlled.
There are two types of air inductive units from the manufacture. 45 air inductive units (with 180-m3/h standard air volume, 120-m3/h induced air volume, 1.2-kW cooling capacity and 20-Pa outside excess pressure) and 120 air inductive units (with 250-m3/h standard air volume, 170-m3/h induced air volume, 1.7-kW cooling capacity and 20-Pa outside excess pressure) are connected alongside to distribute and are utilized as terminal units on two floors of the office building.
The test system is built up to monitor the temperature and humidity indoor and outdoor, supply and return air temperature, indoor air dew point temperature and supply and return water temperature. To obtain the temperature and relative humidity, sensors are applied in this test. Sensors test ranges are as follows: temperature of −20°C to 50°C, humidity of 0% to 99% RH, and the temperature accuracy is ±0.5°C, humidity is ±4.5% RH. In order to obtain the performance of the tested medium-temperature chiller and the entire system, two YD2000 power monitors with the accuracy of ±0.5% are used to monitor the input power of the medium-temperature chiller, which includes the power of compressor, condenser fan, circulation pump and extensive AHU fan. A glass rotameter LZB-50 is used to measure the water flow rate, with an accuracy of ±1.5% within the range of 10 m3h−1 to 100 m3h−1. The abovementioned sensor data collected are automatically recorded via data acquisition switch unit on a computer.
There are four measurements of the medium-temperature chiller: (1) chilled water flow rate, (2) chilled water supply and return temperature and (3) chiller electricity consumption (includes the power of compressor and condenser fan).
$$ \begin{equation} {Q}_0= V\rho c\varDelta t/3600, \end{equation}$$
(3)
Cooling capacity of the medium-temperature chiller is calculated as follows:
where Q0 is the medium-temperature chiller cooling capacity, ρ is mean density of chilled water, c is mean specific heat at constant pressure of the chilled water and △T is supply and return water temperature difference.
$$ \begin{equation} CO{P}_{chiller}={Q}_0/\sum {N}_c \end{equation}$$
(4)
The medium-temperature chiller COP is calculated as follows:
|$\mathrm{COP}=\frac{\mathrm{Qc}}{\mathrm{W}}$|
The whole system COP is calculated as follows:$$ \begin{equation} CO{P}_{system}={Q}_0/\sum {N}_t, \end{equation}$$
(5)
The whole system COP is calculated as follows:
where |$CO{P}_{system}$| is COP of the whole system (kW/kW) and |$\sum {N}_t$| is sum of the whole system energy consumption equipment, includes chiller, circulation pump and fan of extensive type AHU.
] introduced a method to calculate the experimental accuracy: if given the test result r by the following data reduction equation$$ \begin{equation} r=r\left({X}_1,{X}_2,\cdots \cdots {X}_J\right), \end{equation}$$
(6)
1, X2 and XJ are the measured variables, then in the general uncertainty analysis method the 95% expand uncertainty can be determined by the following equation:$$ \begin{equation} {U}_r\!=\!{\left[{\left(\frac{\partial r}{\partial {X}_1}{U}_{X_1}\right)}^2\!+\!{\left(\frac{\partial r}{\partial {X}_2}{U}_{X_2}\right)}^2\!+\cdots \cdots +{\left(\frac{\partial r}{\partial {X}_J}{U}_{X_J}\right)}^2\right]}^{1/2}, \end{equation}$$
(7)
1, Ux2,… and UxJ are the independent variables uncertainties.The uncertainty in test results was caused by measurement errors. Coleman and Steele [ 14 ] introduced a method to calculate the experimental accuracy: if given the test result r by the following data reduction equationwhere X, Xand Xare the measured variables, then in the general uncertainty analysis method the 95% expand uncertainty can be determined by the following equation:where Ur is the result uncertainty and Ux, Ux,… and Uxare the independent variables uncertainties.
The data collected to evaluate the performance indices are measured by temperature sensors, power sensors and water flow-meter. The uncertainties associated with the experiments were calculated for Eqs. (3), (4) and (5), and the results are 6.2%, 9.7% and 12.6,% respectively.
The performance tests for the medium-temperature chiller with induction air-conditioning system were performed from 9:00 a.m. to 5:00 p.m. (17:00) every day except Saturday and Sunday during summer (from 26 April to 15 September 2020). The test items include the temperature and humidity indoors and outdoors, the performance of the medium-temperature chiller and the performance of the circulation pump and extensive AHU fan.
The room temperature was kept within the range of 24°C–27°C and indoor air relative humidity was kept within the range of 40–65% during summer operation when outdoor temperature remains at the same level. The arrangement test point is illustrated in Figure 6. Figure 7 and Figure 8 show the temperature and relative humidity profile, respectively, of the typical room 203 on 31 July. It is apparent from the figures that both temperatures and relative humidity of the room are within the range of thermal comfort in spite of high outdoor temperature. Although thermal comfort requirement are met, the room temperatures are a little high because the sensible load is relatively large on that day due to relatively high outdoor air temperature. Room relative humidity is sensible to the office staff with a little fluctuation.
Figure 6
Open in new tabDownload slideTest point of the room 203.
Figure 7
Open in new tabDownload slideTemperature values of room 203 on 31July.
Figure 8
Open in new tabDownload slide` humidity values of room 203 on 31 July.
Figure 9
Open in new tabDownload slideAir, water supply and return temperature and room air dew point temperature.
Figure 10
Open in new tabDownload slideVariation of the chiller and system COP on July 31.
As can be seen from Figure 7, room temperatures are nearly the same, except point 2 near the window affected by the outdoor temperature, points 1 and 2 near the air inductive unit and point 3 near the floor. In practical engineering, this influence should be considered.
In order to verify the outlet surface of the inductive unit, room 203 is chosen as a sample to test the air supply temperature and the room air dew point temperature on 31 July. As shown in Figure 9, air supply temperature keeps at ~20°C, which is much higher than the indoor dew point temperature at the same time. So, no condensed water or dew would be formed on surface of the air inductive unit.
Test results of fan power consumption per unit air volume during the tests are shown in Table 2.
Table 2
Fan frequency(Hz).
20.
30.
35.
40.
45.
50.
Air volume(m3/h) 3734 5696 6999 7813 8672 9811 Fan input power(kW) 0.22 0.80 1.26 2.03 2.51 3.44 Fan power consumption of unit air volume W/(m3/h) 0.06 0.14 0.18 0.26 0.29 0.3 Fan frequency(Hz).
20.
30.
35.
40.
45.
50.
Air volume(m3/h) 3734 5696 6999 7813 8672 9811 Fan input power(kW) 0.22 0.80 1.26 2.03 2.51 3.44 Fan power consumption of unit air volume W/(m3/h) 0.06 0.14 0.18 0.26 0.29 0.3 Open in new tabTable 2
Fan frequency(Hz).
20.
30.
35.
40.
45.
50.
Air volume(m3/h) 3734 5696 6999 7813 8672 9811 Fan input power(kW) 0.22 0.80 1.26 2.03 2.51 3.44 Fan power consumption of unit air volume W/(m3/h) 0.06 0.14 0.18 0.26 0.29 0.3 Fan frequency(Hz).
20.
30.
35.
40.
45.
50.
Air volume(m3/h) 3734 5696 6999 7813 8672 9811 Fan input power(kW) 0.22 0.80 1.26 2.03 2.51 3.44 Fan power consumption of unit air volume W/(m3/h) 0.06 0.14 0.18 0.26 0.29 0.3 Open in new tabWhen the extensive type AHU fan running at 30 Hz for the most of time, the fan power consumption per unit air volume W(m3/h) is only 0.14, which is much lower than the national standard value 0.29 for a variable air volume system. So, it saves a lot of energy if we use the national standard to evaluate the supply air system.
During the whole test period, the temperature difference and pump head are kept by the constant pressure variable frequency technique. The lowest water pump efficiency is 0.65,
ER = 0.002342 × 20/(10 × 0.65) = 0.00721.
The running state value is much less than the national standard requirement of 0.0241.
Figure 9 also presents the hourly variations of the supply and return water temperatures for medium-temperature chiller. Temperature difference is kept at ~10°C by the frequency converters controlled circulation pump.
As shown in Figure 9, owing to the high water supply temperature of 10°C and large temperature difference of 10°C, evaporating temperature for cooling can be increased considerably, for example, from current 2°C to 7.5°C, so the performance of chillers is improved. That can be seen from Figure 10: variation of the chiller COP on 31 July. In this new air conditioning system, the average COP of the medium-temperature chiller achieves 3.6, which is higher than 2.9 (with 7/12°C chilled water under normal conditions), the public buildings requirement value of Chinese standard ‘GB 50189-2015 Design standard’.
The variation of the system |$CO{P}_{system}$| during the whole test process on 31 July is shown in Figure 10. From the figure, the variation of the |$CO{P}_{system}$| is consistent with that of the COP because energy consumption proportion of the fan of extensive type AHU (31%) and circulation pump (10%) of the whole summer is consistent with the chiller’s energy consumption (59%) in this system owing to the frequency conversion technique of the fan and pump.
Unit office building energy consumption of medium-temperature chiller with induction air-conditioning system is 28.8 kWh/m2 annually. The data of the metering system in Hangzhou office building with the same building envelope and similar staff occupancy of all air system at the same time are ~38.4 kWh/m2. Office building that adapts medium-temperature chiller with induction air-conditioning system can save energy compared to conventional all air system. The initial cost is 1.4 times more than the all air system. The payback period of the added cost is calculated to be ~4.5 years. It is a promising alternative mode to the all air system.
A new air-conditioning system, the medium-temperature chiller with induction air-conditioning system with extensive AHU of fin oval tube heat exchanger, is introduced in this paper. According to the results from the performance tests, it is found that temperature and humidity conditions can be maintained on a relatively steady level.
Based on the medium-temperature chiller with induction air-conditioning system designed and installed in the office building, performance measurements were carried out in summer in 2020. By analyzing the measurement results, we obtained the following conclusions:
(1) With the variable frequency technique of fan and pump, air supply temperature of the extensive type AHU with fin oval heat exchanger of heat transfer enhancement and low-pressure drop can keep at a low air supply temperature 13°C, and temperature difference of water supply and return can keep at 10°C during the experimental tests. The fan and pump power consumption of unit air volume is much less than the national standard. Thus they are more energy efficient.
(2) The medium-temperature chiller COP is in the range of 3.1–4.2, much higher than the national standard requirement value of 2.9.
(3) The average COP of the medium-temperature chiller with induction air-conditioning system is 3.6. It is an energy efficient air conditioning system and therefore is worth being broadening.
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