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Optimizing energy consumption of a water-loop variable-speed heat pump system
Applied Thermal Engineering 31 (2011) 894e901
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Optimizing energy consumption of a water-loop variable-speed heat pump system Shui Yuan a, *, Michel Grabon b,1 a b
United Technologies Research Center, MS129-85, 411 Silver Lane, East Hartford, CT 06108, USA Carrier Corporation, Route de Thil, 01122 Montluel, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2010 Accepted 1 November 2010 Available online 20 November 2010
Water-loop heat pump (WLHP) systems have been widely used for decades. The most appealing feature of WLHP is its capability of handling simultaneous heating and cooling demands in a building and recovering heat generated from zones in cooling. The typical heat sink and source are cooling tower and boiler. WHLPs discussed in this paper are equipped with a variable-speed compressor; the cooling tower comes with one variable-speed fan; and the boiler has a continuously-adjustable capacity. This paper analyzes how the energy efficiency of individual component responds to variations in loop water temperature; demonstrates that there exists a unique loop-water temperature that minimizes overall energy consumption given a certain building load; proposes a practical strategy to find the optimal temperature in a real-time manner. 2010 United Technologies Corporation. Published by Elsevier Ltd. All rights reserved.
Keywords: Water-loop heat pump Optimization Energy efficiency
1. Introduction Water-loop heat pump (WLHP) systems have been widely used for decades in commercial, residential and institutional buildings. As shown in Fig. 1, a WLHP system is composed of a set of watersource heat pumps and a pair of heat source and sink. The watersource heat pumps are installed in zones of a building to maintain zonal air temperature; the heat source provides the energy to meet the building’s heating demand, while the heat extracted from the building is rejected into the heat sink. All the components are connected with a closed two-pipe water loop, in which the water is circulated by a pump. Depending on the zonal demand e heating or cooling, some heat pumps increase water temperature by releasing heat into the loop, while some tend to lower the water temperature by taking out heat. The change of loop water temperature reflects the summed thermal demand of the building. If water temperature keeps decreasing, it indicates that heating load in building is dominant, so the heat source should be turned on to add heat; if water temperature keeps rising, it means the building has a cooling load larger than heating, the heat sink should run to remove heat. One of the most appealing features is the ability to satisfy the cooling and
* Corresponding author. Tel.: þ1 860 610 7483; fax: þ1 860 610 7134. E-mail addresses: [email protected] (S. Yuan), michel.grabon@carrier. utc.com (M. Grabon). 1 Tel.: þ33 (0)4 72 25 22 15.
heating demand simultaneously with heat recovery. The heat generated from cooling zones can be utilized to heat up air in heating zones or to make domestic hot water, such that thermal loads on the heat source/sink reduces. WLHP systems work friendly with various types of HVAC components. The typical heat-source sink are a cooling tower plus a fossil-fuel boiler, but other options are acceptable such as a groundsource heat exchanger, an absorption chiller, a boiler, a solar thermal collector, an air/water-cooled/ground-source heat pumps, etc. The water-source heat pumps can be either water-to-air or waterto-water. The latter can work with fan-coil units, chilled beams/ panels to implement zonal air conditioning. The configuration investigated in this paper is shown in Fig. 2. A cooling tower with a variable-speed fan is employed as the heat sink; a boiler works as a heat source, capacity of which can be continuously adjusted from 0 to 100%. A feedback temperature controller adjusts the fan speed or the boiler’s capacity to maintain the loop water temperature, Tlp, within a range or at a set point. The water-to-water scroll heat pumps are installed in zones to provide chilled or hot water to chilled beams, which have both heating and cooling capability. The heat pumps are equipped with a variablespeed compressor. A closed-loop controller maintains zonal temperature at its set point by having the chilled beams’ capacity match the load, which is achieved by turning on a subset of heat pumps plus chilled beams and adjusting the temperature of water entering the chilled beams. Each heat pump is controlled by a dedicated controller, which receives the chilled/hot water temperature set point from zonal temperature controller and varies
1359-4311/$ e see front matter 2010 United Technologies Corporation. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.11.012
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
Nomenclature c cpw CAP CAPFT
coefficients in heat pump and cooling tower model specific heat of water capacity (kW) a correlation equation denoting ratio of maximal capacity under certain load-side and source-side temperature to reference capacity reference capacity (kW) CAPref maximal capacity under certain load-side and sourceCAPT side temperature (kW) COP coefficient of performance dTlpmax allowed maximal change of Tlp ( C) e coefficients in heat pump model EIRFT a correlation equation denoting efficiency at maximal capacity under certain load-side and source-side temperature to reference efficiency EIRFPLR a correlation equation denoting power ratio at a certain PLR EWT entering water temperature ( C) flue loss factor of boiler fl coefficient of boiler’s heat loss to ambient (W/ C) fa l coefficients in heat pump model LWT leaving water temperature ( C) LWTbomax maximal leaving water temperature of boiler ( C) _ m mass flow rate of fluid (kg/sec) OATWB outdoor air wet-bulb temperature ( C) p coefficients in heat pump model PLR part load ratio with respect to maximal capacity under certain load-side and source-side temperature
the compressor speed to control the water temperature at the set point. We define the side of a heat pump connecting with chilled beams as load side, and the part linked to the water loop as source side. The heat pump has a constant water flow rate on both load and source sides when it operates, while the total loop water flow rate is variable depending on the number of running heat pumps. There is no thermal storage in the studied system, but the thermal mass of loop water is taken into account. The loop pump is of variable speed, which is controlled to maintain a preset supply water pressure. The boiler and cooling tower usually requires a minimum water flow rate to operate effectively. A bypass line and a flow control valve are installed for this purpose. The return water flow rate is maintained no lower than the minimal by a feedback controller [14].
POW POWref POW T Q Tam Tlp Tlb Tstpt Tub Ta, Tb T1, T2
h
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power (kW) reference power (kW) maximal power under certain load-side and sourceside temperature (kW) load ambient temperature of boiler ( C) supply water temperature of the loop ( C) low limit on supply water temperature of the loop ( C) supply water temperature set point of the loop ( C) upper limit on supply water temperature of the loop ( C) Endpoint temperatures of bracket embracing minimum, Ta < Tb, ( C) Midpoint temperatures during a minimum searching iteration ( C) efficiency
Subscripts a air bo boiler c cooling ct cooling tower h heating hp heat pump l load side lp water loop old value from previous control interval r rated s source side w water
Although WLHP systems enjoy an inherent capability of heat recovery, there is still much room for improving the overall energy efficiency. This topic has been investigated from various aspects. Kush and Brunner [11] conducted a field study to identify the ways to reduce capital and operation cost. Howell and Zaidi [6] proposed a method to determine if heat recovery potential of a building can make the system work without a boiler. Howell and Zaidi [7,8] demonstrated with simulations that a maximal heat recovery rate of 76% and sensitivity of energy savings to various building characteristics. Woller [16] presented an application of water-loop heat pumps plus ground-source heat exchangers. Henderson et al. [5] presented the energy savings of applying variable-speed water pumps in a WLHP system. Marseille and Schliesing [13] analyzed the performance and applicability of a WLHP system using building
Fig. 1. Schematic of a WHLP system.
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S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
Fig. 2. Configuration of the studied WHLP system.
structure as thermal storage. Lian et al. [12] evaluated the energy performance of WLHP systems under different climate conditions and building types. In typical applications of WLHP systems, the loop water temperature is controlled within a band, which usually is between 16 C and 32 C [1]. When Tlp reaches 32 C, the cooling tower will be on to remove heat out of the water so that Tlp is no higher than 32 C. The boiler will be on if Tlp lower than 16 C. The loop water temperature is allowed to vary free between 16 C and 32 C. Since the energy efficiency of heat pumps, cooling tower and boiler are a function of Tlp, the above strategy may not achieve the best overall energy performance in many situations. Pietsch [15] analyzed the effect of loop water temperature on energy consumption using hypothetical load profiles and simplified component models. The relationship between efficiency of a fossil-fuel boiler and Tlp was not considered. The trends of energy consumption with respect to Tlp were discussed, and optimal loop water temperature was proposed in a qualitative way. This paper analyzes the impacts of Tlp on the energy usage of a WLHP system without limitations on load conditions; demonstrates that there exists a unique loop water temperature to minimize the overall energy consumption; and proposes a simple but effective strategy to find the optimal loop water temperature in a real-time manner. Simulation-based comparisons show that the proposed loop water control strategy outperforms the conventional one in cooling mode.
modeling chillers in various configurations in terms of capacity, compressor types, working fluid, etc. This paper uses this model for both cooling and heating modes of the water-to-water heat pump. We notice that the original model predicts the power well when the compressor runs at full speed, while degradation occurs as the speed reduces, i.e. in partial load conditions. The solution we propose in this paper is to include the compressor speed and water temperatures of load and source sides in the calculation of PLR and EIRFPLR. Eqs. (1)e(6) are the model, and Eqs. (1), (2), (5) and (6) are in their original form in [9], while Eqs. (3) and (4) are revised. Fig. 3 shows the improvement of power prediction from the revised model. The average percentage error of the original model is 5.08% at 750 RPM in cooling while that of the revised one is 0.8%. Similar results are reached for other speeds (full speed ¼ 3000 RPM).
CAPFT ¼
CAPT ¼ c1 þ c2 LWTl þ c3 LWT2l þ c4 EWTs CAPref þ c5 EWT2s þ c6 LWTl EWTs
ð1Þ
2. Energy performance vs. loop water temperature Tlp The following analysis assumes a quasi-static system, which means all the components move from an equilibrium state to another during its course of evolvement and transients are not considered. This assumption is justified by the fact that building envelopes have a slow thermodynamics with a time constant in an order of hour while the response of HVAC equipment is much faster [3]. 2.1. Water-to-water heat pump The chiller model used in DOE2 simulation program and discussed in [9] is adopted in this study, which is applicable to
Fig. 3. Comparison of original and revised water-to-water heat pump models.
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
Fig. 4. (a) Input power vs. EWTS in cooling mode. (b) Input power vs. EWTS in heating mode.
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S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
EIRFT ¼
þ e4 EWTs þ e5 EWT2s þ e6 LWTl EWTs PLR ¼
here: vPOWhp/vEWTs > 0 in cooling, vPOWhp/vEWTs < 0 in heating, and v2POWhp/vEWTs2 0.
POWT =CAPT ¼ e1 þ e2 LWTl þ e3 LWT2l POWref =CAPref ð2Þ
CAP ¼ l1 þ l2 N þ l3 LWTl þ l4 EWTs þ l5 NLWTl CAPT þ l6 NLWTl
2.2. Cooling tower The capacity model of cooling tower in [10] is used in this paper.
ð3Þ
CAPct ¼
_ w c3 c1 m c3 ðEWTct OATWBÞ _w 1 þ c2 m _ m
(8)
a
EIRFPLR ¼
POW ¼ p1 þ p2 LWTl þ p3 EWTs þ p4 LWTl EWTs þ p5 LWT2l þ p6 EWT2s POWT þ N p7 þ p8 LWTl þ p9 EWTs þ p10 LWTl EWTs þ p11 LWT2l þ p12 EWT2s þ N 2 p13 þ p14 LWTl þ p15 EWTs þ p16 LWTl EWTs þ p17 LWT2l þ p18 EWT2s
CAP ¼ CAPref $CAPFT$PLR
(5)
POW ¼ POWref $CAPFT$EIRFT$EIRFPLR
(6)
_ l $jEWTl LWTl j Q ¼ CAP ¼ 4:186$m
(7)
Eq. (4) defines EIRFPLR, a factor representing power level at a certain temperature condition and compressor speed. Eqs. (5) and (6) are used to calculate a heat pump’s capacity and power
POWct ¼ POWr
_w m _ ar m
3
The power model is built based on fan laws.
Eq. (1) defines CAPFT, a factor representing the ratio of maximal capacity at a certain temperature condition to the reference capacity. Fig. 1 shows temperature and flow rate definitions. It is supposed that temperatures in Fig. 1 are known. Eq. (2) defines EIRFT, a factor representing the ratio of energy efficiency at a certain temperature condition to the reference efficiency. Eq. (3) defines part load ratio at a certain temperature condition and compressor speed. Since the system is quasi-static, the capacity of a heat pump, CAP, is always equal to the load, which is equal to the enthalpy change of load-side water, i.e.
ð4Þ
POWct ¼ POWr
_a m _ ar m
3 (9)
Given a cooling load, which is supposed to be matched by cooling tower’s capacity, from Eq. (8) the air mass flow rate should be:
_a ¼ m
c2 CAPct _ w Þc3 ðEWTct OATWBÞ CAPct c1 ðm
1
c3
_w m
(10)
Based on energy balance and ignore the contribution from the make-up water,
EWTct ¼ LWTct þ
CAPct _w cpw m
(11)
Plug Eqs. (10) and (11) into (9),
0
13 c3 c2 CAPct @ A _ w Þc3 LWTct þ CAP_ ct OATWB CAPct c 1 ðm c m pw
(12)
w
consumption, where CAPref and POWref are constant. Note that the coefficients, c, e, l and p, are different in heating and cooling. In cooling mode, given a certain load, lowering EWTs leads to a smaller temperature lift (the temperature difference between condensing and evaporating refrigerant), which increases efficiency so that a heat pump consumes less electricity [2]. Thus vPOW/vEWTs is always positive in cooling model. In heating mode the trend is opposite. Fig. 4 shows the trends of input power in cooling (a) and heating (b) mode of the model built based on manufacturer’s data. The surface in each plot is the input power, and the lines are composed of points on the surface with the same capacity. We also examined v2POW/vEWTs2, given a certain load, by calculating both the second-order difference using the manufacturer’s data and analytical values from the model. It is found that, when EWTs is [10 C, 40 C], v2POW/vEWTs2 is always nonnegative (positive in most cases) in both cooling and heating modes. The plots in Fig. 4 coincide with the conclusions, which are summarized
Take the derivative of POWct with respect to LWTct using Eq. (12),
c3 _a 3c1 m vPOWct ¼ ð1ÞPOWct <0 vLWTct c2 c3 CAPct
(13)
According to [10], c1 and c2 are a function of physical properties of water and air, thus both are positive. c3 must be positive due to the fact that increasing air flow rate leads to a higher cooling capacity, which requires a positive c3 in Eq. (8). So Eq. (13) is always negative. Take the second derivative using Eq. (13),
v2 POWct vLWT2ct
¼ POWct
2c3 2 _a m c1 3 3 CAPct2
c2
c3 c3
þ1 >0
It is apparent that the second derivative is positive.
(14)
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
2.3. Boiler The fossil-fuel boiler model in [17] is employed in this paper. The original model is dynamic with thermal capacitance of the boiler considered, but the dynamics is ignored in this quasi-static analysis.
f LWTbo ¼ CAPbo þ fa ðLWTbo Tam Þ POWbo 1 l LWTbomax
(15)
Take the derivative of POWbo with respect to LWTbo using Eq.(15),
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Based on the analysis above, Eq. (20) is nonnegative. So POWtotal(Tlp) is a convex function on (Tlb, Tub), any local minimum of which is also a global minimum. If d2POWtotal/dT2lp>0, then it is a strictly convex, which means there is a unique minimum on [Tlb, Tub]. If d2POWtotal/dT2lp ¼ 0, then: if dPOWtotal/dTlp ¼ 0 as well, POWtotal is constant on [Tlb, Tub], i.e. its value is independent of Tlp; if dPOWtotal/dTlp s 0, POWtotal is a straight line on [Tlb, Tub], which means the minimum is at either Tlb or Tub.
dPOWbo f LWTbomax CAPbo þ fa LWTbomax ðLWTbomax fl LWTbo Þ þ fa LWTbomax ðLWTbo Tam Þ ¼ l >0 dLWTbo ðLWTbomax fl LWTbo Þ2
Since fl ˛½0; 1 and practically LWTbo > Ta, all the three terms in the numerator on the right-side of Eq. (16) is positive, which in turn makes Eq. (16) positive. Take the second-order derivative using Eq. (16),
d2 POWbo dLWT2bo
¼
fa LWTbomax ð1 fl Þ þ ðLWTbo Tam Þ ðLWTbomax fl LWTbo Þ2
þ
(16)
3. A strategy to find the optimal Tlp that minimizes POWtotal Since a unique minimum of POWtotal on [Tlb, Tub] exists, an algorithm called Golden Section Search [4] is employed in this paper
2fl LWTbomax CAPbo þ ðLWTbomax fl LWTbo Þ þ ðLWTbo Tam Þ ðLWTbomax fl LWTbo Þ3
>0
(17)
to find the optimal Tlp, which is a simple and effective minimumbracketing technique without need of calculating gradients and can be easily applied to real-time applications.
It is apparent that the second derivative is positive.
2.4. System When the system is in operation, the load of each heat pump can be calculated using Eq. (7); the power can be found _ lp and Tlp are using Eqs. (1)e(6). On the water-loop side, m measured directly, EWTlp can be either measured or calculated based on energy and mass balance, so the load imposed on the cooling tower or boiler can be found, i.e. CAPct or CAPbo. The power of cooling tower or boiler can be found using Eq. (12) or (15). Note that we let EWTct and EWTbo equal to Tlp in analysis, but in simulations the heat added due to water pump inefficiency is included. The total powers has Tlp as the only independent variable, which is:
X POWtotal Tlp ¼ POWhpi Tlp þ POWct Tlp þ POWbo Tlp i
(18) Actually the cooling tower or boiler’s load is affected by variations in Tlp because it changes heat pumps’ efficiency, but we have the behavior of cooling tower and boiler decoupled from that of heat pump in order to simplify the situation, i.e. the load appears as a constant instead of a variable in Eq. (18). Eq. (18) is continuous on [Tlb, Tub], so POWtotal(Tlp) has its minimum and maximum according to the extreme value theorem. Eq. (18) is also twice differentiable on (Tlb, Tub),
X dPOWhp dPOWct dPOW dPOWtotal bo ¼ þ þ dTlp dTlp dTlp dTlp
(19)
X d2 POWhp d2 POWct d2 POW d2 POWtotal bo ¼ þ þ 0 2 2 2 2 dTlp dT dT dT lp lp lp i
(20)
(1) Identify all the parameters in Eqs. (1)e(6), (12) and (15) in an offline manner; (2) t each control interval, (1) Acquire a set of system operating conditions, which _ l of includes the mode (heating or cooling), LWTl, and m each operating heat pump; variables related to the performance of the cooling and boiler eTlp, Tstptold, EWTlp, _ lp , OATWB. m (2) Calculate the current load of each heat pump, load on cooling tower or boiler, determine whether cooling tower or boiler should be in operation; (3) Search the optimal Tlp by using the Golden Section Search algorithm to minimize POWtotal(Tlp) on [Tlb, Tub]; (4) Limit Tlpstpt between Tlpstptold dTlpmax and Tlpstptold þ dTlpmax, and send Tlpstpt and operation mode (heating or cooling) to the Tlp controller (see Fig. 2). The procedure at each of iteration to find the optimal Tlp (see Fig. 5) is described here.
i
Fig. 5. Golden Section Search algorithm.
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S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
If |Tb Ta| is less than a predefined tolerance, the search terminates successfully. Otherwise, (a) if POWtotal(T1) > POWtotal(T2), Ta ¼ T1, T1 ¼ T2, POWtotal(T1) ¼ POWtotal(T2), T2 ¼ Ta þ 0.618(Tb Ta), and update POWtotal(T2); (b) if POWtotal(T1) POWtotal(T2), Tb ¼ T2, T2 ¼ T1, POWtotal(T2) ¼ POWtotal(T1), T1 ¼ Ta þ 0.382(Tb Ta), and update POWtotal(T1). Note that at the very first iteration, Ta ¼ Tlb and Tb ¼ Tub.
loop side, each operating water-to-water heat pump has a fixed water flow of 0.15 kg/s, and there is a minimal water flow of 3 kg/s through the boiler and cooling tower. The loop holds a total of 300 kg of water. The water pressure of the loop pump is maintained at 75 kPa. All the water pumps have an overall efficiency of 75%. The boiler has a maximal input energy rate of 35 kW, an fl of 0.1 and an fa of 0.0737 kW/ C. The cooling tower has an evaporator cooling capacity of 52.8 kw (15 ton), a rated fan power of 1.5 kW, and c1 ¼ 6.6, c2 ¼ 1.92, c3 ¼ 0.94. The model has a simulation time step of 1 min. The module of optimal control strategy updates Tlp set point every 5 min and passes it to the Tlp controller. The parameter settings are: Tlb ¼ 16 C, Tub ¼ 32 C, dTlpmax ¼ 1 C. A one-year-long simulation is conducted.
4. Evaluating the optimal control strategy on Tlp 4.1. Cooling We need to compare the performance of the optimal Tlp reset strategy with the conventional one, i.e. injecting heat into the loop when Tlp 16 C by turning on the boiler and removing heat when Tlp 32 C by running the cooling tower. A fair comparison requires the exactly same operating conditions applied to both cases in terms of load profile, weather, etc., so computer simulations are employed to conduct the analysis. The building and WLHP system is modeled with TRNSYS e a leading simulation platform for buildings and HVAC (http://sel. me.wisc.edu/trnsys/). The modeled building has 6 zones with a floor area of 991 m2. Multiple terminal units are installed in every zone, each of which consists of two active chilled beams and one water-to-water heat pump. Each terminal unit has a nominal capacity of 2.3 kW in cooling and 3.0 kW in heating. The quantity of terminal units in a zone is determined based on the zonal peak sensible load, which is 5, 4, 5, 7, 7 and 8 respectively for each zone. The latent cooling load and ventilation are handled by an airhandling unit, which is out of the scope of this study. On the water-
The performance between hour 4137 and 4146 is illustrated in Fig. 6. Fig. 6a shows the cooling load profile applied to both the conventional and optimal cases. Fig. 6b shows that the set points in the optimal case are lower than the conventional 32 C. The power consumption of heat pumps and fans in the cooling tower is presented in Fig. 6c. The optimal case consumes more fan power to achieve a lower Tlp so that even more energy can be saved on heat pumps. The plots in Fig. 6d are accumulative energy consumption of the two cases, which are the integration of the total power in Fig. 6c. It clearly shows that the optimal outperforms the conventional case by using less energy, which is 23.4% actually. Table 1 summarizes the energy performance when the system has a net cooling load over a year. 4.2. Heating The optimal strategy generates a constant of Tlp set point e 16 C, which is identical to the conventional set point, so that no
Fig. 6. Performance in cooling between hour 4137 and 4146.
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901 Table 1 Energy performance comparison of the two Tlp control strategy. Water-to-water heat pump Water pump Fan in cooling tower Total Energy savings
MWh MWh MWh MWh %
20.04 3.07 0.30 23.41
14.43 3.07 1.62 19.12 18.3%
energy improvement is found in heating mode. The reasons are as follows. Assuming that Qc and Qh are the cooling and heating loads from a building, and total power consumption of water-to-water heat pumps are POWc in cooling and POWh in heating, if the net load is heating (Qh > Qc 0), we have the heat injected into the loop water by heat pumps as:
ðQc þ POWc Þ ðQh þ POWh Þ;
(21)
which is negative. The boiler’s capacity must match the load, thus,
POWbo h þ ðQc þ POWc Þ ðQh POWh Þ ¼ 0
(22)
Rearrange the terms in Eq. (22),
POWbo h þ POWc þ POWh ¼ Qh Qc
(23)
If we have a boiler without a heat loss, i.e. h ¼ 1, the total power consumption will be constant since the right side of Eq. (23) is constant, which means manipulating Tlp will only change the ratio of the terms on the left side of Eq. (23). If h < 1, i.e. fl s 0 and/or fa s 0, the boiler’s efficiency can be improved by keeping Tlp as low as possible. 5. Conclusions This paper analyzes the impact of loop water temperature on energy performance of a WLHP system at both component and system levels, and demonstrates that there exists a unique temperature minimizing the energy consumption. An optimal loop water temperature reset strategy has been proposed and evaluated
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to be effective with simulations for cooling, while the optimal loop water temperature in heating mode is the same as the conventional one so that no energy can be further saved. References [1] ASHRAE, ASHRAE Handbook e 2000 HVAC Systems and Equipment. American Society of Heating, Refrigerating, and Air-Conditioning Engineerings, Atlanta, 2000, [Chapter 8]. [2] J.F. Kreider, Handbook of Heating, Ventilation, and Air Conditioning. CRC Press, 2000. [3] J. Bourdouxhe, M. Grodent, J. Lebrun, Reference Guide for Dynamic Models of HVAC Equipment. American Society of Heating, Refrigerating, and AirConditioning Engineerings, Atlanta, 1998. [4] C.F. Gerald, P.O. Wheatley, Applied Numerical Analysis. Addison Wesley, 2004. [5] H.I.J. Henderson, M.K. Khattar, S.W. Carlson, A.C. Walburger, The implications of the measured performance of variable flow pumping systems in geothermal and water loop heat pump applications, ASHRAE Trans 106 (2) (2006) 750e757. [6] R.H. Howell, J.H. Zaidi, Analysis of heat recovery in water-loop heat pump systems, ASHRAE Trans 96 (1) (1990) 1039e1047. [7] R.H. Howell, J.H. Zaidi, Heat recovery in buildings using water-loop heat pump systems: part I e Energy requirements and savings, ASHRAE Trans 97 (2) (1991) 736e749. [8] R.H. Howell, J.H. Zaidi, Heat recovery in buildings using water-loop heat pump systems - part II - sensitivity analysis, ASHRAE Trans 97 (2) (1991) 750e757. [9] M. Hydeman, K.L.J. Gillespie, Tools and techniques to calibrate electric chiller component models, ASHRAE Trans 108 (1) (2002) 733e741. [10] G. Jin, W. Cai, L. Lu, E.L. Lee, A. Chiang, A simplified modeling of mechanical cooling tower for control and optimization of HVAC systems. [11] E.A. Kush, C.A. Brunner, Field test results applied to optimizing water-loop heat pump design and performance, ASHRAE Trans 97 (2) (1991) 727e735. [12] Z. Lian, S. Park, H. Qi, Analysis on energy consumption of water-loop heat pump system in China, Appl Thermal Eng 25 (2005) 73e85. [13] T.J. Marseille, J.S. Schliesing, The Integration of Water Loop Heat Pump a.d Building Structural Thermal Storage Systems. Pacific Northwest Laboratory, Richland, 1991. [14] PG&E, CoolTools Chilled Water Plant Design and Specification Guide. Pacific Gas and Electric Company, 2000. [15] J.A. Pietsch, Optimization of loop temperatures in water-loop heat pump systems, ASHRAE Trans 97 (2) (1991) 713e726. [16] B.E. Woller, Design and operation of a commercial water-loop heat pump system with a ground-loop heat exchanger, ASHRAE Trans 100 (1) (1994) 1577e1587. [17] M. Zaheer-Uddin, Decentralized control systems for HVAC, ASHRAE Trans 98 (2) (1992) 114e126.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Optimizing energy consumption of a water-loop variable-speed heat pump system Shui Yuan a, *, Michel Grabon b,1 a b
United Technologies Research Center, MS129-85, 411 Silver Lane, East Hartford, CT 06108, USA Carrier Corporation, Route de Thil, 01122 Montluel, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2010 Accepted 1 November 2010 Available online 20 November 2010
Water-loop heat pump (WLHP) systems have been widely used for decades. The most appealing feature of WLHP is its capability of handling simultaneous heating and cooling demands in a building and recovering heat generated from zones in cooling. The typical heat sink and source are cooling tower and boiler. WHLPs discussed in this paper are equipped with a variable-speed compressor; the cooling tower comes with one variable-speed fan; and the boiler has a continuously-adjustable capacity. This paper analyzes how the energy efficiency of individual component responds to variations in loop water temperature; demonstrates that there exists a unique loop-water temperature that minimizes overall energy consumption given a certain building load; proposes a practical strategy to find the optimal temperature in a real-time manner. 2010 United Technologies Corporation. Published by Elsevier Ltd. All rights reserved.
Keywords: Water-loop heat pump Optimization Energy efficiency
1. Introduction Water-loop heat pump (WLHP) systems have been widely used for decades in commercial, residential and institutional buildings. As shown in Fig. 1, a WLHP system is composed of a set of watersource heat pumps and a pair of heat source and sink. The watersource heat pumps are installed in zones of a building to maintain zonal air temperature; the heat source provides the energy to meet the building’s heating demand, while the heat extracted from the building is rejected into the heat sink. All the components are connected with a closed two-pipe water loop, in which the water is circulated by a pump. Depending on the zonal demand e heating or cooling, some heat pumps increase water temperature by releasing heat into the loop, while some tend to lower the water temperature by taking out heat. The change of loop water temperature reflects the summed thermal demand of the building. If water temperature keeps decreasing, it indicates that heating load in building is dominant, so the heat source should be turned on to add heat; if water temperature keeps rising, it means the building has a cooling load larger than heating, the heat sink should run to remove heat. One of the most appealing features is the ability to satisfy the cooling and
* Corresponding author. Tel.: þ1 860 610 7483; fax: þ1 860 610 7134. E-mail addresses: [email protected] (S. Yuan), michel.grabon@carrier. utc.com (M. Grabon). 1 Tel.: þ33 (0)4 72 25 22 15.
heating demand simultaneously with heat recovery. The heat generated from cooling zones can be utilized to heat up air in heating zones or to make domestic hot water, such that thermal loads on the heat source/sink reduces. WLHP systems work friendly with various types of HVAC components. The typical heat-source sink are a cooling tower plus a fossil-fuel boiler, but other options are acceptable such as a groundsource heat exchanger, an absorption chiller, a boiler, a solar thermal collector, an air/water-cooled/ground-source heat pumps, etc. The water-source heat pumps can be either water-to-air or waterto-water. The latter can work with fan-coil units, chilled beams/ panels to implement zonal air conditioning. The configuration investigated in this paper is shown in Fig. 2. A cooling tower with a variable-speed fan is employed as the heat sink; a boiler works as a heat source, capacity of which can be continuously adjusted from 0 to 100%. A feedback temperature controller adjusts the fan speed or the boiler’s capacity to maintain the loop water temperature, Tlp, within a range or at a set point. The water-to-water scroll heat pumps are installed in zones to provide chilled or hot water to chilled beams, which have both heating and cooling capability. The heat pumps are equipped with a variablespeed compressor. A closed-loop controller maintains zonal temperature at its set point by having the chilled beams’ capacity match the load, which is achieved by turning on a subset of heat pumps plus chilled beams and adjusting the temperature of water entering the chilled beams. Each heat pump is controlled by a dedicated controller, which receives the chilled/hot water temperature set point from zonal temperature controller and varies
1359-4311/$ e see front matter 2010 United Technologies Corporation. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.11.012
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
Nomenclature c cpw CAP CAPFT
coefficients in heat pump and cooling tower model specific heat of water capacity (kW) a correlation equation denoting ratio of maximal capacity under certain load-side and source-side temperature to reference capacity reference capacity (kW) CAPref maximal capacity under certain load-side and sourceCAPT side temperature (kW) COP coefficient of performance dTlpmax allowed maximal change of Tlp ( C) e coefficients in heat pump model EIRFT a correlation equation denoting efficiency at maximal capacity under certain load-side and source-side temperature to reference efficiency EIRFPLR a correlation equation denoting power ratio at a certain PLR EWT entering water temperature ( C) flue loss factor of boiler fl coefficient of boiler’s heat loss to ambient (W/ C) fa l coefficients in heat pump model LWT leaving water temperature ( C) LWTbomax maximal leaving water temperature of boiler ( C) _ m mass flow rate of fluid (kg/sec) OATWB outdoor air wet-bulb temperature ( C) p coefficients in heat pump model PLR part load ratio with respect to maximal capacity under certain load-side and source-side temperature
the compressor speed to control the water temperature at the set point. We define the side of a heat pump connecting with chilled beams as load side, and the part linked to the water loop as source side. The heat pump has a constant water flow rate on both load and source sides when it operates, while the total loop water flow rate is variable depending on the number of running heat pumps. There is no thermal storage in the studied system, but the thermal mass of loop water is taken into account. The loop pump is of variable speed, which is controlled to maintain a preset supply water pressure. The boiler and cooling tower usually requires a minimum water flow rate to operate effectively. A bypass line and a flow control valve are installed for this purpose. The return water flow rate is maintained no lower than the minimal by a feedback controller [14].
POW POWref POW T Q Tam Tlp Tlb Tstpt Tub Ta, Tb T1, T2
h
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power (kW) reference power (kW) maximal power under certain load-side and sourceside temperature (kW) load ambient temperature of boiler ( C) supply water temperature of the loop ( C) low limit on supply water temperature of the loop ( C) supply water temperature set point of the loop ( C) upper limit on supply water temperature of the loop ( C) Endpoint temperatures of bracket embracing minimum, Ta < Tb, ( C) Midpoint temperatures during a minimum searching iteration ( C) efficiency
Subscripts a air bo boiler c cooling ct cooling tower h heating hp heat pump l load side lp water loop old value from previous control interval r rated s source side w water
Although WLHP systems enjoy an inherent capability of heat recovery, there is still much room for improving the overall energy efficiency. This topic has been investigated from various aspects. Kush and Brunner [11] conducted a field study to identify the ways to reduce capital and operation cost. Howell and Zaidi [6] proposed a method to determine if heat recovery potential of a building can make the system work without a boiler. Howell and Zaidi [7,8] demonstrated with simulations that a maximal heat recovery rate of 76% and sensitivity of energy savings to various building characteristics. Woller [16] presented an application of water-loop heat pumps plus ground-source heat exchangers. Henderson et al. [5] presented the energy savings of applying variable-speed water pumps in a WLHP system. Marseille and Schliesing [13] analyzed the performance and applicability of a WLHP system using building
Fig. 1. Schematic of a WHLP system.
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Fig. 2. Configuration of the studied WHLP system.
structure as thermal storage. Lian et al. [12] evaluated the energy performance of WLHP systems under different climate conditions and building types. In typical applications of WLHP systems, the loop water temperature is controlled within a band, which usually is between 16 C and 32 C [1]. When Tlp reaches 32 C, the cooling tower will be on to remove heat out of the water so that Tlp is no higher than 32 C. The boiler will be on if Tlp lower than 16 C. The loop water temperature is allowed to vary free between 16 C and 32 C. Since the energy efficiency of heat pumps, cooling tower and boiler are a function of Tlp, the above strategy may not achieve the best overall energy performance in many situations. Pietsch [15] analyzed the effect of loop water temperature on energy consumption using hypothetical load profiles and simplified component models. The relationship between efficiency of a fossil-fuel boiler and Tlp was not considered. The trends of energy consumption with respect to Tlp were discussed, and optimal loop water temperature was proposed in a qualitative way. This paper analyzes the impacts of Tlp on the energy usage of a WLHP system without limitations on load conditions; demonstrates that there exists a unique loop water temperature to minimize the overall energy consumption; and proposes a simple but effective strategy to find the optimal loop water temperature in a real-time manner. Simulation-based comparisons show that the proposed loop water control strategy outperforms the conventional one in cooling mode.
modeling chillers in various configurations in terms of capacity, compressor types, working fluid, etc. This paper uses this model for both cooling and heating modes of the water-to-water heat pump. We notice that the original model predicts the power well when the compressor runs at full speed, while degradation occurs as the speed reduces, i.e. in partial load conditions. The solution we propose in this paper is to include the compressor speed and water temperatures of load and source sides in the calculation of PLR and EIRFPLR. Eqs. (1)e(6) are the model, and Eqs. (1), (2), (5) and (6) are in their original form in [9], while Eqs. (3) and (4) are revised. Fig. 3 shows the improvement of power prediction from the revised model. The average percentage error of the original model is 5.08% at 750 RPM in cooling while that of the revised one is 0.8%. Similar results are reached for other speeds (full speed ¼ 3000 RPM).
CAPFT ¼
CAPT ¼ c1 þ c2 LWTl þ c3 LWT2l þ c4 EWTs CAPref þ c5 EWT2s þ c6 LWTl EWTs
ð1Þ
2. Energy performance vs. loop water temperature Tlp The following analysis assumes a quasi-static system, which means all the components move from an equilibrium state to another during its course of evolvement and transients are not considered. This assumption is justified by the fact that building envelopes have a slow thermodynamics with a time constant in an order of hour while the response of HVAC equipment is much faster [3]. 2.1. Water-to-water heat pump The chiller model used in DOE2 simulation program and discussed in [9] is adopted in this study, which is applicable to
Fig. 3. Comparison of original and revised water-to-water heat pump models.
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
Fig. 4. (a) Input power vs. EWTS in cooling mode. (b) Input power vs. EWTS in heating mode.
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S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
EIRFT ¼
þ e4 EWTs þ e5 EWT2s þ e6 LWTl EWTs PLR ¼
here: vPOWhp/vEWTs > 0 in cooling, vPOWhp/vEWTs < 0 in heating, and v2POWhp/vEWTs2 0.
POWT =CAPT ¼ e1 þ e2 LWTl þ e3 LWT2l POWref =CAPref ð2Þ
CAP ¼ l1 þ l2 N þ l3 LWTl þ l4 EWTs þ l5 NLWTl CAPT þ l6 NLWTl
2.2. Cooling tower The capacity model of cooling tower in [10] is used in this paper.
ð3Þ
CAPct ¼
_ w c3 c1 m c3 ðEWTct OATWBÞ _w 1 þ c2 m _ m
(8)
a
EIRFPLR ¼
POW ¼ p1 þ p2 LWTl þ p3 EWTs þ p4 LWTl EWTs þ p5 LWT2l þ p6 EWT2s POWT þ N p7 þ p8 LWTl þ p9 EWTs þ p10 LWTl EWTs þ p11 LWT2l þ p12 EWT2s þ N 2 p13 þ p14 LWTl þ p15 EWTs þ p16 LWTl EWTs þ p17 LWT2l þ p18 EWT2s
CAP ¼ CAPref $CAPFT$PLR
(5)
POW ¼ POWref $CAPFT$EIRFT$EIRFPLR
(6)
_ l $jEWTl LWTl j Q ¼ CAP ¼ 4:186$m
(7)
Eq. (4) defines EIRFPLR, a factor representing power level at a certain temperature condition and compressor speed. Eqs. (5) and (6) are used to calculate a heat pump’s capacity and power
POWct ¼ POWr
_w m _ ar m
3
The power model is built based on fan laws.
Eq. (1) defines CAPFT, a factor representing the ratio of maximal capacity at a certain temperature condition to the reference capacity. Fig. 1 shows temperature and flow rate definitions. It is supposed that temperatures in Fig. 1 are known. Eq. (2) defines EIRFT, a factor representing the ratio of energy efficiency at a certain temperature condition to the reference efficiency. Eq. (3) defines part load ratio at a certain temperature condition and compressor speed. Since the system is quasi-static, the capacity of a heat pump, CAP, is always equal to the load, which is equal to the enthalpy change of load-side water, i.e.
ð4Þ
POWct ¼ POWr
_a m _ ar m
3 (9)
Given a cooling load, which is supposed to be matched by cooling tower’s capacity, from Eq. (8) the air mass flow rate should be:
_a ¼ m
c2 CAPct _ w Þc3 ðEWTct OATWBÞ CAPct c1 ðm
1
c3
_w m
(10)
Based on energy balance and ignore the contribution from the make-up water,
EWTct ¼ LWTct þ
CAPct _w cpw m
(11)
Plug Eqs. (10) and (11) into (9),
0
13 c3 c2 CAPct @ A _ w Þc3 LWTct þ CAP_ ct OATWB CAPct c 1 ðm c m pw
(12)
w
consumption, where CAPref and POWref are constant. Note that the coefficients, c, e, l and p, are different in heating and cooling. In cooling mode, given a certain load, lowering EWTs leads to a smaller temperature lift (the temperature difference between condensing and evaporating refrigerant), which increases efficiency so that a heat pump consumes less electricity [2]. Thus vPOW/vEWTs is always positive in cooling model. In heating mode the trend is opposite. Fig. 4 shows the trends of input power in cooling (a) and heating (b) mode of the model built based on manufacturer’s data. The surface in each plot is the input power, and the lines are composed of points on the surface with the same capacity. We also examined v2POW/vEWTs2, given a certain load, by calculating both the second-order difference using the manufacturer’s data and analytical values from the model. It is found that, when EWTs is [10 C, 40 C], v2POW/vEWTs2 is always nonnegative (positive in most cases) in both cooling and heating modes. The plots in Fig. 4 coincide with the conclusions, which are summarized
Take the derivative of POWct with respect to LWTct using Eq. (12),
c3 _a 3c1 m vPOWct ¼ ð1ÞPOWct <0 vLWTct c2 c3 CAPct
(13)
According to [10], c1 and c2 are a function of physical properties of water and air, thus both are positive. c3 must be positive due to the fact that increasing air flow rate leads to a higher cooling capacity, which requires a positive c3 in Eq. (8). So Eq. (13) is always negative. Take the second derivative using Eq. (13),
v2 POWct vLWT2ct
¼ POWct
2c3 2 _a m c1 3 3 CAPct2
c2
c3 c3
þ1 >0
It is apparent that the second derivative is positive.
(14)
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
2.3. Boiler The fossil-fuel boiler model in [17] is employed in this paper. The original model is dynamic with thermal capacitance of the boiler considered, but the dynamics is ignored in this quasi-static analysis.
f LWTbo ¼ CAPbo þ fa ðLWTbo Tam Þ POWbo 1 l LWTbomax
(15)
Take the derivative of POWbo with respect to LWTbo using Eq.(15),
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Based on the analysis above, Eq. (20) is nonnegative. So POWtotal(Tlp) is a convex function on (Tlb, Tub), any local minimum of which is also a global minimum. If d2POWtotal/dT2lp>0, then it is a strictly convex, which means there is a unique minimum on [Tlb, Tub]. If d2POWtotal/dT2lp ¼ 0, then: if dPOWtotal/dTlp ¼ 0 as well, POWtotal is constant on [Tlb, Tub], i.e. its value is independent of Tlp; if dPOWtotal/dTlp s 0, POWtotal is a straight line on [Tlb, Tub], which means the minimum is at either Tlb or Tub.
dPOWbo f LWTbomax CAPbo þ fa LWTbomax ðLWTbomax fl LWTbo Þ þ fa LWTbomax ðLWTbo Tam Þ ¼ l >0 dLWTbo ðLWTbomax fl LWTbo Þ2
Since fl ˛½0; 1 and practically LWTbo > Ta, all the three terms in the numerator on the right-side of Eq. (16) is positive, which in turn makes Eq. (16) positive. Take the second-order derivative using Eq. (16),
d2 POWbo dLWT2bo
¼
fa LWTbomax ð1 fl Þ þ ðLWTbo Tam Þ ðLWTbomax fl LWTbo Þ2
þ
(16)
3. A strategy to find the optimal Tlp that minimizes POWtotal Since a unique minimum of POWtotal on [Tlb, Tub] exists, an algorithm called Golden Section Search [4] is employed in this paper
2fl LWTbomax CAPbo þ ðLWTbomax fl LWTbo Þ þ ðLWTbo Tam Þ ðLWTbomax fl LWTbo Þ3
>0
(17)
to find the optimal Tlp, which is a simple and effective minimumbracketing technique without need of calculating gradients and can be easily applied to real-time applications.
It is apparent that the second derivative is positive.
2.4. System When the system is in operation, the load of each heat pump can be calculated using Eq. (7); the power can be found _ lp and Tlp are using Eqs. (1)e(6). On the water-loop side, m measured directly, EWTlp can be either measured or calculated based on energy and mass balance, so the load imposed on the cooling tower or boiler can be found, i.e. CAPct or CAPbo. The power of cooling tower or boiler can be found using Eq. (12) or (15). Note that we let EWTct and EWTbo equal to Tlp in analysis, but in simulations the heat added due to water pump inefficiency is included. The total powers has Tlp as the only independent variable, which is:
X POWtotal Tlp ¼ POWhpi Tlp þ POWct Tlp þ POWbo Tlp i
(18) Actually the cooling tower or boiler’s load is affected by variations in Tlp because it changes heat pumps’ efficiency, but we have the behavior of cooling tower and boiler decoupled from that of heat pump in order to simplify the situation, i.e. the load appears as a constant instead of a variable in Eq. (18). Eq. (18) is continuous on [Tlb, Tub], so POWtotal(Tlp) has its minimum and maximum according to the extreme value theorem. Eq. (18) is also twice differentiable on (Tlb, Tub),
X dPOWhp dPOWct dPOW dPOWtotal bo ¼ þ þ dTlp dTlp dTlp dTlp
(19)
X d2 POWhp d2 POWct d2 POW d2 POWtotal bo ¼ þ þ 0 2 2 2 2 dTlp dT dT dT lp lp lp i
(20)
(1) Identify all the parameters in Eqs. (1)e(6), (12) and (15) in an offline manner; (2) t each control interval, (1) Acquire a set of system operating conditions, which _ l of includes the mode (heating or cooling), LWTl, and m each operating heat pump; variables related to the performance of the cooling and boiler eTlp, Tstptold, EWTlp, _ lp , OATWB. m (2) Calculate the current load of each heat pump, load on cooling tower or boiler, determine whether cooling tower or boiler should be in operation; (3) Search the optimal Tlp by using the Golden Section Search algorithm to minimize POWtotal(Tlp) on [Tlb, Tub]; (4) Limit Tlpstpt between Tlpstptold dTlpmax and Tlpstptold þ dTlpmax, and send Tlpstpt and operation mode (heating or cooling) to the Tlp controller (see Fig. 2). The procedure at each of iteration to find the optimal Tlp (see Fig. 5) is described here.
i
Fig. 5. Golden Section Search algorithm.
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S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901
If |Tb Ta| is less than a predefined tolerance, the search terminates successfully. Otherwise, (a) if POWtotal(T1) > POWtotal(T2), Ta ¼ T1, T1 ¼ T2, POWtotal(T1) ¼ POWtotal(T2), T2 ¼ Ta þ 0.618(Tb Ta), and update POWtotal(T2); (b) if POWtotal(T1) POWtotal(T2), Tb ¼ T2, T2 ¼ T1, POWtotal(T2) ¼ POWtotal(T1), T1 ¼ Ta þ 0.382(Tb Ta), and update POWtotal(T1). Note that at the very first iteration, Ta ¼ Tlb and Tb ¼ Tub.
loop side, each operating water-to-water heat pump has a fixed water flow of 0.15 kg/s, and there is a minimal water flow of 3 kg/s through the boiler and cooling tower. The loop holds a total of 300 kg of water. The water pressure of the loop pump is maintained at 75 kPa. All the water pumps have an overall efficiency of 75%. The boiler has a maximal input energy rate of 35 kW, an fl of 0.1 and an fa of 0.0737 kW/ C. The cooling tower has an evaporator cooling capacity of 52.8 kw (15 ton), a rated fan power of 1.5 kW, and c1 ¼ 6.6, c2 ¼ 1.92, c3 ¼ 0.94. The model has a simulation time step of 1 min. The module of optimal control strategy updates Tlp set point every 5 min and passes it to the Tlp controller. The parameter settings are: Tlb ¼ 16 C, Tub ¼ 32 C, dTlpmax ¼ 1 C. A one-year-long simulation is conducted.
4. Evaluating the optimal control strategy on Tlp 4.1. Cooling We need to compare the performance of the optimal Tlp reset strategy with the conventional one, i.e. injecting heat into the loop when Tlp 16 C by turning on the boiler and removing heat when Tlp 32 C by running the cooling tower. A fair comparison requires the exactly same operating conditions applied to both cases in terms of load profile, weather, etc., so computer simulations are employed to conduct the analysis. The building and WLHP system is modeled with TRNSYS e a leading simulation platform for buildings and HVAC (http://sel. me.wisc.edu/trnsys/). The modeled building has 6 zones with a floor area of 991 m2. Multiple terminal units are installed in every zone, each of which consists of two active chilled beams and one water-to-water heat pump. Each terminal unit has a nominal capacity of 2.3 kW in cooling and 3.0 kW in heating. The quantity of terminal units in a zone is determined based on the zonal peak sensible load, which is 5, 4, 5, 7, 7 and 8 respectively for each zone. The latent cooling load and ventilation are handled by an airhandling unit, which is out of the scope of this study. On the water-
The performance between hour 4137 and 4146 is illustrated in Fig. 6. Fig. 6a shows the cooling load profile applied to both the conventional and optimal cases. Fig. 6b shows that the set points in the optimal case are lower than the conventional 32 C. The power consumption of heat pumps and fans in the cooling tower is presented in Fig. 6c. The optimal case consumes more fan power to achieve a lower Tlp so that even more energy can be saved on heat pumps. The plots in Fig. 6d are accumulative energy consumption of the two cases, which are the integration of the total power in Fig. 6c. It clearly shows that the optimal outperforms the conventional case by using less energy, which is 23.4% actually. Table 1 summarizes the energy performance when the system has a net cooling load over a year. 4.2. Heating The optimal strategy generates a constant of Tlp set point e 16 C, which is identical to the conventional set point, so that no
Fig. 6. Performance in cooling between hour 4137 and 4146.
S. Yuan, M. Grabon / Applied Thermal Engineering 31 (2011) 894e901 Table 1 Energy performance comparison of the two Tlp control strategy. Water-to-water heat pump Water pump Fan in cooling tower Total Energy savings
MWh MWh MWh MWh %
20.04 3.07 0.30 23.41
14.43 3.07 1.62 19.12 18.3%
energy improvement is found in heating mode. The reasons are as follows. Assuming that Qc and Qh are the cooling and heating loads from a building, and total power consumption of water-to-water heat pumps are POWc in cooling and POWh in heating, if the net load is heating (Qh > Qc 0), we have the heat injected into the loop water by heat pumps as:
ðQc þ POWc Þ ðQh þ POWh Þ;
(21)
which is negative. The boiler’s capacity must match the load, thus,
POWbo h þ ðQc þ POWc Þ ðQh POWh Þ ¼ 0
(22)
Rearrange the terms in Eq. (22),
POWbo h þ POWc þ POWh ¼ Qh Qc
(23)
If we have a boiler without a heat loss, i.e. h ¼ 1, the total power consumption will be constant since the right side of Eq. (23) is constant, which means manipulating Tlp will only change the ratio of the terms on the left side of Eq. (23). If h < 1, i.e. fl s 0 and/or fa s 0, the boiler’s efficiency can be improved by keeping Tlp as low as possible. 5. Conclusions This paper analyzes the impact of loop water temperature on energy performance of a WLHP system at both component and system levels, and demonstrates that there exists a unique temperature minimizing the energy consumption. An optimal loop water temperature reset strategy has been proposed and evaluated
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to be effective with simulations for cooling, while the optimal loop water temperature in heating mode is the same as the conventional one so that no energy can be further saved. References [1] ASHRAE, ASHRAE Handbook e 2000 HVAC Systems and Equipment. American Society of Heating, Refrigerating, and Air-Conditioning Engineerings, Atlanta, 2000, [Chapter 8]. [2] J.F. Kreider, Handbook of Heating, Ventilation, and Air Conditioning. CRC Press, 2000. [3] J. Bourdouxhe, M. Grodent, J. Lebrun, Reference Guide for Dynamic Models of HVAC Equipment. American Society of Heating, Refrigerating, and AirConditioning Engineerings, Atlanta, 1998. [4] C.F. Gerald, P.O. Wheatley, Applied Numerical Analysis. Addison Wesley, 2004. [5] H.I.J. Henderson, M.K. Khattar, S.W. Carlson, A.C. Walburger, The implications of the measured performance of variable flow pumping systems in geothermal and water loop heat pump applications, ASHRAE Trans 106 (2) (2006) 750e757. [6] R.H. Howell, J.H. Zaidi, Analysis of heat recovery in water-loop heat pump systems, ASHRAE Trans 96 (1) (1990) 1039e1047. [7] R.H. Howell, J.H. Zaidi, Heat recovery in buildings using water-loop heat pump systems: part I e Energy requirements and savings, ASHRAE Trans 97 (2) (1991) 736e749. [8] R.H. Howell, J.H. Zaidi, Heat recovery in buildings using water-loop heat pump systems - part II - sensitivity analysis, ASHRAE Trans 97 (2) (1991) 750e757. [9] M. Hydeman, K.L.J. Gillespie, Tools and techniques to calibrate electric chiller component models, ASHRAE Trans 108 (1) (2002) 733e741. [10] G. Jin, W. Cai, L. Lu, E.L. Lee, A. Chiang, A simplified modeling of mechanical cooling tower for control and optimization of HVAC systems. [11] E.A. Kush, C.A. Brunner, Field test results applied to optimizing water-loop heat pump design and performance, ASHRAE Trans 97 (2) (1991) 727e735. [12] Z. Lian, S. Park, H. Qi, Analysis on energy consumption of water-loop heat pump system in China, Appl Thermal Eng 25 (2005) 73e85. [13] T.J. Marseille, J.S. Schliesing, The Integration of Water Loop Heat Pump a.d Building Structural Thermal Storage Systems. Pacific Northwest Laboratory, Richland, 1991. [14] PG&E, CoolTools Chilled Water Plant Design and Specification Guide. Pacific Gas and Electric Company, 2000. [15] J.A. Pietsch, Optimization of loop temperatures in water-loop heat pump systems, ASHRAE Trans 97 (2) (1991) 713e726. [16] B.E. Woller, Design and operation of a commercial water-loop heat pump system with a ground-loop heat exchanger, ASHRAE Trans 100 (1) (1994) 1577e1587. [17] M. Zaheer-Uddin, Decentralized control systems for HVAC, ASHRAE Trans 98 (2) (1992) 114e126.