Elsevier

Applied Thermal Engineering

Volume 112, 5 February 2017, Pages 201-207
Applied Thermal Engineering

Research Paper
Simplified model for coefficient of performance calculation of surface water source heat pump

https://doi.org/10.1016/j.applthermaleng.2016.10.081Get rights and content

Highlights

Abstract

A simple and accurate method to calculate the energy consumption of a water source heat pump system is important for analyzing its economic and social benefits. This paper presents a study on performance of a lake-water source heat pump unit, the calculation models of inlet water temperature on lake water side was obtained. Our results have shown that the hourly temperature of inlet water on lake water side and outlet water on user side would change a little within a day, and there was a linear relationship between daily average lake-water temperature and daily average air dry-bulb temperature. The measured daily average outlet water temperature on user side during a whole cooling/heating season changed in a little margin. Then a simplified model for coefficient of performance (COP) calculation of unit was proposed, the feasibility was validated through data analysis. A simplified method has therefore been developed to calculate and predict energy consumption of surface water source heat pump (SWHP) unit and will therefore enable energy efficient operation of water source heat pump for HVAC (heating, ventilating and air-conditioning) applications in buildings.

Introduction

The energy consumption for air-conditioning system accounted for a large part of building energy consumption, and this is an important consideration for energy reduction strategy for buildings [1], [2]. The majority of energy consumption in HVAC system is due to the water chiller which accounts for 50–60% of electric loads [3]. The prediction and calculation of energy consumption is the main purpose of this feasibility study of central air-conditioning system, which is an important consideration to determine the social and economic benefits for the society [4].

The application of water source heat pump central air-conditioning system is now widespread in the world due to the possible energy saving of utilization natural water as for heating and cooling [5]. Water source heat pump can be classified into ground-water source heat pump [6], [7] and surface-water source heat pump (river-water, sea-water, lake-water, etc.) [8], [9]. The calculation of the system energy consumption would be more precise by a consideration of various factors that could affect the energy consumption of central air-conditioning system, such as the performance of water chiller, the unit control strategy, the cooling and heating load of building, etc. However, the uncertainty of environmental conditions, the complex coupling relationship among these factors and the collection of all kinds of parameters could make the calculation process become complicated. So a simple and accurate method for energy consumption calculation of central air-conditioning system is necessary.

Section snippets

Background of calculation model

Coefficient of Performance (COP) is commonly used to evaluate the unit performance of in an air-conditioning system. Under rated condition, COP of unit is defined as [10]:COP=Q/Pwhere Q is rated heating/cooling capacity, kW; P is the rated input power of unit, kW. Because of the effect of external and internal factors, the unit cannot be operated under the rated condition all the time, when the unit is running under variable conditions, COP will change with the change of operating conditions.

Project description

Hunan University of Science and Technology is located in Xiangtan City of Hunan Province, China, the city is in the area with hot-summer and cold-winter climate. The heating season is usually from the beginning of December to the end of March of next year, and the cooling season is from the beginning of June to the end of September. An office building which has a water source heat pump air-conditioning system, is situated in the new district of development at the university. The water source is

Model for water temperature and COP calculation

The outdoor dry-bulb temperature, inlet/outlet temperature and water flow rate of condenser and evaporator, input power of the unit were monitored during machine operation from the beginning of June 2014 to the end of March 2015. In this case, the lake-water inlet is about 3 m from the surface, so the temperature of the lake-water is collected by measuring the inlet water temperature of the unit on water source side. The dry-bulb temperature was recorded by an electronic temperature recorder.

Verification of prediction model for water temperature calculation

In order to verify the feasibility of the simplified model, in two periods of June to September, 2015 (cooling season) and December, 2015 to March, 2016 (heating season), the outdoor dry-bulb temperature, inlet/outlet temperature and water flow rate of condenser and evaporator, input power of the unit were monitored continuously, So the data achieved could be used for the verification.

Si et al. [18] proposed a river-water temperature calculation model by fitting the data collected from two

Energy consumption calculation procedure

The model shown in Eq. (5) is for the determination of COP of chiller unit, but the correlation coefficients should be obtained by fitting a series of data which include inlet water temperature on water source side, outlet water temperature on user side and COP. The data is laboriously obtained by changing inlet water temperature on water source side of the water chiller, and different chillers have different performance and correlation coefficients. Our model of Eq. (5) would enable the

Conclusions

Based on our study of a lake-water source heat pump unit, the following conclusions are drawn:

  • 1.

    The hourly water temperature vary a little within a day, and therefore this could be replaced by daily average temperature to determine the COP of a unit.

  • 2.

    A linear relationship between the daily average lake-water temperature and daily air temperature was established by our study. The seasonal average outlet temperature of the chiller on the user side is close to the rated average value. The prediction

Acknowledgement

This work is financial supported by the National Natural Science Foundation of China (Projects No. 51274098 and No. 51134005).

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