The water source heat pump technology is a technology that utilizes the low-temperature and low-level heat energy in the shallow geothermal resources of the earth's surface (such as groundwater, lakes, rivers or oceans), and uses the heat pump principle to transfer low-level heat energy to high-level heat energy by inputting a small amount of high-level electric energy. Due to the diversity of water sources and the excavation of groundwater wells and the uncertainty of geological conditions, the water outlet temperature varies, which in turn affects the choice of unit type. The same unit has different input power and coefficient of performance when using different water sources. This paper mainly studies the influence of temperature and mass flow of water source on unit characteristics.

1 Steady-state mathematical model of water source heat pump unit The typical heat pump unit consists of four components: compressor, evaporator, condenser and expansion valve. Each component affects the state of the system during operation of the unit, and the parameters of each component interact with and correlate with the parameters of other components. Therefore, the mathematical model of the heat pump unit must be composed of mathematical models of these components, namely the compressor model, the evaporator model, the condenser model and the expansion valve model. The parameters of these models are coupled to each other, and the compressor input power, evaporator cooling capacity, and condenser heating can be iteratively calculated by given compressor inlet pressure, outlet pressure, and evaporator outlet superheat.

1. 1 scroll compressor mathematical model theoretical displacement, actual displacement and volumetric efficiency

q= n P t(P t - 2 ) ( 2N - 1) h /60( 1)qr = q V( 2) where q theoretical displacement, m 3 /sn compressor speed, r/m in P t vortex pitch, m vortex wall thickness, m N compression chamber number h vortex height, mqr actual displacement, m 3 / s V scroll compressor volumetric efficiency refrigeration capacity! e = qm( h 1 - h 4)( 3) where! e cooling capacity, kW qm mass flow, kg/sh 1 refrigeration compressor ratio at the suction point of the scroll compressor, kJ/kg h 4 refrigerant compressor throttle valve ratio ç„“, kJ/kg Scroll compressor exhaust temperature T d = T spdps? - 1

(4) where T d compressor discharge temperature, KT s suction temperature, K pd compressor discharge pressure, Pa ps compressor suction pressure, Pa

The isentropic index of the working fluid? The compressor input power does not account for the heat loss during the heat exchange and exhaust process of the compression process, and considers that the internal and external pressure ratios of the compressor are equal, that is, the compression process is regarded as the ideal gas isentropic compression, and the indicated power is: P i =- 1 Psqrpdps? - 1- 1( 5)P mO = P z mO = P im mO( 6) where P i compressor indicates power, WP mO motor power, WP z compressor shaft power, W mO motor efficiency m mechanical efficiency, 0. 78 1. 2 Heat exchanger model In order to simplify the heat exchanger model, the following assumptions are made: the refrigerant flow is a one-dimensional flow, the fluid pressure is equal in any flow section, and the influence of the pressure change in the flow channel on the refrigerant flow rate is ignored. Neglecting the influence of refrigerant viscous force, the velocity component is zero in the section perpendicular to the main flow direction, and the dryness of the refrigerant inside the evaporator and the condenser is linear. The heat exchanger model is: ( Ac p)wT wx + ( qmcp) w? T w? x + % wde, w(T w - T ht) = 0( 7) medium density, kg /m 3 A area, m 2 cp than constant pressure heat capacity, J/( kg! K )T w water temperature , K % w surface heat transfer coefficient, W / ( m 2! K ) de, w flow end equivalent diameter, m T ht heat exchanger wall temperature, K subscript subscript w represents water.

1. The capillary model assumes that the flow of refrigerant in the adiabatic capillary is a one-dimensional homogeneous flow under thermodynamic equilibrium. Capillary models include continuity, energy, and momentum equations. Continuity equation: qm = 4 d 2 G = constant (8) energy equation (adiabatic): h + 1 2G 2 v 2 = constant (9) momentum equation: - dp = G 2 dv + 1 2!

Fd vG 2 dL( 10) where d capillary inner diameter, m G mass flow density, kg /(m 2! s)h ratio ç„“, kJ/kg v specific volume, m 3 /kg p fluid pressure, M Pa f Process friction coefficient L capillary length, m 2 heat pump unit model solution and experimental verification heat pump unit model solution The above four components are connected to form a unit model. In this paper, the corresponding calculation program is developed to solve the discrete model, input the initial given compressor inlet pressure, outlet pressure and evaporator outlet superheat. Through the coupling calculation, the compressor input power, evaporator cooling capacity and condensation are obtained. Heat production.

Experimental device The residential water source heat pump unit is mainly composed of a refrigerant circulation system, a water source simulation system and an indoor fan coil system.

a. The water source simulation system uses a 20 kW air-cooled heat pump unit and a 10 kW electric heater in series to provide 12 32% circulating water. It is mainly used to simulate the source water of a 10 kW water source heat pump unit to provide water for the heat pump unit. At the same time, some water is used to simulate the user and provide the required indoor cold and heat load for the fan coil system.

b. The indoor fan coil system consists of a 10 kW fan coil, a set of open electric heating water tanks and an indoor circulating water system for simulating indoor working conditions.

Experimental verification of the performance of the water source heat pump unit Due to the limitations of the experimental conditions, only the inlet and outlet state parameters were tested for the compressor, and the refrigerant flow rate was not measured. Compressor input power comparison. It can be seen that there is a certain deviation between the simulation results and the experimental results, but the deviation is within an acceptable range, and the reason is analyzed, mainly caused by errors in the experimental process. Therefore, the compressor model established in this paper has certain accuracy.

The evaporator cooling capacity is compared with the condenser heating capacity.

It can be seen that the measured value and the simulated value are basically consistent, and the individual points deviate greatly, mainly due to the reason that the thermocouple contacts the tube wall when measuring the temperature. From the obtained data, the average error of the heat of the condenser is 1.84%, and the maximum error is 3.94%. Therefore, the heat exchanger model can be applied to the analysis and calculation of the unit model. In the condenser model, most of the measured values ​​are smaller than the analog value, mainly because the heat exchange between the compressor, the refrigerant pipe and the surrounding environment is neglected in the simulation, resulting in a large analog value. From the perspective of error analysis, the obtained data can also meet the accuracy requirements, so that the model analysis is not affected.

3 Experimental analysis of characteristics of water source heat pump unit The influence of chilled water temperature on unit performance When the cooling water inlet temperature is 40% and the mass flow of cooling water and chilled water is kept constant, the performance parameters of the unit change with the chilled water inlet temperature. 6. As the chilled water inlet temperature increases, the compressor input power P el (in kW), evaporator refrigeration capacity! Evap (in kW) and condenser heat!

cO nd (in kW) is approximately linearly rising, and the coefficient of performance I cO p of the unit also increases. This is mainly because when the chilled water inlet temperature rises, the unit evaporating temperature increases and the refrigerant flow rate increases, so the evaporator cooling capacity and the compressor input power increase. The increase in cooling capacity causes the condenser heating capacity and the unit performance coefficient to increase.

The influence of the cooling water inlet temperature twc on the performance of the unit. When the chilled water inlet temperature is 12% and the mass flow of the cooling water and chilled water is kept constant, the performance parameters of the unit vary with the temperature of the cooling water inlet. 8. It can be seen that As the inlet temperature of the cooling water increases, the cooling capacity of the evaporator decreases rapidly, and the coefficient of performance of the unit and the heat of the condenser also decrease, but the input power of the compressor increases accordingly. This is mainly because the temperature of the cooling water inlet rises and the condensing temperature of the unit rises. When the chilled water inlet temperature and the mass flow rate are constant, the refrigerant flow rate in the unit is reduced, resulting in a decrease in the evaporator cooling capacity and the condenser heating capacity. As the compression ratio increases, the compressor input power also increases, and the unit performance factor decreases as the cooling capacity decreases and the compressor input power increases. Therefore, the temperature rise of the cooling water inlet is unfavorable to the performance of the unit. The cooling water inlet temperature should be reduced as much as possible in the project.

Influence of water source mass flow on unit performance The effect of water source mass flow change on unit performance. It can be seen that when the mass flow rate of chilled water is constant, as the mass flow of cooling water increases, the evaporator cooling capacity, the unit performance coefficient and the condenser heating capacity increase slightly, but the compressor input power varies with the cooling water mass flow. The increase is reduced. This is because when the mass flow of the cooling water increases, the temperature of the cooling water outlet decreases, resulting in a decrease in the condensing temperature of the unit, an increase in the cooling capacity of the evaporator, and a decrease in the input power of the compressor, so that the coefficient of performance of the unit increases. When the mass flow rate of the cooling water is constant, as the mass flow rate of the chilled water increases, the evaporator cooling capacity, the condenser heating capacity, the compressor input power and the unit performance coefficient increase accordingly. The main reason is that the increase of the chilled water flow increases the evaporating temperature of the unit, and the condensing temperature is basically unchanged, so the refrigerant flow rate increases, so the compressor input power, the evaporator cooling capacity, the condenser heating capacity and the unit performance coefficient increase. It can also be seen from the figure that the evaporator cooling capacity and the condenser heating amount change little with the increase of the cooling water mass flow rate, but slightly change with the increase of the chilled water mass flow rate.

4 Conclusions The steady-state mathematical model of the water source heat pump unit was established, and the characteristics of the unit were simulated. The matching of the model was verified by experiments.

The effects of water source temperature and mass flow on the characteristics of the unit were studied experimentally. a. When the chilled water inlet temperature rises, the input power of the compressor, the cooling capacity of the evaporator and the heat of the condenser all increase, and the coefficient of performance of the unit increases. b. When the temperature of the cooling water inlet rises, the evaporator cooling capacity, the condenser heating capacity and the unit performance coefficient decrease, and the compressor input power rises, which is unfavorable to the performance of the unit.

c. When the mass flow rate of chilled water is constant, as the mass flow of cooling water increases, the evaporator cooling capacity, condenser heat generation and unit performance coefficient increase slightly, and the compressor input power decreases as the cooling water mass flow increases. d. When the mass flow rate of the cooling water is constant, as the mass flow rate of the chilled water increases, the evaporator cooling capacity, the condenser heating capacity, the compressor input power and the unit performance coefficient increase.

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