Cascade Heat Pump Research Articles

Domestic Hot Water Production with Ground Source Heat Pump in Apartment Buildings

Producing domestic hot water (DHW) with a ground source heat pump (GSHP) is challenging due to the high temperature (HT) of DHW. There are many studies proving the better performance of cascade heat pumps compared to single-stage heat pumps when the difference between the condensing and the evaporation temperature is large. In this system approach study, different GSHP arrangements are described and computationally compared. A two-stage heat pump arrangement is introduced in which water tanks of the heating system are utilized for warming up the DHW in two stages. It is shown that the electricity consumption with this two-stage system is approximately 31% less than with the single-stage heat pump and 12% less than with the cascade system. Further, both low temperature (LT) and HT heat pumps can run alone, which is not common in cascade or other two-stage heat pumps. This is advantageous because the high loads of the space heating and DHW production are not simultaneous. Proper insulation of the DHW and recirculation pipe network is essential, and drying towel rails or other heating coils should be avoided when aiming for a high efficiency. The refrigerants in the calculations are R407C for the LT heat pump and R134a for the HT heat pump. Investment costs are excluded from calculations.

캐스케이드 열펌프의 저단 사이클 충전량 변화에 따른 성능 특성

Abstract In this study, the optimization and performance characteristics of a cascade heat pump system wasanalyzed with the variation of low stage refrigerant charge amount. The cascade heat pump wasdesigned and constructed with R134a and R410A as the refrigerant for high stage and low stage cycle,respectively. Experiments were conducted by varying the low stage charge amount and the performancecharacteristics of the cascade heat pump were studied. The refrigerant charge amount of the low stagecycle was varied between the ranges of -15% and +10% of the optimum charge amount. Theperformance variation experienced in the cascade heat pump due to the variation of refrigerant chargeamount shows greater effect in the undercharge regions than the overcharge regions. COP reduction inthe undercharge region is larger than the decrease in the overcharge region. Some cycle variation suchas power consumption and cycle pressure according to low stage refrigerant charge amount showeddifferent trends comparing with those according to high stage refrrgerant charge amount. Therefore, theoptimum charge amount of the cascade heat pump should be determined based on the experimentaldata obtained by the variation of high and low stage refrigerant charge amount. Keywords: Cascade heat pump (캐스케이드 열펌프 ), Charge amount(충전량 ), COP(성적계수 )

Capacity modulation of a cascade heat pump with the variation of compressor speed and electronic expansion valve opening

With the increased awareness on excessive energy consumption, increasing fuel cost and negative environmental impact (ozone depletion and global warming) of the major heat generation systems and processes, the ability of heat pumps to generate high temperature with very efficient use of energy and minimal environmental impact has gained lots of awareness over the years. Some domestic and most industrial heating and refrigeration applications demand very high temperature lifts. Single-stage cycles do not practically support these applications since their performance and reliability decrease greatly. High temperature generation can be successfully executed through the use of cascade heat pump. This study analyzes the effects of compressor speed and electronic expansion valve (EEV) opening on the performance of the cascade heat pump in heating mode. The rate of increase of capacity with high-stage (HS) EEV opening at higher HS compressor speed is larger than it at high low-stage (LS) compressor speed. The slope of the capacity with the HS EEV opening was much steeper at small opening range than that at large opening range. When the LS compressor speed increased, the maximum Coefficient of Performance (COP) occurred at larger HS EEV opening than the standard opening. The COP variation at the range of low HS compressor speed had smaller dependency on the LS EEV opening than that at the range of high HS compressor speed.

Evaluation of the impacts of high stage refrigerant charge on cascade heat pump performance

Heat pump systems offer economical alternatives for recovering heat from different sources for use in domestic and industrial refrigeration, space heating, and air conditioning. Some of these domestic and industrial applications require very low evaporating temperatures and very high condensing temperatures which induce high compressor pressure ratios beyond the practical range for single-stage heat pump cycles. This challenge can be overcome by adopting cascade heat pump cycles. In this study, a water-to-water cascade heat pump is tested to investigate the effects of high stage refrigerant charge amount on the performance in a steady state and heating mode operation. The temperature difference between the condensing temperature of the LS cycle and the evaporating temperature of the HS cycle at cascade heat exchanger was increased by reducing the HS refrigerant charge amount, while the heat transfer rate between HS and LS cycles decreased due to a decreasing of refrigerant flow rate. Finally, COP showed lower value at the undercharged condition than that at the fully charged condition. The slope of the capacity with a HS charge amount was much steeper at undercharged conditions than that at overcharged conditions. For HS undercharged conditions, the heating capacity decreased greatly, because heat transfer rate from LS cycle to HS cycle reduced.

The effect of water temperature lift on the performance of cascade heat pump system

The mass flow rate of secondary fluid in a water heater heat pump determines not only the discharge water temperature but also the heat pump performances. As water flow rate decreases, water discharge temperature increases; however, the COP (coefficient of performance) deteriorates due to the increase of high side pressure. In this study, cascade cycle which is used for a water heater heat pump was studied to verify the effect of the water temperature lift in terms of pressure and performance change. Also, the optimal intermediate temperature of cascade cycle with respect to temperature lift was studied and this temperature was adopted for a heat pump transient operation. The heat pump was operated to meet the constant heating capacity and target intermediate temperature by PID control of compressor speed. The transient results showed that water temperature lift determines not only the vertical temperature profile of water reservoir but also the transient performance.

Frosting characteristics of fin-tube heat exchanger at temperature range of −18 to 6 °C of a cascade heat pump

Most studies on frosting characteristics were conducted between −6 and 6 °C, which is the most common heat pump operation temperature region under frosting condition. But the cascade heat pump can operate at the temperature as low as −20 °C, thus, the frosting characteristics of low temperature environment should be studied. The frosting experiments are carried out in a climate chamber on a cascade type air-source heat pump. The outdoor temperature in the study varies from −18 °C to 6 °C and outdoor relative humidity varies from 70% to 90%. From the experimental results, it is found that the amount of frosting almost changes linearly with the frosting time, and the outdoor relative humidity has a great influence of the quantity. At low temperature conditions, frost still exists with lower the frosting rate. A new index called “frosting factor” is brought forth, which considers the moisture content difference as the frosting potential. Finally, it is found that the frosting factor is from 0.0389 to 0.05 g m−2 s−1 (g/kg)−1 and the average value is 0.0446 g m−2 s−1 (g/kg)−1, which can be utilized to estimate the frosting quantity under different outdoor environment conditions.

Performance optimization of a cascade multi-functional heat pump in various operation modes

A multi-heat pump has been widely used to provide cooling and heating in a building. In addition, the demands for a cascade heat pump water heater have been increased due to its higher energy efficiency with high water temperature. In this study, a cascade multi-functional heat pump, which combines a multi-heat pump using R410A for air heating and cooling with a cascade heat pump using R134a for water heating, is considered to provide simultaneous air heating, air cooling, and water heating in a building. The performance of the cascade multi-functional heat pump was measured and analyzed in four operation modes: full-heating hot water, heating-main hot water, full-cooling hot water, and cooling-main hot water. The performance of the cascade multi-functional heat pump in each of the four operation modes was optimized by adjusting control parameters such as the compressor rotation speed and EEV opening and by modifying the refrigerant flow path.

Performance investigation of a cascade heat pump water heating system with a quasi-steady state analysis

Abstract The cascade refrigeration system has been applied to heat pump water heaters to overcome the problems which arise from the high compression ratio at low ambient temperature. In this study, the performance of a cascade heat pump water heating system is investigated with a quasi-steady state analysis to find out the transient behavior of the system. The water-heating system consists of the cascade heat pump water heater, which uses R134a and R410A as refrigerants, and a water storage tank. The steady-state cascade heat pump model is developed based on experimental results and the dynamic storage tank model is created by thermodynamic equations. The model computes the several major parameters related with the system performance, such as the heating capacity, the power consumption, the COP (coefficient of performance), and the temperature distribution in a storage tank. Moreover, the performance of the system is calculated for various conditions to research the influence of ambient temperature and hot water demand.

The effect of the refrigerant charge amount on single and cascade cycle heat pump systems

The refrigerant charge amount is a key factor for heat pump system optimization, and normally determines the condensing pressure, which affects the subcooling at the exit of the condenser. Heating capacity increases as subcooling increases, however, there is an optimum charge amount for the best coefficient of performance (COP), since the compressor work increases as the charge amount increases. Many studies have focused on optimization of the charge amount for a single-stage heat pump system, while few investigations have been carried out on the charge amount for a cascade heat pump system. The cascade cycle has been suggested as an alternative to a single cycle in extremely cold regions. In this study, air-to-water single and cascade heat pumps were studied to experimentally and numerically verify the effect of the charge amount on the heat pumps. Numerical results showed a good accuracy of experimental results in terms of optimal subcooling.

Effect of vapor-injection technique on the performance of a cascade heat pump water heater

In this study, we applied a vapor-injection (VI) technique in a cascade heat pump system. The VI was applied to both upper and lower stage cycles. Test results showed that heating and cooling capacities increased by using the VI technique (12% and 6%, respectively); however, the system COP decreased (6.6% at the injection ratio of 16.7%). The cascade system which has a small compression ratio and a cascade condenser, cannot fully utilize the VI's advantages to improve the system COP. However, the VI is effective for the system reliability and capacity improvement. We also found that the VI in the upper and lower stage cycles had different effect on overall cycle operating characteristic.

Two-phase adiabatic frictional pressure gradients for R410A and CO2 in a macro channel: Experiments and a simplified predictive method for annular flow from low to medium reduced pressures

The use of refrigerants at evaporating temperatures much higher than those of the refrigeration field is an emerging need in the industry related to energy conversion systems, such as Organic Rankine Cycles and cascade heat pumps for sanitary hot water production.In this work the effect of reduced pressure is experimentally investigated for two-phase adiabatic frictional pressure gradients for CO2 and R410A in a wide range of reduced pressure (0.19–0.52 for R410A and 0.57–0.64 for CO2) and mass flux (150–500kgm−2s−1) in a adiabatic horizontal smooth tube of 6.00mm of inner diameter, collecting 1214 new data points. A simplified phenomenological method for annular flow was proposed and validated with the present and existing databases by the same authors and independent databases for several fluids (CO2, R410A, R134a and also the new fluid R1234ze(E)).

운전조건에 따른 가변속 캐스케이드 열펌프의 성능 특성 연구

Most researches done on heat pumps have been on heat pumps for refrigeration, cooling and heating. There is therefore the need for more research on hot water heat pumps, especially for high temperature. Even though the cascade heat pump cycle has a great potential more efficient hot water generation even at low evaporating temperatures, it has been researched least for this purpose. In this study, the heating performance of a variable speed cascade heat pump was investigated by varying operating conditions. For the same heating capacity values, it was found that increasing the low stage compressor speed was more suitable for enhancing the performance of the system to get a higher temperature.

저온 외기조건에서 케스케이드 급탕열펌프의 성능특성에 관한 해석적 연구

Heat pump systems have been widely adopted in buildings for cooling and heating, due to their higher energy efficiency. Recently, the demand for hot water supply from the heat pump system has been increasing. To increase the water supply temperature with higher system efficiency and reliability, a heat pump water heater adopting cascade cycle was investigated in this study. The cascade heat pump water heater consisted of a low-stage cycle using R410A, and a high-stage cycle using R134a. A simulation program for the cascade heat pump water heater was developed, and verified by comparison with experimental data. The performance of the cascade heat pump water heater was optimized, by varying the compressor rotating speeds of the low- and high-stage cycles. At low ambient temperatures, the performance of the cascade cycle was compared with that of the single-stage cycle. The system efficiency of the cascade cycle was higher than that of the single-stage cycle, showing a lower compression ratio and compressor discharge temperature.

Optimal temperature between high and low stage cycles for R134a/R410A cascade heat pump based water heater system

Single-stage air-source heat pumps have some disadvantages such as a deteriorated heating capacity and low coefficient of performance at low ambient temperatures. To overcome these disadvantages, a cascade cycle has been suggested; this cycle exhibits a better performance at low ambient temperatures. Many studies have investigated the cascade heat pump system. However, little information is available regarding experimental verification of the optimal intermediate temperature levels based on experimental results. In this study, an AWHP (air-to-water heat pump) was studied numerically and experimentally. The high and low cycles of the AWHP adopted R134a and R410A as refrigerants, respectively. Numerical analysis of the optimal intermediate temperature was performed and the experiment was conducted under various operating conditions: ambient temperature, condenser water inlet temperature, and heating capacity. The numerically optimized intermediate temperature accurately predicted the experimental data.

Transient behavior and dynamic performance of cascade heat pump water heater with thermal storage system

At low ambient temperature, air-source heat pump water heater suffers from decrease of both heating capacity and coefficient of performance, and increase in compressor’s pressure ratio and discharge temperature. A cascade air-source heat pump water heater with phase change material (PCM) for thermal storage application was designed to ensure the reliable operation under various weather conditions and enhance the system performance at low ambient temperature. Dynamic experiments were carried out under various operating conditions in accordance with China National Standard GB/T23137-2008. Transient operating characteristics were adopted to analyze the performance of cascade heat pump system. Dynamic performance of the heat pump water heater in single stage mode and cascade mode was compared and discussed. The heating COP values in single stage mode ranged from 1.5 to 3.05, while in cascade mode, the heating COP values ranged from 1.74 to 2.55. Based on the transient heating COP values, critical switching curve from single stage mode to cascade mode was founded for the code of the system controller. Furthermore, energy performance between water tanks with and without PCM was compared to clarify the contribution of PCM.

Second law based performance analysis of seawater source cascade heat pump system: a case study

This paper presents the second law based analysis of a seawater source cascade heat pump system operating with two different refrigerants. The present cascade heat pump system consists of two different heat pumps attached in such a way that the condenser of the first stage heat pump is the evaporator for the second heat pump and the condenser of the second heat pump is attached to the place to be heated. The rate of irreversibility for the low pressure compressor is found to be higher than that of the high pressure compressor.

4391104 Cascade heat pump for heating water and for cooling or heating a comfort zone : James C. Wendschlag, assigned to The Trane Company

4391104 Cascade heat pump for heating water and for cooling or heating a comfort zone : James C. Wendschlag, assigned to The Trane Company