Combination of air-source heat pumps with liquid desiccant dehumidification of air

https://doi.org/10.1016/j.enconman.2011.12.023Get rights and content

Abstract

This paper proposes a frost-free air source heat pump system with integrated liquid desiccant dehumidification, in which frosting can be retarded by dehumidifying air before entering an outdoor heat exchanger. And the water removed from the air is used to humidify a room. Simulation is carried out at a dry-bulb temperature of −7 to 5.5 °C and a relative humidity of 80% depending on the frosting conditions. The results show that the coefficient of performance (COP) is in the range of 2.6–2.9, which is 30–40% higher than that of heat pump heating integrated with an electric heater humidifying system. And it is found that the optimum value of the concentration of lithium chloride aqueous solution is 37% for the frost-free operation mode. Experiments are conducted for liquid desiccant system under low air temperature and high relative humidity conditions. Experimental results show that the dew point of the dehumidified air is decreased by 8 °C and the humidity ratio of the humidified air is kept at 8.1 g kg−1, which ensures the frost-free operation of the heat pump evaporator and the comfortable level of room humidity simultaneously. The heating load of solution is 3–4.5 times larger than cooling load of solution, which agrees with the assumption given at the part of the simulation. Furthermore, the deviations between the calculated COPLHRU and the experimental results are within 33%.

Highlights

► We propose a frost-free air-source heat pump system with integrated desiccant. ► The system can provide heating load continuously and humidify room. ► The coefficient of performance of the system is 2.6 at Ta = −7 °C and RH = 80%. ► The heating load of solution is 3–4 times larger than cooling load of solution.

Introduction

An air-source heat pump that uses air as a heat source offers an economic form of heating. Depending on the climate, air-source heat pumps are about 1.5–3 times more efficient than electric resistance heating alone. However, one of the problems is evaporator frosting, and the subsequent need for defrosting, which occurs when an air-source heat pump is operating at low ambient temperature and high relative humidity. Frost normally accumulates when air temperature is between −7 °C and 5 °C and relative humidity is over 60% [1].

Frost accumulation on an evaporator has several consequences. First, the accumulation of large amounts of frost will reduce the air flow rate by blocking air passages and increasing resistance to heat transfer [2], [3]. This causes the heating capacity and COP to be reduced [4], [5], [6]. Other problems caused by frosting include increased equipment cost due to the addition of auxiliary heating elements, reduced equipment reliability, and increased loss of the operational load during the defrosting process. Therefore, preventing or delaying the frosting process is an important concern in designing an air-source heat pump. There are many factors that influence the frost formation and deposition process; these may include the temperature and characteristics of a cold surface and humidity, temperature, and velocity of air. Many efforts have been made to find various functional surfaces that can reduce frost deposition or from which it is easy to remove the frost layer, such as hydrophilic polymeric coatings [7], [8] and hydrophobic and hydrophilic surfaces [9].

When frost accumulated on the evaporator coil exceeds a certain level, the performance of the unit degrades to such a level that frost must be removed from the evaporator surface on either a continuous or intermittent basis. Defrosting can be achieved by supplying heat to an outdoor heat exchanger by various methods, including reversing the cycle [6], [10], defrosting with warm air using an electric resistance heater [1], and hot-gas refrigerant bypass [11], [12], [13].

As for the reverse-cycle defrosting method, by using a 4-way valve, the normal heating operation and the refrigerant flow are reversed. During the defrosting process, hot gas is pumped into an outdoor coil so as to melt the frost. However, reversing the cycle has a disadvantage that heat is often extracted from the conditioned space, which makes users feel uncomfortable. Another disadvantage is that some melted water remains on the heat-exchanger surfaces. As the defrost cycle ends and the heat pump changes to its heating mode, this water freezes to form a high-density frost, which is slow to melt during subsequent defrosts [1]. In a warm-air defrosting system, the air before the evaporator is heated in order to melt the frost on the coil surface. Defrosting with warm air is not commonly employed in heat-pump operations because heating from electrical resistive heating is inefficient. As for the hot-gas bypass defrosting method, the superheated refrigerant from the compressor directly flows to the evaporator, bypassing the condenser and the expansive device. This method is considered as one of the most effective defrosting methods. The average COP and integrated heating capacity are improved by 8.5% and 5.7%, respectively, as compared to heat pumps where no defrosting method is used [11].

These abovementioned defrosting methods are based on increasing the temperature of the outdoor coil surface in order to melt the frost formed on it. One common problem caused by those defrosting methods is heating shutdown. The sorption dehumidification of air by a liquid desiccant is an interesting method to humidity control [14], [15]. In this study, we propose a frost-free air-source heat pump combined with a liquid desiccant system. In this system, frost formation is retarded by dehumidifying air before entering the outdoor coil using liquid desiccants. And the water removed from air is used to humidify room. As a result, the system proposed in our research cannot only provide heating load continuously, but also humidify room without extra water source.

Section snippets

System description

Fig. 1 shows the schematic diagram of the frost-free air-source heat pump system, which consists of two major units: a sensible heat removal unit (SHRU) and a latent heat removal unit (LHRU). The SHRU comprises a vapor-compressing heat pump (HPSHRU), which employs R410A as the refrigerant. The task of the SHRU is to handle the sensible heat of return air from the air conditioning room. The LHRU comprises a liquid desiccant system, which uses a lithium chloride aqueous solution as the desiccant

Mathematic model

Before modeling, some performance indexes are listed as follows:

  • (1)

    Mass transfer coefficients for a structured packing dehumidifier (Kdeh) and regenerator (Kreg) can be determined employing the empirical correlations by Zhang et al. [19]:

KdehdeDaρa=0.0038Rea0.52Sca0.33Res0.28Scs0.33KregdeDaρa=0.0038Rea0.39Sca0.33Res0.39Scs0.33

  • (2)

    Temperature effectiveness of dehumidifier/regenerator (ηT) is defined as the ratio of actual change in temperature of the air across the dehumidifier/regenerator to the

Simulation results and analysis

Simulations were carried out under the operating conditions shown in Table 1.

The thermophysical properties of the aqueous solutions of lithium chloride were calculated from equations developed and collected by [17]. The thermophysical properties of R410A were calculated using REFPROP7.0 [18]. A flow chart for the cycle simulation is given in Fig. 5.

Calculations were performed on each element with the abovementioned methods, and the results corresponding to the two following operation conditions

Experimental analysis

The frost-free hybrid heat pump system proposed in this work is composed of a vapor-compressing heat pump and a liquid desiccant system. The vapor-compressing heat pump in the SHRU is almost no difference compared with traditional heat pumps. However, to the liquid desiccant system in the LHRU, it should remove water from the ambient air and humidify room air simultaneously in the winter operation conditions. A literature survey shows that almost all experimental studies have concerned on

Experimental results

Fig. 11 shows the measured air humidity ratio at the inlet and outlet of the dehumidifier. The outlet humidity ratio reduced from 2.6 g kg−1 to 1.5 g kg−1 when the inlet humidity ratio decreased from 4.5 g kg−1 to 2.5 g kg−1. Whereas the dew point difference between the air at the inlet and outlet of the dehumidifier was kept constant at 8 °C as shown in Fig. 11. Regardless of air humidity ratio at the inlet of the regenerator, the outlet humidity ratio was kept constant at 8.1 g kg−1, as shown in Fig. 12

Conclusions

A new type of frost-free air-source heat pump system with integrated liquid desiccant dehumidification is proposed in which frost formation is retarded by decreasing the dew point of the air passing through the HPSHRU evaporator. And the water removed from the air is used to humidify a room. From the results of simulation and experiments carried out in this research, the following conclusions can be drawn:

  • (1)

    The COPtotal of the frost-free heat pump system varied from 2.6 to 2.9 when the

Acknowledgment

This research was supported by Tokyo Electric Power Company. The support is gratefully acknowledged.

References (22)

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