Defrosting for Air Source Heat Pump

Research, Analysis and Methods

Mengjie Song, Shiming Deng

• Publisher: Woodhead Publishing
• Year: 2019
• Print ISBN: 9780081025178
• eBook ISBN: 9780081025185
• Topic: Tecnología e ingeniería
• Subtopic: Energía

Defrosting for Air Source Heat Pumps: Research, Analysis and Methods presents a detailed analysis of the methods, processes and problems relating to defrosting, a necessary requirement to maintain the performance of ASHP units. Readers will gain a deeper understanding of control strategies and system design optimization methods that improve the performance and reliability of units. The book discusses the most recent experimental and numerical studies of reverse cycle defrosting and the most widely used defrosting method for ASHP. Techno-economic considerations are also presented, as is the outlook for the future.This book is a valuable resource for research students and academics of thermal energy and mechanical engineering, especially those focusing on defrosting for ASHP, heating, ventilation and energy efficiency, as well as engineers and professionals engaged in the development and management of heat pump machinery.- Includes MATLAB codes that allow the reader to implement the knowledge they have acquired in their own simulations and projects- Discusses experimental and numerical studies to provide a well-rounded analysis of technologies, methods and available systems- Presents techno-economic considerations and a look to the future

1

Introduction

Abstract

Heat pumps are widely used to transfer thermal energy in the opposite direction of spontaneous heat transfer by absorbing heat from a colder space and releasing it to a warmer one. A heat pump unit commonly consists of four basic components: a condenser, an expansion valve, an evaporator, and a compressor. For an air source heat pump unit, when it works at the heating mode at an ambient environment with a low air temperature and high air humidity, frost can form and accumulate on the surface of its evaporator or outdoor coil, which is usually of a multirefrigerant circuit. Frosting can reduce the output heating capacity and operational performance of the ASHP unit, and thus defrosting becomes necessary. In this chapter, the background of this book is first given, followed by presenting the seven objectives of the book. Finally, an outline of the book consisting of 11 chapters and 6 appendices is also presented.

Keywords

Heat pump; Air source heat pump; Multicircuit outdoor coil; Frosting; Defrosting

1.1 Background

Similar to a water pump that could move fluids (liquids or gases) or sometimes slurries against gravity, a heat pump can transfer heat from the heat source at a low temperature to a heat sink at a high temperature by consuming power, which is against the direction of heat transfer from high temperature to low temperature. In applications, heat pumps are widely used to transfer thermal energy in the opposite direction of spontaneous heat transfer by absorbing heat from a cold space and releasing it to a warmer one. The most common design of a heat pump includes four basic components: a condenser, an expansion valve, an evaporator, and a compressor. Refrigerant is the heat transfer medium being circulated through these components in a heat pump.

There are two main types of heat pumps: absorption and compression. An absorption heat pump could be powered by oil, gas, or solar energy. The fuel utilization efficiency in an absorption heat pump is evaluated by the ratio of the energy supplied to that consumed. For example, when gas is used to power an absorption heat pump, the utilization efficiency could reach 1.0–1.5. Compression heat pumps, on the other hand, are most likely powered by electricity. When comparing the performance of a compression heat pump, the term “coefficient of performance (COP)” as the ratio of useful heat to work input is commonly used. For a compression-type heat pump, although it has the obvious advantage of higher energy performance than that of an absorption-type heat pump, its disadvantage of consuming high-quality electricity limits its application in waste energy recovery.

Heat pumps might also be categorized by their heat sources, such as air, ground soil, and water, into air-source heat pumps (ASHPs), ground-source heat pumps (GSHPs), and water-source heat pumps (WSHPs). Clearly, air is everywhere, and is the most common, safe, stable, and cheapest thermal source. An ASHP extracts heat from the ambient air and transfers the heat to the indoor air as an air-air heat pump, or to hot water in a domestic hot water tank as an air-water heat pump. In practice, air-air heat pumps are widely found; science air conditioners just work following the same principle. When an air conditioner is used in summer, it removes thermal energy from the indoor air to the outdoor air. However, when it reversely operates in winter, thermal energy is moved from the outside air for heating the indoor air. ASHPs are the most common type of heat pumps with the lowest investment cost. Air-water heat pumps may also easily be found as water heaters, by transferring the extracted heat from the outside air for water heating all year round. In winter, the hot water provided by an air-water heat pump may also be partly or wholly used in a water-based space-heating system, such as a floor radiant heating system, which is efficient and enables a comfortable indoor environment. ASHPs may also be used to extract thermal energy from exhaust air, and they are thus sometimes called exhaust air heat pumps. The exhaust air can be from buildings due to ventilation or from some industrial production processes. It is easy to understand that the evaporator side heat exchanger might be specially made.

The use of ASHPs is advantageous. Compared with other space heating methods, an ASHP unit does not need a plant room, but could be placed on a roof or ground at will to save floor area and reduce the construction and installation costs. As compared to a boiler-based space heating system, ASHPs are safe and reliable and do not produce environmental pollution. However, the performances of ASHP units would vary with the changes in outdoor weather conditions. When the unit extracts heat from low-temperature air, frost may be formed on the surface of its outdoor coil. To maintain normal operation of the ASHP unit, defrosting will be necessary once adequate frost has been accumulated, which reduces the operating efficiency and output heating capacity of the ASHP unit.

On the other hand, a GSHP unit converts low-grade shallow geothermal energy to high-grade thermal energy by consuming a small amount of electricity. A GSHP unit usually consumes 1 kWh of electrical energy to produce 4.4 kWh of thermal energy. In fact, the heat drawn from soil is in most cases the stored solar heat, and hence should not be confused with that in direct geothermal heating. In a geothermal heating system, a circulation water pump but not a heat pump is required because the soil temperature is higher than that in a space to be heated. Therefore, such technology relies only upon convective heat transfer. As an efficient measure against rising energy costs, GSHP technology has attracted worldwide attention since the 1980s and has been a hot topic in China since the late 1990s. Earlier investigations on GSHP technology focused on the performance tests for experimental GSHP systems, and technical and economic comparisons with traditional ASHP units. Later, the complex heat and mass transfer between ground heat exchangers used in a GSHP system and subsurface rock and soil was investigated in great detail, with a large number of heat transfer models developed and reported. Recently, novel hybrid GSHP units and the determination of soil thermal properties became hot research topics.

Furthermore, a WSHP utilizes energy resources in shallow water on the Earth's surface, such as the solar energy and geothermal energy absorbed by groundwater, rivers, streams, and lakes. However, river/seawater heat pumps and wastewater heat pump systems are the most widely used systems. When the river/seawater is used as a heat source or sink, it is always stable with an almost unlimited capacity. For example, several big data centers were built near rivers/seas, and released heat to the water using a heat pump or direct sea/river water cooling. However, in recent years, more and more environmental problems, such as the death of fish and water grass or the changes and migration of microorganisms, emerged. These further destroyed local ecological environments. On the other hand, wastewater heat pump systems take thermal energy from treated domestic wastewater, and would thus have no adverse impacts on local environments.

Hybrid heat pump systems have also been widely studied and reported. A solar-assisted heat pump integrates a heat pump and thermal solar panels in a single integrated system. Typically, the two technologies are used independently to produce hot water. However, in this integrated system, the solar thermal panel functions as a low-temperature heat source and the heat collected in the panel is fed to the heat pump's evaporator. A hybrid system can produce thermal energy in a more efficient and less expensive way. A further typical hybrid system is an air/water-brine/water heat pump. Unlike other hybrid systems, this system usually utilizes both conventional and renewable energy sources. For example, it uses air and geothermal heat in a single compact device. There are two evaporators in an air/water-brine/water heat pump: an outdoor air evaporator and a brine evaporator. Both evaporators are connected to the heat pump cycle to allow the use of the most economical heating source according to the actual operating conditions, air, geothermal heat, or both.

1.2 Frosting and defrosting

As previously mentioned, when an ASHP unit works at heating mode in winter to take thermal energy from the ambient air, the surface temperature of its outdoor coil can be much lower than both the dewpoint of air and the freezing temperature of water. Therefore, frost would form and accumulate on an outdoor coil surface. In fact, the formation of frost on the surfaces of plant leaves is a well-known phenomenon in nature. This phenomenon was recorded in an ancient Chinese poem, The Reed. The second verse of this love poem reads, “Dew and frost gleam,” meaning that the dew on the surface of reed leaves was changed into frost. The fourth verse reads, “Beyond the stream,” meaning that the reed was near a river where the air humidity was high. The poem may describe the natural phenomenon on an autumn day at dusk, when air temperature was low. Clearly, the following three conditions–cold surface, high air humidity, and low air temperature–are the prerequisites for frost to be formed. Therefore, it can be easily understood that frosting is commonly observed in refrigeration fields because of operating conditions [1].

Frost deposition is inevitable once moist air is exposed to a cold surface having a temperature below the water triple point and the air dewpoint [2]. To clearly understand the frosting mechanism, the process of frost formation on a cold flat plate surface is shown in Fig. 1.1 [3]. It could be divided into four periods according to the growth timeline: (1) the droplet condensation period, (2) the solidified liquid tip growth period, (3) the frost layer growth period, and (4) the frost layer full growth period. During the droplet condensation period, the condensing droplets at a subcooling state are formed on the cold surface. The droplet nucleation occurs first, followed by the coalescence of the droplets. As the vapor-liquid and liquid-solid phase changes take place, the droplets merge and solidify; the diameters of the solid droplets increase significantly. After reaching a critical time, t_c, all coalescent droplets turn into ice particles or crystals. The length of this period is determined by the ambient and surface conditions, including the surrounding air temperature/relative humidity, the natural or forced convection affected by air velocity, the cold surface temperature, and its roughness. The second period, the solidified liquid tip-growth period, starts from the critical time to a transitional time, t_t, when a relatively uniform porous layer of frost is formed. At this period, nonuniform tip growth occurs on individual droplets.

Fig. 1.1 The four periods in a frost growth process on the surface of a cold plate.

The entire process of frost branch formation is referred to as the third period, the frost layer growth period, when frost branches would form at the top of ice crystals. These frost branches grow in three dimensions and connect to the neighboring frost branches, thus forming a flat frost layer. During the fourth period, the frost layer full growth period, the interface between the frost layer and the ambient air is at a temperature of 0°C, owing to the thermal resistance of the porous frost layer and the upper part of the frost layer being melted into water. The melted water penetrates the frost layer and freezes, thus forming a new, considerably thicker ice layer. When ice layers form inside the frost layer, the density of the frost layer increases, and the conduction thermal resistance decreases. As a result, the surface temperature of the frost layer is lower than 0°C, causing the frost layer to grow on the surface again. After ice crystals form on the surface, the water vapor on the surface that surrounds the humid air becomes frost, which branches on the top of the ice crystals and no further mass transfer take places between the surface and the humid air. In fact, all previously mentioned ambient and surface conditions may affect a frost formation process. Such has been extensively studied on the surfaces of plates and heat exchangers. In addition, frosting characteristics on hydrophobic and superhydrophobic surfaces are different from those on ordinary surfaces. The changes in surface properties affect the frosting behaviors at the early stage, from a dry surface to the formation of ice crystals. The surface properties are hardly possible to influence the growth of the frost layer, and the change in the surface properties is considered only prior to the crystal growth period or the droplet condensation period.

Frosting has caused severe negative effects in various application fields. Therefore, numerous theoretical and experimental studies have been conducted, aiming at the mitigation and control of frost formation on various cold surfaces. For example, in the field of aerospace technology, the phenomenon of frost deposition is harmful. For the airplanes that fly at night, frost deposits may occur on their wings owing to low air temperature. The frost deposition increases the surface friction drag during take off or navigation, thus affecting safety [4]. In launching rockets, there are similar frosting problems. Similar to that on the surface of airplane wings, frost may deposit on the rocket surface, thus causing satellites to fail to enter a correct synchronous orbit [5]. For liquid-fueled rockets, frost may also deposit on the surface of cryogenic oxidizer tanks of very low temperature, as rockets fly through the atmosphere. The frost deposition may change both the shape and the weight of the oxidizer tanks, thus influencing the aviation performance of the rocket. In addition, in the field of LNG production and application, frost may deposit on the surfaces of LNG evaporators, and thus affect the so-called passive-evaporation technology that uses air as a heat source [6]. Harmful frosting phenomena mainly occur in various industry processes that occur under normal low-temperature conditions. Hence, most theoretical and experimental research on frost deposition has focused on normal low-temperature conditions. However, because of the increased practical and engineering applications, frost formation on the surfaces with a very low temperature has been receiving increasing attention. In addition, in the ca...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• 1: Introduction
• 2: Previous related work: A review
• 3: Uneven defrosting on the outdoor coil in an ASHP
• 4: Modeling study on uneven defrosting
• 5: Investigation of effect on uneven defrosting performance
• 6: Frosting evenness coefficient
• 7: The influence of refrigerant distribution on defrosting
• 8: Energy transfer during defrosting
• 9: Defrosting control strategy
• 10: Technoeconomic performances
• 11: Conclusions and future work
• Appendices
• Index