By Sachin R P Harshith AChannabasaveshwara Institute of Technology, gubbi

Introduction to Heat pumpParts of a Heat pumpWorking of Heat pumpThermodynamics Process Terms associated with Heat pumpApplication of Heat pumpTypes of RefrigerantWhy CO2 ? Properties of CO2 as a Refrigerant Advantages of heat pumps with CO2 refrigerantConclusion

OVERVIEW

A heat pump is a device that transfers heat energy from a heat source to a heat sink against a temperature gradient. Heat pumps are designed to move thermal energy opposite the direction of spontaneous heat flow. A heat pump uses some amount of external high-grade energy to accomplish the desired transfer of thermal energy from heat source to heat sink.

Reversing valve

Evaporator

compressor Condenser

Expansion

The Refrigerant is the liquid/gaseous substance that circulates through the heat pump alternately absorbing transporting and releasing heat.The Reversing valve controls the direction of flow of the refrigerant in the heat pump and changes the heat pump from heating to cooling mode or vice versa.

The Evaporator is a coil in which the refrigerant absorbs heat from its surroundings and boils to become a low-temperature vapour.

The compressor squeezes the molecules of the refrigerant gas together increasing the temperature of the refrigerant.

The Condenser is a coil in which the refrigerant gives off heat to its surroundings and becomes a liquid.

The Expansion valve lowers the pressure created by the compressor. This causes the temperature to drop, and the refrigerant becomes a low-temperature.

Heat pumps mainly consists of operation a)Heating cycleb)Cooling cycle c)Defrost cycle

During the heating cycle, heat is taken from outdoor air and "pumped" indoors. The liquid refrigerant passes through the expansion device, changing to a low-pressure liquid/vapour mixture. This vapour passes through the reversing valve to the accumulator, which collects any remaining liquid before the vapour enters the compressor. The reversing valve sends the gas, which is now hot, to the indoor coil, which is the condenser. Below this outdoor ambient temperature, the heat pump can supply only part of the heat required to keep the living space comfortable, and supplementary heat is required.

The cycle described above is reversed to cool the house during the summer. The unit takes heat out of the indoor air and rejects it outside. As in the heating cycle, The liquid refrigerant absorbs heat from the indoor air and boils,. This vapour passes through the reversing valve to the accumulator, which collects any remaining liquid, and then to the compressor.. The heat from the hot gas is transferred to the outdoor air, causing the refrigerant to condense into a liquid. This liquid returns to the expansion device, and the cycle is repeated. The heat pump also dehumidifies the indoor air

If the outdoor temperature falls to near or below freezing when the heat pump is operating in the heating mode, moisture in the air passing over the outside coil will con dense and freeze on it.This frost build up decreases the efficiency of the coil by reducing its ability to transfer heat to the refrigerantWhile heat pump is cooling the air in the ductwork. The heating system would normally warm this air as it is distributed throughout the house.

Reversed Carnot cycle

Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat source and heat sinks.

V

Reverse Brayton cycle

This is an important cycle frequently employed in gas cycle refrigeration systems. This may be thought of as a modification of reversed Carnot cycle, as the two isothermal processes of Carnot cycle are replaced by two isobaric heat transfer processes.

Coefficient of performance (COP): It is the ratio of heat removed from the substance to the work supplied 𝑄𝑊

COP=

It is a measure of a heat pump’s efficiencyThe Higher the COP, the more efficient the heat pump works

Btu/h (British thermal unit per hour) : is a measure of the output of a heating or cooling system, in one hourTon: is a measure of heat pump capacity equivalent to 3.5 kW or 12000 Btu/hBalance point : is the temperature at which the amount of heating provided by the heat pump equals the amount of heat lost from the house

HEATING OF INTERIOR ENVIRONMENTSHeat pumps may be used also solely for heating the internal environment. Space heatingSanitary water heatingProcess heatingDehumidificationHeat recovery

COOLING OF INTERIOR ENVIRONMENTSHeat pumps are mainly used for cooling the internal environment as an alternative source over conventional systemAir-conditionsCold storageIndustrial use

Ammonia Sulphur-di-oxideCarbon-di-oxideFreon i) Freon-12(R-12) ii)Freon-22(R-22)

Carbon dioxide is very abundant in the environment. It is a natural refrigerant known and used in the past. Carbon dioxide has an ozone depletion potential (ODP) of zero .It has a low replacement cost. In addition to its basic environmental properties, carbon dioxide is non- toxic. It carries an A1 safety classification (the same as most fluorocarbon refrigerants), indicating that it has low toxicity and is non-flammable

It is an inert product, compatible with all common materials encountered in a refrigerating circuit, both metals and plastics or elastomers.the density of carbon dioxide is around 1.98 kg/m3, about 1.67 times that of air.high working pressures At low concentrations, the gas is odourlesshigh discharge temperatureNBP of CO2 is 195.4K

No Fumes, odours and smokeEnergy efficientThe other challenge is that CO2 refrigerant cycles operate at far higher pressure than standard vapour-compression-cycle equipmentA new generation of CO2 based heat pumps could avoid the high global warming potential much higher temperatures

The natural fluid Carbon Dioxide displays some excellent properties in the use as a refrigerant in compression-type refrigerating or heat pump systems: it offers unequalled local and ecological safety, widespread availability at low cost, with no need for recycling and containment. Because of its low critical temperature (around 31 °C), CO2 does not compare favourably against traditional refrigerants, as far as energy efficiency is concerned, when simple theoretical cycle analyses are carried out. But this situation can be mitigated, and in some cases completely reversed, by proper design of the system aimed at fully exploiting the unique characteristics of CO2 and/or the exclusive features of transcritical cycles, which bring about important factors that improve the practical performance of CO2 systems. A widespread research activity is underway world-wide for the application of CO2 in many areas with promising results, including mobile and residential air conditioning, heat pumps, and water chillers, commercial and marine applications).