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8 Water heating heat pumps

8.1 Introduction

The temperature difference between the source and sink has a direct impact on the pressure difference the compressor must meet for a specific refrigerant. In general, heat pump systems will be less efficient under higher temperature difference condition. The required compressor power input is a fraction of the total useful energy delivered. The required input power to the heat pump is mainly dictated by the heating capacity required from the heat pump, the temperature difference between the source and sink, the effectiveness of heat exchangers and compressor/driver efficiency. Required power input to water pumps and fans must be included to determine the power consumption of the total heat pump system. The heat pump overall coefficient of performance (COP) is defined as the useful heat delivered divided by the total power input. Recent standards and regulations are referring to a seasonal coefficient of performance (SCOP) defined as the useful heat delivered during the heating season divided by the corresponding power input.

Heat pumps are in most applications an alternative to fossil fuel gas or oil combustion boilers, often resulting in a significant reduction in CO₂ emission and primary energy consumption. The cost of the equipment and efficiency are most important to compete with fossil fuel solutions.

In 2012 the global air to water heat pump market increased by around 5.4 % to 1,37 million units. Both Europe and China had an increase of respectively 7% and 11%, while Japan had a drop of 12.8% (JARN, 2013/08).

2. Heat pump types, implications and trends

   1. Heat pump types

Most heat pumps are driven by electric motors, but gas engine drives are also used to a small extent. Heat pumps may also be thermally driven, using a sorption or ejector concept.

It is also possible to classify heat pumps by types depending on the usage:

1) Heat pump water heaters (HPWH)

2) Space heating heat pumps

3) Combined water and space heating heat pumps

Heat pump water heaters (HPWH) are a category of heat pumps designed to heat domestic and other service hot water to temperatures between 50 and 90 °C. These operating temperatures must be considered when selecting the refrigerant.

A HPWH basically consists of a water storage tank and a heat pump water heating unit and in some designs an additional heat exchanger. In the heat pump unit, water supplied from the storage tank or directly from the city water supply is heated by the condenser or, for trans-critical cycles using R-744 the gas cooler of the refrigerant circuit or, for absorption cycles by both condenser and absorber, and then returned to the storage tank or used directly for service. Stored hot water is supplied to each tapping point, in response to the demand.

The basic components of the heat pump unit are: a compressor driven by an electric motor or gas engine, a condenser or a gas cooler for heating water, an evaporator to absorb heat from the heat source, refrigerant, a refrigerant expansion device, and a control unit. Heat pump units that use R-744 as a refrigerant typically use an additional internal heat exchanger, and in some cases, an ejector or expander to improve energy efficiency. Different from mechanical compression type heat pumps, there is not any mechanical compressor for absorption types. Instead one or two generators, one or two solution heat exchangers, one or two solution valves and a solution pump are used.

A space heating heat pump is optimised for comfort heating. Comfort heating heats the room by heating water for distribution to an air handling unit, radiator or under floor panel. The required water temperature depends on the types of emitter, low temperature application ranging from 25 to 35°C for under floor heating, for moderate temperature application such as air handling units around 45 °C, for high temperature application such as radiant heating 55 to 60 °C, and for very high temperature application, as high as 65 to 80 °C, such as for the fossil fuel boiler replacement market. The required water temperature affects the selection of refrigerant. Heat pump systems are more efficient at lower sink temperatures, but each product must fulfil the required operating temperature.

A space heating heat pump using water for distribution generally consists of a heat pump unit and often an additional heat exchanger unit and a water storage tank. Several different configurations are used. The basic components of a heat pump space heating unit are similar to those of heat pump water heaters.

In most cases air source heat pumps are used for space heating. Ground and water source heat pumps are also used, especially in colder regions, but they represent a smaller segment of the total heat pump market.

Combined water heating and space heating heat pumps have two functions, supplying hot tap water and providing space heating. Several configurations of combined space and water heating heat pumps exist in order to optimise the seasonal energy efficiency for a specific application. In most configurations a water storage tank is used to store the domestic hot water and also to act as a small heat buffer for the space heating function.

In order to obtain high water temperatures at low outdoor ambient temperatures, air to water heat pump systems can utilise a multistage compression system or a cascade refrigerating system with different refrigerants for each stage in the cascade.

Capacity Ranges of Water and Space Heating Heat Pumps

Table 8-2 lists the most common heating capacity range offered by single units of each type of heat pumps.

2. Heat pump implications and trends

The tendency of decarbonisation of electricity strengthens this positive effect. Also the efficiency levels of heat pumps are being improved year by year. However, the investment in a heat pump is in most cases more expensive than a fossil fuel system because they employ complicated refrigerant circuits, compressors, larger heat exchangers and other special features. The majority of purchasers have not yet accepted the significantly higher first costs of heat pumps as a means to reduce running costs, compared to lower first cost fossil fuel based space and water heating systems, especially in colder regions.

In 2013 the EU commission decided upon a common energy label (commission delegated regulation No 811/2013) for hydronic heating systems comparing the primary energy efficiency of all major heat generators. This may have a positive impact on the heat pump applications in Europe. Minimum efficiency requirements are set separate for heat pumps. Both the energy label and minimum efficiency requirements will have a major impact on the refrigerant choice. Air source heat pumps have become the most popular heat pump type in Europe. They have experienced strong growth over the last several years because of their economic benefits and recognition as renewable energy source. The positive environmental aspects and potential market growth of heat pumps must be taken into account when refrigerant options and limitations are considered

In Japan, heat pump water heaters using the refrigerant R-744 have become very popular in recent years under the trade name “Eco-cute” and are also introduced in Europe. These heat pump water heaters were launched in 2001. Between 2008 and 2011, there were annual sales of around 500,000 units (JARN, 2013/08). An evaluation method for defining the annual performance factor of heat pump water heaters has been established in Japan. This method considers the typical Japanese life style usage of hot tap water.

The general trend when taking new building standards into account is that the relative importance of tap water heating will increase compared to space heating. It is expected that this will be reflected in future systems.