Heat Pump

For the THEMAC system, a novel hybrid solar-assisted heat pump unit is designed and the heat pump performance is significantly enhanced by taking heat from the natural solar energy. The hybrid solar assisted heat pump is aimed to deal with hot water supply as well as space heating.

The system is deemed to be economically superior to electrical heating and solar-only systems and competitive with conventional energy-efficient fuel burning systems. Heat pumps combined with solar thermal have a superior seasonal performance factor for domestic applications. Solar thermal collectors convert solar radiation into thermal energy, in which a fluid such as air, water, glycol and etc., is used as a heat transfer medium.

The solar thermal system collects energy at low temperature and it may not work efficiently in direct heating, while this energy can be utilised as a source to the heat in winter. The solar collector-heat pump coupling results in lowering of the collector temperature which is generally maintained in a range of zero to 20℃ above the ambient air temperature.

A smaller size solar collector with higher efficiency and less cost as in the case of a solar assisted heat pump system collect about the same amount of energy as a bigger size collector with lower efficiency as in the case of a conventional solar heating system.

THEMAC System

The heat pump unit adopted in the THEMAC system is manufactured by Arkaya Ltd

In terms of refrigerants for solar assisted heat pumps, the halogenated refrigerants R134a is used due to good thermodynamic and thermos-physical properties.

This direct-expansion solar assisted heat pump is a system combining both solar thermal collector and heat pump evaporator into one unit, where the refrigerant evaporates by the effect of both incident solar energy and ambient.

The solar thermal collector operates at a temperature low enough to boost the solar energy collection efficiency, yet it is high enough to enhance the heat pump performance compared to an air source heat pump. Moreover, by coupling with PCM ceiling tile, solar assisted heat pump reflects higher energy saving rate and more flexible operating modes.

PCMs can store excess energy for later space heating or unused energy when space heating is not required. The solar-assisted heat pump unit handles both hot water supply and space heating in the winter.

A diagram of the solar-assisted heat pump unit is shown in Figure, with production of hot water and space heating through either radiator or fan coil unit. Additionally, the hot water provided by the heat pump unit is stored in the hot water cylinder and it could be used as the heat source for regenerating absorbents.

Arkaya has developed thermodynamic system for the THEMAC model with 3 things in mind

Increased efficiency and reliability
Smaller upfront cost with wide adaptability
Ease of operation and greater monitoring capabilities

Increased Efficiency and Reliability

Arkaya Thermodynamic System efficiency is increased as it uses the scroll technology
compressors. Scroll technology is a considerable improvement on the older technologies
namely of Piston and Reciprocating type compressors.

The clearance volume which is required in piston and or reciprocating technology is not required in scroll compressors.

This provides a greater efficiency, the spinning wheel axis allows for it to create small pockets with the fixed wheel and in these pockets is where the gas is compressed.

This phenomenon is illustrated in the image which showcases the different stages of
compression seen in a scroll compressor.

Thermodynamic system for the THEMAC model has also made use of the vapor injectection technology to enhance reliability under adverse conditions such as harsh winters. Vapour injection technology allows for vapor to be injected in the compressor through an additional port whilst the compressor is in the middle of the compression cycle.

This technology is especially beneficial under adverse environmental conditions which normally cause pressure drops and this is frequently accompanied by inability for correct flow to be delivered resulting in malfunction.

This has posed a serious threat to operation of heat pumps effectively. Previously mechanisms in place resulted in lower heating/condensing temperatures being attained which are unsatisfactory, or systems set to not run when temperatures get close to zero or freezing.

Generally the lower temperatures are accompanied by higher temperature demand to cope with greater heat loss in the house and also increased heating output requirement.

The user or the designer finds themselves in a situation that for longevity of the product its application needs to be altered or it needs to be linked with a back up system such as a boiler. Most common belief found amongst individuals not wanting to go for heat pumps is either the inability to function in lower temperatures and/or lack of support.

Both issues are overcome with the use of vapour injection technology, it ensures longevity of the system through management of pressure and flow required by the system.

The vapour injection technology also ensures that higher temperatures can be achieved even under harsh winter conditions without compromising the comfort.

The working principle for the vapour injection is simple, the outcoming hot refrigerant from the water heat exchanger is divided into two flow streams. One flow stream pressure is reduced by throttling whereas the other pressure is still maintained high. The reduced pressure in one stream accompanies a reduction in temperature.

The temperature and pressure difference in the two streams allows for exchange of heat and as such allows for compressors to be injected with more gas required for smooth operation ensuring that the flow is maintained and as such smooth operation is maintained.

A simple diagram below showcases this phenomenon.

The efficiency is also increased through this simple exchange of heat between the two streams, as this allows for the compressor to have a reprieve allowing it to compress in two stages as opposed to one. This vapor injection effect is somewhat similar to compression being performed
by 2 individual compressors.

Below the cycle is showcased on a pressure enthalpy diagram, it can be seen by the additional cooling effect created due to this exchange of heat between two streams which results in greater performance due to additional heat gained through the environment