In a basic air conditioning system the heat within a given space is rejected to atmosphere via a refrigerant based air-cooled condenser, therefore offering no energy reclaim benefits.

The reverse cycle water source heat pump system employs water as a transfer medium and is maintained within a predetermined operating band by means of a form of heat injector or rejecter. The system's use of ground loops in geothermal applications means that the water loop temperature is controlled within a predetermined band providing individual units with a transfer medium to allow heating and cooling on an independent basis.

This forms an energy efficient heat transfer system which, assuming the system is within the dead band of operation, requires no form of heat injection or rejection. It has been proven that for 60% of the year the heat pump system can run in a thermal balance situation, thus minimizing running costs by using pre-paid energy.

The system may incorporate any number of individual self-contained water source heat pump air conditioners, connected to a simple two-pipe closed water loop system. This system of air conditioning offers the installer and end user total flexibility with individual unit control, adaptability, energy efficiency, simplicity of servicing, fast track installation and low running costs.

It also offers environmental benefits with a minimal quantity of non-ozone depleting refrigerant, hermetically sealed within each heat pump. This eliminates the need for any on-site refrigerant handling and as the refrigerant is factory encapsulated, minimises the possibility of any leakage. A further advantage is the fact that the system is extremely simple to design and requires only basic pipe work skills for the mechanical installation.

Control flexibility

The system's flexibility enables the occupants of the air-conditioned areas to have individual control of their working environment, irrespective of the requirements of the occupants of adjacent rooms. This type of system also has the advantage that in the very rare event of a unit failure, the breakdown does not affect adjoining areas and does not involve shutting down the entire system as may be the case in the event of a fault in a central plant or multi split type air conditioning system.

The popularity of zoned air conditioning, which allows any room to be air-conditioned independently and tempered to meet requirements different from adjacent areas, contributes to the overall running economics of the system. Many older type hotel and office buildings are being renovated and modernised to meet present day standards.

Fig 2

Such renovation and modernisation has to be carried out in most instances whilst the building is at least partly occupied. It is, therefore, essential that the installation of an air conditioning system or plant should be effected with as little disruption to the building or its occupants as possible.

Because of its simplicity of installation and its use of small-bore uninsulated pipe work and minimal size ductwork, this type of system has many advantages in reducing the installation disruption to a minimum. This can often be an overriding factor when considering air conditioning for buildings such as an hotel, where the system may be installed and operated on a phased basis.

Another consideration is that in many older type buildings, the existing available plant room space is restricted and in new or modern buildings it is always desirable to keep the plant room to a minimum. The heat pump system lends itself to these requirements, as not only is the plant compact but in the older type buildings some of the existing services such as boilers, and in some instances pipework, may be incorporated.

System operation

The system comprises a number of reverse cycle heat pump units interconnected by a two-pipe closed water loop, within this loop are installed supplementary methods of heat rejection and heat injection. The two-pipe water loop is maintained within a controlled temperature band thus providing a heat source or sink from which the reverse cycle heat pumps draw heat or conversely into which they reject heat.

In winter, when a majority of units may be providing heating, the heat being drawn from the water loop may eventually depress the loop temperature below the design limit. Supplementary heat injection would then be activated to raise the loop temperature back to the design temperature band.

Conversely, in summer, when a majority of units may be in cooling, rejecting heat into the water loop, the loop temperature would eventually exceed the design conditions. The method of heat rejection, i.e. dry cooler or similar, would then be activated to reject the excess heat from the water loop and return it to the design temperature band.

From the above explanation it can be seen that only the supplementary heat injector or the heat rejecter, operates at any one time, never together, unlike fan coil type systems where a boiler and chiller would run simultaneously to provide independent cooling and heating.

During the intermediate seasons of the year, ie for approximately eight months of annual operation, the heat rejected by the units operating on cooling is used by those units operating on heating. Thus a thermal balance is attained across the system and neither the heat rejecter nor heat injector is required to operate.

The unit comprises a hermetically sealed refrigeration circuit with refrigerant-to-air and refrigerant-to-water heat exchangers. The refrigerant circuit is reversed automatically by the user-set thermostat, such that each heat exchanger acts either as evaporator or condenser. The air side heat exchanger provides heat or cooling to the room by passing air over the indoor coil, whilst the water heat exchanger either extracts heat from the water or rejects heat to it.

Each unit requires an electrical supply, a condensate drain and connections to the flow and return of the water loop.

Building costs

This is brought about by the compact plant room requirements, which are usually much smaller than that required by most other systems, and the non-requirement of large ductwork.

Installation costs/pipework

The water circuit utilises small-bore pipe work as a result of extensive development in the design of the heat pump. This reduces installation costs significantly, also pump capacities and their costs are decreased. The circulating water pipework in nearly all instances does not require the use of expensive lagging materials, due to the water temperature operating band, thereby offering substantial cost savings against other systems.

Minimal disruption to construction

Because of its unitary design, small pipe work and limited use of ductwork, a system can be installed with the minimum of disruption to the normal everyday running of the building and consequently there is usually little loss of revenue to buildings such as hotels etc.

Running costs

Actual savings in the cost of fuel and power used in the system operation are mainly derived from the very nature of the energy conservation principal on which the system is based. These savings can be listed broadly under the following headings:

· Operates only on demand
· Systems thermally balanced operation
· User selective operation of units
· User environmental preferences
· Overrun capacity
· Energy losses

Fig 3

System balanced running

Here we are mainly concerned with solar gain effects as compared with the effect of shade, both of which will be experienced simultaneously to a greater or lesser extent depending on the aspect of any building. Particularly important are the times of the year when these diverse effects can be in greatest opposition.

In late autumn and winter for instance, when the elevation of the sun is relatively low the normal ambient temperature can be far below the normal comfort condition but the solar gain effect in areas of high glass content will have the pronounced effect upon raising the internal temperature of the building in those areas directly exposed to the sun. Thus an immediate situation is generated whereby some areas of the building will have an excessive heat load, whilst others - those in the shade - will have a demand for heating if comfort is to be maintained.

During normal occupancy, the majority of buildings have variable climatic requirements. Some areas require heating, others cooling, these needs are normally met from the energy within the loop. Furthermore, this effect is perpetuated throughout the day by variable solar gains affecting different areas of the building. Additionally, only a proportion of all units will be in heating or cooling mode at any given time as the units operate with an energy saving dead band.
Hence, we get savings through:

· Reduced total power input
· Cycling of units (reduced consumption of electrical power) during a partial, or total, thermal balance situation
·Intermittent operation of the dry cooler. It is seldom that the dry cooler will be in continuous operation, even in summer, due to variable building orientations experienced. This equally applies to the heat injection plant

User selective operation of units

The use of a decentralized system, enables maximum economy to be realized in terms of switching off the units in unoccupied areas or areas where for some reason air conditioning may not be temporarily required. For example, in an hotel application the units need only be switched on when rooms are occupied and are often activated by the door entry systems. To maintain an acceptable room temperature the units may be controlled to a reduced set point when the rooms are unoccupied with the addition of a room degradation sensor.

User environmental preference

This economy relates to individual occupied areas and their own personal preference for comfort conditions. Whilst, in general, most of these different requirements will tend to compensate each other, it is likely that the individual control feature of the equipment leads to an overall reduction of power requirements as compared with a centrally controlled type of air conditioning or heating and ventilating system.

Overrun capacity

In office block applications there is normally sufficient residual energy within the water loop and building fabric to enable late working to be affected without the need for heat injection from the boiler. If the problem is purely one of cooling, then the heat rejecter will run at the appropriate time.

Energy losses

The rate of loss of heat energy from the pipework is minimal because the band temperature is close to ambient. Hence there is an immediate saving in terms of radiated energy losses.

With thanks to Mike Creamer of Business Edge who revisits his Masterclass series of articles, updating and adding to the information which proved so useful to readers when the series was first published over ten years ago. In this reincarnation, the series will cover both air conditioning and refrigeration and serve as an on-going source of technical reference for experienced personnel as well as providing a solid educational grounding for newcomers to our industry.