Reverse cycle water based heat pump systems

When considering conventional solutions for the air conditioning of buildings, fan coil and static cooling systems tend to dominate. Heat pump systems have traditionally presented a low cost alternative, but their adoption has been held back by constraints on system capacity and reservations on the use of refrigerant in main cooling circuits. Recently, a new system that uses heat pump technology, with water instead of refrigerant as the heat transfer medium, has started to attract the interest of some commercial developers and end-user clients in the UK, as a viable alternative to fan coil units.

Pioneered in Holland, the system is more cost-effective both in terms of initial capital and life cycle costs than an equivalent fan coil system. To date, over 50 installations have been completed, mainly in Holland and Germany, and the UK’s first installation has recently been completed for a new day care unit for a hospital in the south of England.

Principal features of the system

Table 1 sets out a simple analysis of the main selection criteria for the system compared to a standard fan coil solution.

The system draws on existing technologies and components and comprises a series of heat pump units fitted within ceiling voids together with a central, external heat pump unit, all connected by a water loop. Fresh or recirculated air is filtered, cooled or heated as required and supplied through ducting and grilles to the internal spaces via the room units. Each zone or room has its own local unit.

Each heat pump unit includes a refrigerant circuit with both refrigerant to air and refrigerant to water heat exchangers. A small refrigerant charge circulates within each unit. A reversing valve enables each heat exchanger to act as either an evaporator or a condenser, and the system can be configured in such a way that units in the warmer part of the building cool and transfer the thermal energy, via the water loop, to units in the cooler areas, enabling heating and cooling to take place simultaneously in different zones.

One of the key features of the system is that the central heat pump is used in place of both the primary heat source and cooling source found with a comparable fan coil system. This results in substantial cost savings and a significant reduction in plant space requirements.

Water is circulated around the building in a two pipe loop at 15°C-30°C to minimise losses and reduce energy usage. The temperature in the circuit is maintained by the central heat pump unit. The local units in the circuit use this water to either warm up or cool down the air, with the superfluous energy being recycled to the water.

During the summer months, the central heat pump operates like a chiller unit dissipating heat. However, while most water chiller systems operate at around 5°C-14°C this system only starts up when the temperature exceeds 28°C-30°C, saving energy, as it only operates when the water temperature becomes excessively warm.

In winter, as the majority of spaces will require warm air, the local heat pump units will take the heat energy out of the water loop, causing a heat deficiency within the system. As a consequence of this, the system takes the cold external air and cools it further, transferring this heat energy into the loop.

Since all units are connected to the same water loop, mutually exchanging heat, the system can achieve a high recovery of thermal energy. The water loop is expected to remain within the required temperature range for up to 85% of the year, without the requirement for any external energy from the central heat pump.

With an operating water temperature of 15°C-30°C, polyethylene pipework may be used instead of copper, and the pipework does not need to be insulated, resulting in further capital cost savings.

Advantages of the system

The principal advantages of reverse cycle water based heat pump systems are as follows:

• The system achieves high levels of efficiency, with a coefficient of performance score for the overall system of around four to five being claimed by manufacturers.
• Lower capital costs compared to an equivalent fan coil system (as detailed in table 2).
• The system contains a relatively small charge of refrigerant, reducing potential environmental impact.
• Each internal space or room unit is individually controlled to suit the zone requirements. The system also allows different units to heat and cool at the same time to suit the local requirements.
• Distribution is based on two uninsulated polyethylene pipes and so is simple, quick to install and requires less space.
• In the event of a single unit failure, the other units will not be affected and the system will remain operational.
• The decentralised nature of the system means it is easy to modify to suit future internal alterations.
• There is a saving in plant space compared to a fan coil system, due to the reduced requirements for central plant.
• The system qualifies for tax relief under the Enhanced Capital Allowance scheme.
• Lower running costs compared to an equivalent fan coil system, due to the high recovery of thermal energy that is possible, the reduced operation of the central pump unit (compared to that of the primary cooling and heating sources associated with a comparable fan coil system), reduced pumping costs and reduced thermal loss from pipework.
• It is important to note that to date no studies have been carried out to establish the actual running costs of existing installations, so the magnitude of the savings still need to be proven.
• Maintenance requirements are reduced due to the reduced central plant provision.

The design issues that require careful consideration in relation to reverse cycle heat pumps are as follows:

The compressor within the room unit can be noisy. Through careful design of the attenuation within the unit, manufacturers claim that noise levels are comparable to a similar sized fan coil unit.

An effective water treatment regime is required to minimise the risk of blockages to the heat exchangers within the internal units.

Controls. Because this system is a changeover system, it does not allow the sensitivity of temperature control that can be achieved with a properly commissioned fan coil unit, possibly resulting in more variable space conditions.

Should a compressor in a unit break down, it may be more cost effective to replace the entire unit rather than carry out site repairs. This may lead to the unit possibly being replaced sooner than a comparable fan coil unit, so the reliability and longevity of the compressor is an important factor in the unit selection.

The electrical requirements of the internal units are greater than those of a comparable fan coil unit, and so the electrical distribution system would need to be designed accordingly.

Alternative system options
The central heat pump unit or units can be omitted completely if a separate heat injection source, such as a boiler system, and a heat rejection source, such as cooling tower are provided instead. This is particularly relevant on refurbishment projects with existing plant or indeed on large developments where some form of centralised heating system may be being provided to more than one building.

Cost comparison
The costs detailed in this model for the heat pump system are based on the analysis of current proposed speculative commercial projects.

Costs are current as of fourth quarter 2004, based on a location within the London area.

The costs are representative of reasonable sized projects in the region of 10 000 m² gross internal area.

Rates for the shell and core installation are given on the basis of gross floor area, whilst costs for the category A fit out are given on the basis of the net internal area of treated office space.