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Why blowing hot and cold can make sense

Modern flexible workspaces demand modern flexible HVAC systems that don't waste energy. Self-adaptive simultaneous heating and cooling in four-pipe hydronic systems, capable of reducing energy consumption by 40 per cent, is a giant leap in the right direction, says Dean Ward
It is well established that through the course of a day there are many buildings that will require heating in some areas and cooling in others at the same time. This is generally due to changes in solar orientation through the day, resulting in changing patterns of solar heat gains, as well as variation in occupancy.

The latter parameter has also been influenced in recent years by changes in workspace management. Some areas, for instance, may have been changed from housing fixed workstations to a more flexible space with, perhaps, hot desking or 'touchdown' areas.

There may be other areas in the building where staff densities have been increased to optimise space usage, resulting in higher internal heat gains.

It is equally well established that a traditional four-pipe hydronic system design using separate heating and cooling plant is potentially wasteful of energy. Clearly, with so much focus on energy costs and carbon emissions, such waste is unacceptable.

This was certainly the case at both the new Mediaset Studios and La Forgiatura office space in Milan, where there was considerable variation in heating and cooling loads. In both cases the solution proved to be Climaveneta Integra multi-purpose units. In the case of La Forgiatura, water from the water table is used as a heat exchange fluid.

Such systems address this waste issue by providing simultaneous heating and cooling in a four-pipe system, using a single item of central plant. Crucially, they can also use the heat produced by the generation of chilled water for heating - and vice versa. Field tests and live installations have shown that using this approach can reduce primary energy consumption by 40 per cent.

There are three basic operating configurations which are totally independent of external temperature conditions - cooling only, heating only and combined hot and chilled water production. At the heart of this flexibility is the use of two refrigerant circuits that are able to operate independently of each other under the unit's control logic. Inclusion of suitable thermal storage tanks, both on the cold and hot sides, offers effective system operating modularity and optimises running costs.

When only chilled water is required the unit operates like a simple chiller, rejecting heat to the atmosphere through an air-refrigerant condensation coil.

In heating only mode the unit operates as a heat pump, extracting heat from an external source and channelling it through the evaporator to heat water for the building. The external heat source can be air, groundwater or geothermal.

The main difference compared with traditional reverse cycle heat pumps is that the hot water is produced in a different heat exchanger from the one used to produce chilled water, featuring a dedicated evaporator - thus maintaining separation of hot and cold systems.

Where hot and chilled water are required simultaneously, the unit behaves like a water-water unit, managing condensation and evaporation on two separate heat exchangers connected to the two circuits. The cooling and heating energy are provided respectively to evaporator and condenser, via heat exchangers that are hydraulically coupled to the cooling and heating circuits.

Where natural water is used as a heat source, a 'water save' function reduces the water flow rate to the heat exchanger in proportion to the use of the system. In this way the energy required for pumping is minimised, as are the discharge costs for the drained water.

Cost of ownership benefits
Beyond energy savings, there are further cost of ownership benefits. These include reduced installation requirements, as there is only one unit to connect and commission - requiring less plant room space compared to separate boiler and chiller plant, as well as minimising onsite work.

In specifying these all-in-one units, selection is typically based on the maximum peak demand for either heating or cooling, depending on the characteristics of the project. This means that for most of the year the units work at part-load, so efficiency at these part-loads is a critical consideration. To that and, the units are available with inverter control of compressor speeds to ensure efficient response to load variation.

With a unit that provides heating and cooling simultaneously, the traditional ratings such as Coefficient of Performance and Energy Efficiency Ratio become less meaningful. Consequently, Climaveneta has developed an alternative to objectively measure performance under simultaneous load conditions.

The Total Efficiency Ratio (TER) is calculated as the ratio between the sum of the delivered heating and cooling power and electrical power input. The TER reaches its maximum value when the heating and cooling loads are completely balanced.

Irrespective of how the efficiency is measured, though, the important factor is that this approach eliminates the waste inherent in traditional systems, helping to reduce carbon emissions while delivering a flexible system that meets the needs of the building's occupants.

// The author is product manager at Climaveneta UK //

1 March 2012

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