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Sat 19 May 2007 10:54 AM

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Gaining energy

With the focus on energy efficient and sustainable engineering rising in the region, Ilse Dubois, application engineer with Trane Europe, Middle East, Africa and India outlines the benefits of water-side heat recovery.

As energy costs rise and environmental awareness is coming to the industry forefront, the interest in energy recovery is also increasing. In building services systems, heat recovery from the condenser of air or water-cooled chillers can provide significant savings if the setting and building use are suitable for this application.

The feasibility of water-side heat recovery takes into account several factors, such as the number of hours of simultaneous cooling and heating needed and the available space for heat storage equipment. To determine the appropriate water-side heat recovery usage, one should be well aware of all the different chiller configuration possibilities, design system configurations and controls needed to ensure efficient operation. These factors will determine the amount of savings possible while taking into account the initial investment.

Energy recovery: the benefits

But what energy can be recovered? In any cooling system heat is transferred from the load location, whether this load is a result of space temperature conditioning, dehumidification or a process. When air conditioning units are providing this cooling, several sub-processes are involved:

• Heat is removed from the space or process by the evaporator or chilled-water coil in the system;

• Heat is transferred to a condenser in the refrigeration process;

• Heat is transferred from the system's condenser to the outdoor air - this can be done directly through an air-cooled condenser or via the cooling tower or dry cooler connected to the water-cooled condenser.

Recovering otherwise rejected, thus wasted, heat gives a double benefit: it lowers the amount of purchased heat needed and reduces the ancillary power necessary to reject the heat. The annual operating costs to produce the necessary cooling and heating will also be reduced.

Some national or international regulations require condenser heat recovery to be used for domestic hot water pre-heating or heating. Typical applications that meet these criteria include hotels, dormitories and hospitals.

Recovering heat also reduces environmental emissions: burning fossil fuels increases site emissions and recovering heat can satisfy future loads without consuming more fuel on site. Also, some countries are considering emissions trading, which may become extremely valuable.

Feasibility analysis

Carrying out an accurate feasibility analysis of heat recovery systems is critical. Factors to take into account include:

• Coincident cooling and heating loads;

• The fuels used;

• Utility rates: model time-of-day usage and demand charges;

• Chiller heat recovery conditions and energy usage;

• System configurations used in heat-recovery applications;

• Reductions in substances such as CO
2

, NOx and SO
2

when comparing heat recovery with alternative solutions.

Tools that can be used to fulfill these needs include: System Analyzer, Trace700 Chiller Plant Analyser, EnergyPlus and HAP.

The heat recovery formats that are possible include full and partial, plus controlled and uncontrolled solutions. On air-cooled chillers, a heat recovery condenser may be added in parallel or in series with the condenser inside the chiller itself.

With water-cooled chillers there may be only one water-cooled condenser, where the heat recovery will be realised in the hydraulics system. Water-cooled chillers with an additional heat-recovery condenser in parallel (dual) or series (auxiliary) with the water-cooled condenser, may have the heat recovery inside the chiller itself. Depending on the chiller configuration, full or partial heat recovery will be obtained.

But what exactly does full or partial recovery mean? The total (or gross) heat that any chiller must reject is the sum of its cooling capacity and the compressor power input. Depending on the chiller configuration, this full gross heat rejection amount may or may not be recovered. When it cannot, this means that physically the heat recovery condensor can only recover part of the otherwise rejected heat due to its size or configuration.

But is full better than partial recovery? It all depends on the building. What are the simultaneous cooling and heating needs? When cooling is provided, how much heating is needed? As heat can only be stored to a limited extent, why would you recover all rejected heat if it is not needed in the building? Depending on the building size and usage, providing partial heat recovery will be enough to satisfy the heating loads.


Controlling aspects

Another important aspect to distinguish among different heat recovery systems available is the controlled versus uncontrolled methods. A chiller that provides the possibility to operate controlled heat recovery conditions will fix the leaving hot water temperature of the heat recovery condenser, mainly independently of the outside ambient conditions. The advantage of such a system is obvious; the disadvantage may be that the chiller consumes more energy in the heat recovery mode than it would at normal ambient conditions when providing a cooling only function.

However, when the chiller is operating in a very hot climate such as is typical of the Middle East, and the demanded recovered hot water temperature is not extremely high, the chiller will operate in heat recovery mode at higher energy efficiency levels than it would in cooling-only mode. In a hot climate, controlled heat recovery has two advantages: recovering heat and running the chiller at a lower energy consumption.

Fixing the demanded hot water temperature at high level compromises the chiller efficiency, but reduces the consumption of heating devices such as boilers. As the electricity meter is on the building rather than individual equipment, a balance should be found between compromising chiller efficiency versus saving boiler energy.

Uncontrolled heat recovery chillers are less sophisticated. Here the chillers are configured to obtain the lowest energy consumption in cooling-only mode, taking advantage of ambient relief when outside temperatures are lower. Hence the chiller runs at better energy efficiency ratios. The consequence of this, is that the leaving hot water temperature will be high when the ambient temperature is high and vice versa.

The leaving recovered hot water temperature is not guaranteed however. As the heat recovery condenser adds to the total condenser capacity, these chiller types actually have slightly better efficiencies. Typical applications are the pre-heating of domestic hot water so that boiler consumptions are reduced.

System configurations

When a chiller is located on the load-side of the bypass line in a primary-secondary system it is loaded preferentially because it always receives the warmest return-water temperature. The chiller will always work at full load unless system cooling demands are really low, all other chillers are shut down and the preferential chiller also needs to run at part load. Therefore, when operating it rejects as much heat as possible depending on the chiller heat recovery configuration capacity.

A chiller that is piped in this location will also add to the chiller plant flowrate and does not reduce the return water temperature to any other chillers. If the chiller supply water temperature is maintained, the chiller may reject more heat than can be used by the heating load of the system. If it concerns an air-cooled chiller, more heat must be rejected through the standard air-cooled condenser to guarantee that the refrigerant fully condensates. When it concerns a water-cooled chiller, part of the heat must be removed through the cooling tower or dry cooler.

A potential problem that exists in preferential loading is of delivering a greater heat capacity than is needed by the system. One way to reduce the amount of recovered heat would be through resetting the chiller's cold water setpoint. When less cooling is produced, less heat will be recovered. However, changing the chiller's cold water setpoint will have consequences on the system supply temperature, which should be maintained as designed.

Another solution to this issue is moving to the use of the sidestream position. A chiller piped in a sidestream position can be loaded to any capacity by varying its leaving chilled water setpoint. When operating, it will cool the return chilled water temperature to the non-heat recovery chillers, which will then operate in part load.

The non-heat-recovery chiller pumps must supply the entire flow demand of the cooling load. These chillers must be designed for total design cooling capacity. Cooling capacity of the sidestream chiller is an add-on, but it cannot be relied on to deliver the required cooling capacity. When both heat recovery and standard chillers are operating, the standard chillers will experience a reduced change in temperature and will not be able to load to their full capacity. This can result in an operating condition where more than one standard chiller will be operating to satisfy flow requirements, even though one could meet the load.

One advantage of the sidestream configuration is that the chiller does not need to produce the design system supply water temperature; it can produce the exact water temperature that is necessary to meet the required heating load of the application. This will allow the chiller to operate more efficiently because the cooling is produced at a higher chilled water temperature. Also, even if the sidestream chiller leaving water temperature is higher than the system supply temperature, it will be lower than the system return temperature. Hence, the entering water temperature of non-heat recovery chillers will be pre-cooled.

Sidestream is an effective configuration to use for a heat recovery chiller in a variable primary flow system. Also, a water-cooled chiller positioned in sidestream will function as a water-to-water heat pump with no cooling tower needed. The heat pump is controlled on leaving condenser water temperature and will be fully loaded when the system demands full heat capacity. When less heat is needed, the heat pump will operate in part load and always delivers the amount of heat needed by the system, never more.

Heat recovery benefits in brief

• Water-side heat recovery is a viable option to address the goals of reduced energy consumption and environmental emissions. With an increasing awareness of operating costs and return on investment calculations, heat recovery is ideal for applications on (pre-)heating domestic hot water, process water or comfort heating.

• Typically hotels, hospitals and dormitories will benefit from the use of water-side heat recovery systems.

• The amount of heat that can be recovered varies according to the system configuration and equipment choice.

• Each of the heat recovery chiller configurations and system configurations has advantages and disadvantages. The system and chiller type that is right for a building depends on the specific project conditions.

• Where the chiller is placed in the system, hydraulics can give some flexibility into how it is then controlled.

• The electricity consumption meter will be on the building rather than on individual equipment within the system.

• The energy costs of the total system should be investigated. Low energy efficiency performance on one unit, for example due to forced heat recovery, may lead to lower global energy costs in an application due to for instance lower boiler consumptions.

• It is important to find the right balance of all factors to suit the application.

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