Absorbing energy

With an increasing focus on creating energy efficient buildings, all aspects of the MEP systems must be considered. MEP Middle East examines the potential application of absorption chillers in the region.

Building Services Research and Information Associa, NEWS, MEP

With an increasing focus on creating energy efficient buildings, all aspects of the MEP systems must be considered. MEP Middle East examines the potential application of absorption chillers in the region.

Ensuring that all MEP systems are designed to operate at maximum effectiveness is vital to the provision of an energy efficient building. And with the cooling systems in the Middle East accounting for 60-70% of the overall building operational energy use, a strong focus in this area can bring substantial savings.

One technology that has proven successful in other areas of the world but has yet to be used on any significant scale for air conditioning systems in the Middle East is that of absorption chillers. But signs are that this is likely to change in the near future, with a retro-fit project at one of Dubai's largest hotels set to be the latest project to include the chillers.

Absorption chillers use heat to provide cooling, using gas rather than electricity as the source of energy to produce chilled water. Benefits of the systems include greatly reduced operating costs in relation to comparable electrically powered chillers, plus lower maintenance costs due to the low number of moving mechanical parts. The units are also proven to be environmentally friendly.

Technology uptake

Currently a few absorption chillers are installed in the region every year, mostly for industrial application, reports the UK-based Building Services Research and Information Association (BSRIA).

The limited gas mains network is cited as a possible cause for this lack of uptake, however many buildings in the region do have gas storage in their basements and the inclusion of a gas mains infrastructure in the Abu Dhabi city masterplan means that the options for application of such chillers are likely to rise substantially in the Emirate.

Indeed, the latest BSRIA World Market for Air Conditioning report is predicting a rise of 5.1% in the volume of absorption chillers with capacities of more than 350kW to be installed in the UAE alone between 2005-2010. This equates to a 7.1% rise in commercial value for the market.

In terms of comfort cooling, there are few examples to date of such systems, however the ones that exist have proven to be successful.

The Pacific Controls Headquarters in Dubai makes use of an absorption chiller as part of the comfort cooling system, with the technology adding to the green credentials of the building.

The application of such systems in the building helped it to achieve the Platinum rating under the US Green Building Council's Leadership in Energy and Environmental Design (LEED) system. In this case the chiller is combined with solar power to provide an energy efficient means of cooling the building.

The system comprises of a total of 240 solar thermal panels, which are arranged in banks of five panels connected in series. Around 300 gallons/minute of hot water is created in the panels and pumped to a vapour absorption chiller.

When the hot water enters the chiller's vapour chamber at 91°C it evaporates. The condensed water then drops onto the heat exchanger tubes of the internal evaporator, cooling the chilled water running through the tubes due to the latent heat of the vaporised refrigerant.

The solar system has a 720kW thermal energy capacity, with 90 tonnes of air conditioning provided to the building using the system. A diesel-powered co-generation plant has been installed as backup for the building's cooling.

In July, Dubai-based firm Save Energy announced the latest project to uptake the use of absorption chillers: Le Meridien Dubai hotel. The firm has won a US $27.2 million (AED100 million) build-operate-transfer (BOT) contract to update the cooling system at the hotel. EtaOne Orient is acting as its co-partner in providing the co-generation power units and the designed system will reduce the hotel's carbon footprint by more than 65%.

Under the contract, the firm will provide a tri-generation system that will use the generator exhaust to provide heat to a steam boiler that provides hot water at 195°C to an absorption chiller and the laundry, plus water for general hotel use at up to 60°C. The heat to the absorption chiller will provide cold air for air conditioning, with lithium bromide used as the absorbent in the unit.

The tri-generation system will be combined with solar thermal energy to reduce the bottom line energy costs of the main hotel by 54% based on 2006 audits (excluding capital costs).

Based on the 2006 audit, the electricity-steam hot water and air conditioning system designed by the firm will provide electricity at 12 fils/kWh. As a result of an overall energy audit carried out, recycled water will be used in the cooling towers, saving an estimated $136,000/month in peak periods at the hotel.

The tri-generation system designed for Le Meridien Dubai will provide up to 4,000 tonnes of air conditioning and 4.5MW of electricity for the hotel. It will use three gas generators producing 4.8MW nominal capacity, with four absorption chillers producing 1,323TR each.

This enables room for future expansion, the net available air conditioning load allowing for ambient temperatures being 4,128 tonnes. The 22 month project is scheduled to begin in September.

Growing options

In Egypt meanwhile, Egypt Kuwait Holding subsidiary Gas Chill established in late 2007 in order to cover the demand for absorption chillers in the country. The firm provides solutions that rely primarily on natural gas as an energy source, however it has also supplied systems using solar energy, steam and high temperature industrial waste as the primary energy source.

The firm offers standard packaged absorption systems in addition to bespoke systems. Projects it has undertaken to date include the British Egyptian Hospital and Misr University.

Further firms such as McQuay International offer absorption chillers in direct and steam-fired versions. The firm's 100-1,500 tonne chillers include advanced PID controls and inverter drive solution pumps.

As the need to provide environmentally friendly building solutions increases along with a reduced availability of electrical power supplies, absorption chillers could prove to be one of the more successful technologies of the future in the region.

ABSORPTION CHILLERS: The technology explained

Absorption chillers use heat to provide cooling, using gas rather than electricity as the source of energy to produce chilled water. The absorption cycle uses a heat-driven concentration difference to move refrigerant vapours (normally water) from an evaporator to a condenser.

The high concentration side of the cycle absorbs refrigerant vapours, heat is then used to drive off the vapours, hence increasing the concentration.

Two types of absorption chillers are available: single-effect (one-stage) and double-effect (two-stage). The first operates under low pressure; the two-stage units are available as gas-fired or steam-driven with high pressure steam.

Lithium bromide is the most common absorbent used in such commercial cooling plant, with water used as the refrigerant. Smaller absorption chillers may use water as the absorbent and ammonia as the refrigerant.

The process involves four main stages:

  • Evaporator - the water refrigerant absorbs heat from the chilled water and evaporates;
 
  • absorber - the lithium bromide absorbs the water vapour;
 
  • Generator - the water is separated from the lithium bromide solution at normal pressure;
 
  •   Condenser - the cold water produced from cooling towers cools the hot water vapour, condenses then returns to the evaporator.
Absorption chillers can save more than 65% of the operating costs compared to a standard electrically-operated chiller. As there are no compressors or major moving mechanical parts, maintenance costs are reduced and life spans are long. The chillers also have very low noise and vibration levels.

 

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