District Cooling

Theory and Practice

Alaa A. Olama

• Éditeur: CRC Press
• Année: 2016
• ISBN:
  9781315354408
• Édition: 1
• Sujet: Commerce
• Sous-sujet: Développement durable en entreprise

DISTRICT COOLING: THEORY and PRACTICE provides a unique study of an energy cogeneration system, set up to bring chilled water to buildings (offices, apartment houses, and factories) needing cooling for air conditioning and refrigeration. In winter, the source for the cooling can often be sea water, so it is a cheaper resource than using electricity to run compressors for cooling. The related technology of District Heating has been an established engineering practice for many years, but District Cooling is a relatively new technology now being implemented in various parts of the world, including the USA, Arab Emirates and Kuwait, and Saudi Arabia. Existing books in the area are scarce, and do not address many of the crucial issues facing nations with high overall air temperatures, many of which are developing District Cooling plans using sea water. DISTRICT COOLING: THEORY & PRACTICE integrates the theory behind district cooling planning with the practical engineering approaches, so it can serve the policy makers, engineers, and planners whose efforts have to be coordinated and closely managed to make such systems effective and affordable. In times of rising worldwide temperatures, District Cooling is a way to provide needed cooling with energy conservation and sustainability. This book will be the most up-to-date and comprehensive study on the subject, with Case Studies describing real projects in detail.

1	Introduction to District Cooling

1.1 DEFINING DISTRICT COOLING

A district cooling (DC) system is a central air-conditioning system that produces and distributes chilled water from a plant(s) to buildings, thus centralizing the production of chilled water and maximizing economy of scale.

District cooling systems produce and deliver chilled water, or a secondary fluid, from a central source to consumers in a more efficient, reliable, and environment-friendly way than in-building air-conditioning stations. Consumers may be residential, commercial, industrial, or other users in need of chiller water. Comfort air-conditioning or process cooling systems use chilled media to operate their systems. Figure 1.1 shows a district cooling system connected to consumers.

To maintain comfort conditions, individual room air-conditioners generate cooling energy locally for one room. Central cooling systems generate cooling energy in one or more central places within a building and distribute it to more than one room within the building.

District cooling systems generate cooling energy centrally and distribute it to various users’ buildings by utilizing a piping network.

The capital cost of individual air-conditioners is normally low when compared to central air-conditioners, but its operating costs are high because their energy efficiency is low. Capital costs of central air-conditioners are higher, but so is their energy efficiency making their operating costs lower. Over their operating lifetime, the overall cost of central air-conditioners becomes lower than individual air-conditioners.

This analogy applies to a comparison between district cooling systems and central air-conditioners. District cooling systems have a higher capital cost than central airconditioners and a higher energy efficiency. This is particularly so when buildings are situated in a dense area where cooling loads are high per surface area and have a diversified use. If a small number of rooms in a building need air-conditioning, it may be best to use individual air-conditioners; density of cooling load per surface area is usually a deciding factor.

Across the board, utilization of district cooling for all buildings is not usually a sound proposition because some applications may best be served with individual airconditioners. For one or more building in a heavily populated area, district cooling is usually a good option.

Inside a building, the air-side equipment, such as air handlers, fan coil units, terminal units, and chilled water distribution systems, remain the same when a district cooling system supplies chilled water instead of a local in-building chilled water plant.

FIGURE 1.1 A district cooling system connected to end users.

New control systems, pumps, chilled water heat exchangers, and energy metering systems are some of the additions needed for district cooling connected buildings.

An important issue for a district cooling system is its higher energy efficiency. This is especially important in countries where energy supply is a factor. The reduction of carbon dioxide emissions of district cooling systems compared to individual or central air-conditioners is another important issue.

District cooling systems are experiencing a rapid growth in several developing countries, especially in the Middle East, where the largest growth in DC occurred during the past 10 years. The total installed air-conditioning capacity in the Gulf exceeds 8,800,000 kW (2,500,000 TR [tons of refrigeration]), making it the largest installed capacity in the world.¹

1.2 THE ECONOMIC AND ENVIRONMENTAL BENEFITS OF DISTRICT COOLING FOR A CITY

The Sankey diagram in Figure 1.2 shows two scenarios to provide heating, cooling, and electricity to a city.² One scenario uses a traditional coal-fired power station, business as usual (BAU) scenario, whereas the second scenario uses natural gas in a modern combined heat and power (CHP) station.

In the first scenario with a conventional power station, the typical average thermal efficiency of this simple cycle power station is around 35%. More advanced power stations with combined cycles have thermal efficiencies around 45%.

Natural gas-fired CHP stations that recover exhaust gases have overall thermal efficiencies of 80%–90%, and sometimes even higher.

FIGURE 1.2 Economic and environmental benefits of district cooling for a city. (From UNEP, District Energy in Cities—Unlocking the Potential of Energy Efficiency and Renewable Energy, United Nations Environmental Programme, 2015.)

This is why the total primary energy utilized in BAU scenarios shown in Figure 1.2 is 601.6 GWh compared to a primary energy utilization of 308.2 GWh with a CHP station. This is a savings of 293.4 GWh or 48.8% compared to BAU, although in both cases, the same energy is produced and taken up by end users: 100 GWh of heat, 100 GWh of cooling, and 100 GWh of electricity.

Table 1.1 explains how these figures are derived with efficiencies stated for each process.

TABLE 1.1
Comparison of Energy Consumption between a Traditional Power Station and a Modern Power Station Using CHP to Produce a Fixed Quantity of Heating, Cooling, and Electric Energy

...

Case		(1)	(2)
Power Station		Traditional Power Station	Modern CHP Station
Primary energy used, Source	GWh	601.6	308.2
	Coal, GWh	501.6	–
	Natural Gas, GWh	100	308.2
Heating energy	Efficiency	99%	Wind energy	8.3
	Coal, GWh	100	Waste heat	25.0
		N/A