District cooling has the potential to improve the energy efficiency while also lowering the carbon footprint of comfort cooling in urban areas. The following discussion demonstrates how district cooling technology can be used to achieve these two important objectives:
How does district cooling improve the energy efficiency of cooling energy production?
Due to economies of scale, the district cooling system can adopt energy-efficient technology such as industrial grade high-efficiency chillers, series-connected chiller modules, thermal energy storage, and cogeneration or combined heat and power.
Thermal energy storage improves the efficiency of cooling energy production by doing the following:
- Increased on-site energy efficiency
- The load levelling or peak shaving operation mode shifts a significant portion of chiller operation from peak hours to off-peak hours. Because of the lower ambient temperature, chillers operate more efficiently during off-peak hours than during peak hours.
- Thermal energy storage allows chillers to operate continuously at close to full capacity and optimum efficiency, improving chiller energy utilization even further.
- The low flow, high chilled water delta T design also helps to improve chilled water pumping efficiency.
- Increased source energy efficiency
- From the standpoint of the power grid, shifting power demand from peak hours to off-peak hours is beneficial for a number of reasons.
- Lowering peak power demand reduces power generation from the inefficient peaking power generators. With the shift in demand to off-peak hours, there is greater demand for power generation from the base load power generators, which are significantly more energy efficient than peak generators.
- Furthermore, transmission and distribution losses are lower during off-peak hours, contributing to the energy efficiency improvement of the power grid.
- Lowering CO2 emissions
- Increased energy efficiency, both on-site and at the source, will result in lower CO2 emissions.
Series counter-flow chillers improve the energy efficiency of chilled water production by reducing the compressor lift of the series-connected chillers.
Cogeneration district cooling is the simultaneous generation of electrical power and cooling energy from a single fuel source. The most common fuel used for cogeneration district cooling is natural gas, which is considered the cleanest form of fossil fuel. Cogeneration improves the energy efficiency of chilled water production by doing the following:
- Generating chilled water from waste heat recovered from the exhaust gas of the prime mover.
- On-site power generation avoids the transmission and distribution losses associated with central power generation in the traditional power grid.
B) How does district cooling reduce the carbon footprint of cooling energy production?:
According to the discussion above, district cooling can improve the energy efficiency of cooling energy production. In terms of energy utilization, this alone helps to reduce the carbon footprint. Furthermore, district cooling technology can help reduce the carbon footprint in other stages of the plant’s life cycle, such as construction, operation and maintenance, repair, and disposal.
The district cooling system will “see” load diversity between many different end-user buildings by aggregating the cooling load from these same buildings, resulting in a total cooling demand which is significantly less than the total sum of the individual building peak cooling loads. At the same time, the use of thermal energy storage technology allows the district cooling plant to effectively decouple cooling energy production from the end-user demand. The required chiller operating capacity of a district cooling plant with thermal energy storage can be sized based on the average of the total 24-hour cooling load. When compared to a central chiller plant without thermal energy storage, this results in significantly lower installed chiller capacity.
As a result, a district cooling system will require significantly lower installed chiller capacity than the total sum of the installed cooling capacity for the individual in-building chiller plants. Lower installed chiller capacity translates to lower resource utilization throughout the district cooling system’s life cycle, including construction, operation and maintenance, repair, and disposal. Consequently, a district cooling system will have a lower life-cycle carbon footprint than individual in-building chiller plants serving the same end-user space.
To summarize, the preceding discussion has shown that district cooling technology holds the promise of increasing the energy efficiency while lowering the carbon footprint of comfort cooling in urban areas.