3.3 Potential Environmental Benefits Associated with DHC Systems

In Section 3.2, the basic components of DHC systems were discussed. In this section, the potential environmental benefits of DHC systems, compared to conventional or non-district systems, are identified considering the negative environmental impacts identified in Section 2.0. These benefits are derived, partly due to the difference between district and conventional systems and, partly due to stand-alone features of DHC systems.

Partial Load Efficiency
In general, DHC plants operate at higher efficiencies under partial thermal load conditions, compared to conventional systems. This is because conventional systems typically employ only one boiler and chiller unit. While such units must be rated for peak seasonal and hourly loads, they actually operate most of the time at much lower partial loads. Operation at these lower loads can, depending on the class of equipment used, result in much lower operating efficiencies. District systems on the other hand, with multiple units can optimize overall plant efficiency by selectively operating fewer units at or near maximum efficiency during partial load conditions. Further, DHC systems that comprise several different types of thermal energy generation plants can optimize plant and system efficiency by utilizing, whenever possible, the thermal energy sources with the highest energy conversion efficiencies for base and other partial load conditions. The sources with the poorer conversion efficiencies can then be utilized only to meet peak loads. Ultimately, improved efficiency means use of less fuel for the same amount of energy produced which in turn results in the conservation of fossil fuels, reduced emissions of Foes (products of combustion) such as those described in Section 2.0, improved air quality, and reduced use of refrigerants (CFCs or replacements HCFCs or HFCs) in cooling applications.

DHC Integration with Power Generation
District systems are well suited to combine with electric power production facilities forming what are known as combined heat and power (CUP) plants or cogeneration plants. As discussed in Section 3.2, the amalgamation of these two energy production/utilization schemes results in a substantial improvement in overall energy conversion efficiency since district heating systems can effectively utilize the otherwise wasted heat associated with the electric power production process. A district system meeting much or all of its load requirements with waste heat from power generation facilities will have a positive environmental impact as fuel consumption within the community is reduced considerably. Conservation of fossil fuels and a reduction of combustion-related emissions are resultant direct benefits of such DHC system.

Biomass Combustion
Biomass combustion is considered by many as a means of zero production of Coy when combined with reforestation. The underlying principle Is that by burning blomass, Col is released but with reforestation the CO! is absorbed in the new growth provided the rates of each activity are balanced. Case Study No.8 regarding the Prince Edward Island DH system discusses this approach.

Limited Number of Emission Sources
The centralized nature of DHC energy production plants results in a reduced number of emissions sources in a community. This introduces the potential for several direct benefits.

Firstly, large facilities are much more capable of, and likely to, incorporate sophisticated state- of-the-art pollution control technologies than individual buildings (particularly households, commercial establishments and small industrial complexes). To incorporate such equipment on a small scale basis, due to the general lack of low cost effective pollution control equipment, is normally impractical. In comparison, therefore, large scale district systems, which in many cases have included best available control technology (BACT) are capable of significantly reducing the emissions to the environment on an equivalent energy production basis.

Secondly, the exhaust stacks, characteristic of large energy production facilities, are relatively high and therefore the exhaust gases that are discharged from the stack are well mixed with large volumes of the ambient air before the pollutants can reach the surrounding population, structures or plant life. The resultant improved dispersion introduces the benefit of minimizing low level pollutant concentrations and deposition in the immediate zone of greatest potential pollutant fall out (i.e. near the source), compared to the numerous lower stacks required of a non-district system. While local air quality can benefit significantly from DHC, it should be noted that long range pollutant transport is a subject of continuing debate. While high stacks are an effective means of discharging pollutants so that high concentrations are not experienced locally, they do permit the pollutants to migrate long distances. However, the problems associated with these dispersed pollutants are still related more to the total quantity of pollutants that are emitted to the atmosphere, regardless of stack height.

Superior Operating and Maintenance
Large, centralized plants, such as DHC facilities, typically use better operating and maintenance practices than do small individual building systems. Large facilities have trained staff, as well as sophisticated computerized monitoring equipment available to continuously monitor system operations, ensuring performance specifications are being met on a long-term basis. When such specifications are not met, prompt maintenance can be administered, or operating changes or upgrades introduced, as necessary. Regularly scheduled maintenance is a normal function of facilities of this scale.

With large DHC systems, the incentives to maintain a high level of operability, with little downtime or drop in operating efficiency, are economically based and are often critical to maintain the overall viability of a plant. Individual building systems, on the other hand, can not always afford sophisticated and continuous monitoring equipment (or to upgrade existing obsolete equipment), or permanent maintenance staff. The result is many such operations deteriorate because of the poor maintenance, with operating efficiencies subsequently dropping well below optimum levels. The higher operating efficiency afforded larger, well maintained, facilities translates directly to reduced fuel consumption which in turn results in conservation of fossil fuels and reduced emissions. Higher operating efficiency of the combustion process (where parameters such as temperature, combustion air and fuel input levels, residence time, etc, are closely monitored) also impacts emission production in that the concentration of certain pollutants produced, particularly Coy and NO*, is reduced.

Technical Upgrades
Centralized DHC facilities permit developing thermal energy production and emission reduction technologies to be adopted at the earliest possible date. Such technology improvements usually have significant positive environmental impacts. Examples of such developments include:

With DHC systems, new technologies can be implemented at much reduced cost and much more practically, even when compared to the same emission reduction effort being achieved at an equivalent number of conventional facilities.

With the ability to implement new technologies on older existing DHC systems, a great opportunity is available to system operators in areas of emission-related "non-attainment". Such facilities can continue to achieve the most recent regulatory based emission levels, as quickly as possible, after such regulations are enacted.

In comparison, implementing such techniques on the multitude of smaller sources that exist when DHC is not adopted is not a realistic alternative. Thus, the emissions from existing decentralized system sources cannot be reduced effectively over time.

Higher Design Efficiencies
In many cases, the relatively high capacity equipment associated with DHC facilities inherently operates at higher efficiencies than similar lower capacity units. This is particularly true of large centrifugal chillers which have coefficients of performance (COPS) of more than 5.0. This compares with the smaller units, such as those installed in individual buildings, which have COPS in the range of 3.0 to 4.0. The COP is the ratio of the refrigerating effect or cooling capability of the unit to the power input required to achieve this capability. COP provides a means of comparing the performance of various chiller types.

Other Environmental Benefits
There are many indirect environmental benefits of DHC plants which may not have as much impact as the benefits described above but which are worth noting.

The noise associated with the operation of heating and cooling equipment is concentrated at a single source with a centralized facility. Sophisticated noise control measures to minimize noise impacts on the surrounding neighbourhood can be applied more practically and cost effectively at a central facility than at numerous individual buildings,

With the concentration of fuel oil storage at central facilities, the potential risks associated with leakage are reduced since centralization implies elimination of multiple smaller oil storage vessels which deteriorate with time and lack of supervisory care. Storage vessels at centralized facilities are more likely to be regularly inspected for leaks or deterioration.

For liquid and solid fuels associated with DHC systems, the reductions in fuel use identified above will indirectly reduce vehicle emissions associated with fuel shipment as the requirement for delivering such fuels will also be reduced.

Where local air quality is a significant problem, the type of fuel burned can be upgraded in many DHC applications, with significant environmental benefits. For example, a plant burning coal or even relatively clean burning fuel oil can reduce its emissions simply by converting the operation to natural gas firing. Without DHC alternative fuel options are impractical in most communities.

Finally, considering cooling systems, with DHC, the conversion from CFCs is simplified and a practical option. Also, the use of cooling water from local rivers or lakes in lieu of cooling towers is a realistic alternative with DHC systems. The flexibility, offered by DHC systems, to pursue such environmentally beneficial alternatives is virtually non-existent with decentralized systems with their multitude of small units and owners.

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