3.2 District Heating and Cooling System Components

3.2.1 System Prerequisites

Although varying from country to country and city to city, certain conditions must generally prevail in order for a DHC system to be viable compared to conventional systems.

Heating and cooling load densities, that is the heating/cooling requirements per unit area, should be relatively high. The very nature of a DHC system dictates this criterion since it becomes uneconomical to distribute energy to sparsely populated areas where distribution piping costs and thermal "losses" become comparatively high.

Generally speaking, a relatively high total heating/cooling load is preferred since improved operating efficiencies can be realized at larger facilities, and since economies of scale favour larger installations.

Apartment complexes, hospitals, universities, groups of office buildings, and factories are all energy user candidates which meet the above prerequisites well. Many major cities around the world meet much of their heating requirements through district heating. DHC systems that service areas of the City beyond the high density building zones typically result when adjacent housing densities are fairly high and/or several inexpensive sources of thermal energy are available. Examples of relatively inexpensive thermal energy include waste heat recovery from energy-from-waste facilities, from large power generation plants, and from gas turbine combined cycle cogeneration plants. Without such local opportunities for DHC supply and utilization, city-wide applications become borderline candidates at best.

A partial list of cities with well developed district heating systems would include Paris, Helsinki, Stockholm, Copenhagen, Moscow, New York, Boston, Son Francisco, Toronto and Tokyo. In Sweden, Finland and Denmark, district heating supplies 30, 39 and 42 percent, respectively, of the entire countries hearing demand serving downtown core areas to urban and suburban residential areas.

3.2.2 Thermal Energy Generation

DHC systems, owing to the fact that they are usually connected to a diverse group of customers with varying load requirements, must typically accommodate a relatively large total heating/cooling load with potentially wide variations from season to season. Since individual customers often experience their peak loads at different times of the day, the central production plant's daily characteristic load curve tends to be smoothed out, with the peak demand reduced, compared to the sum of all the individual peak loads. Thus, the installed total capacity of a DHC system can be less than that of conventional decentralized systems - a distinct advantage of a district system.

Figures 1 and 2 show actual hourly demand profiles for two large buildings in Toronto, one in a commercial office tower and the other a large hotel. Both buildings demonstrate significant demand during normal daytime hours and minimal off hour demand. Figure 3 shows the demand profile of the Toronto District Heating Corporation's major customer and demonstrates the flattening effect on peak demand when used by a variety of customer types.

Depending on total system peak and average load requirements and the load variations from day- to-day and season-to-season, DHC plants of varying complexity can and have been developed. A relatively simple DHC system might utilize a single energy production facility, comprising for example an oil or gas fired boiler (heating) and an electrically driven centrifugal chiller (cooling). Multiple units may also be selected to more efficiently meet base, intermediate and peak loads, as well as providing standby capacity and increased system reliability. More complicated DHC systems might utilize several different energy production facilities such as EFW (energy-from-waste - normally from municipal, commercial and industrial waste incineration), waste heat from manufacturing plant processes, absorption chillers, heat pumps, coal fired boilers. Other sources of heat for DH system include geothermal, cement kilns, biomass (burning of woodpulp, peat, straw, etc.) and solar collectors. In the case of these more complicated thermal energy production systems, the energy sources selected and the manner in which they are used depend on local fuel prices, availability of such alternatives, proximity of the load to such sources, environmental sensitivities, and other factors.

Promising Energy Production Alternatives
A very promising thermal energy source being used more and more is combined heat and power (CUP), or cogeneration. Energy from a cogeneration plant is normally extracted in one of two ways; heat is produced and used in a process while exhaust heat from the process is utilized to drive a turbine and produce electric power, or conversely electric power is first produced and exhaust heat from this production is then recovered for other uses. Although system efficiencies depend on the overall energy production capacity and the type, capacity and efficiency of the individual cogeneration components, typical cogeneration energy conversion efficiencies can be as high as 85-90%. This compares favourably with typical electric power generation facility efficiencies of 30-35%. The efficiency of the cogeneration plant is only this high if all of the waste heat associated with the electrical power production facility is utilized. This can be the case with DHC facilities utilizing heat from cogeneration plants for heating purposes and/or when absorption cooling systems are used for cooling purposes. Absorption systems utilize steam or hot water to pressurize and vaporize the refrigerant and the refrigerant, after condensation and expansion, chills the cooling system recirculating water (i.e., heat from space or equipment transferred to chilled water and ultimately to the refrigerant).

DHC systems need not confine themselves to heat utilization from central heating plants. Indeed district systems, because of their centralized and arterial nature, are well suited to becoming energy "brokers", collecting thermal energy from whatever sources have waste heat or unused capacity are available, and distributing the thermal energy to wherever it is needed.

A promising concept for a district heating and cooling system, acting in an energy broker capacity and enabling waste heat to be utilized, is through the extraction of heat from wastewater using a heat pump system. Possible applications include municipal waste treatment plant effluents and industrial waste treatment plant effluents. With such applications, during heating periods, heat would be extracted from the wastewater using heat pumps. The heat pump converts the low temperature heat extracted to a temperature that can be used in heating applications. During cooling periods, these same heat pumps, operating in reverse, extract heat from the space and/or equipment being cooled and transfer the heat collected into the wastewater.

Another promising concept that is receiving attention for district cooling applications is the utilization of, as a thermal energy source, cold lake water. Depending on the capacity of the source and depth at which the cold water is extracted, the temperature of the water remains at a relatively constant "cold" temperature. Such a system, requiring only pumping through the distribution and heat exchanger systems, use as little as 5% of the electricity used by electrically driven chillers. This concept is currently being studied in Toronto, Canada and is referred to as the Deep Lake Water Cooling (DLWC) project. At the present time, studies are underway to determine if any environmental impacts can be expected from the use of this potentially renewable thermal energy source, to establish the viability of the scheme, and to identify how the scheme should be developed.

Peak Shaving Thermal Energy Storage Concept
Thermal Energy Storage (TES) is another developing concept. TES offers the potential for economic and indirect environmental benefits. TES was developed in response to the very nature of typical cooling and heating load demands experienced by district energy production systems. Most systems, regardless of scale, are characterized by periods during the day when demand is quite low and other "peak" periods when demand rises considerably. The energy production required to meet the sum of the (more or less) coincidental peaks requires additional installed thermal energy production capacity with the resulting increases in capital and operating costs and may stress the local utility's resources, discouraging expansion of existing DHC systems. Because the daily peak demand is short-term and the thermal energy production equipment that is provided to meet such demands is used infrequently, utility rates are often considerably higher for peak loads to provide incentive to the users to reduce their short-term peak loads.

The principle behind TES Is to produce surplus quantities and store thermal energy during periods of low demand and subsequently utilize, when necessary, the stored energy to meet peak demands. The thermal energy storage medium may simply be hot water or cold water and ice. With adoption ofTES, the daily peaks of the typical DHC demand curve can be reduced so that the hourly energy production varies less. This means that the energy production equipment can be reduced in size, still be capable of meeting the lower maximum capacity, and can operate closer to a peak efficiency point throughout the day.

DHC systems are well suited to incorporating TES. In general, compared to individual building systems, DHC systems have more flexibility to reduce installed capacity by using TES, without losing system reliability, and are more capable of covering the higher capital costs involved and distributing the recovery of such costs over longer periods. In addition, because district systems normally cater to a diverse group of users with varying peak load requirements, the DHC system's characteristic load curve tends to be smoothed out, with the result that the total TES capacity requirements are proportionately lower than ifTES was considered at the individual building level.

With large TES systems in place, DHC systems that utilize waste heat from power generation plants can also implement load-management, supplying TES based heat during peak power production periods. This reduces the demand for waste heat at extraction plants, permitting production of more power, thereby reducing the peak power demand of the power generation utility.

On the district cooling load side, thermal storage systems using ice formation and storage technology can be utilized to reduce chiller capacity and meet peak short term demands. As with the heat storage system, during low demands, ice is made in the storage system, with the ice subsequently melted and cooling capacity released when demands peak.

Emission Considerations
A wide variety of fuels are used at DHC plants including various grades of oil and coal, natural gas, refuse and other biofuels such as wood chips, peat and straw. The combustion of these fuels may, as indicated in Section 2.0, produce environmentally hazardous products of combustion (Foes) thus flue gas cleaning devices and other emission reduction measures are often incorporated. Such measures are usually required under increasingly strict legislation, before approval to operate a facility is granted. Examples of pollution control equipment used at DHC plants include acid gas scrubbers. These systems typically utilize hydrated lime to react with the moisture, SO, and other acid gases in the flue gases discharged from the combustion system. With such systems, the lime-acid gas-water vapour reaction products are efficiently collected by electrostatic precipitators as particulate matter. Bag filters are also utilized in many applications to capture the particulate matter as well as the acid gas scrubbing reaction products. Conventional oil/gas fired boilers utilizing low NO, burners to dramatically reduce NO* emissions are also becoming more common. Flue gas recirculation to reduce NO, emissions has also been proven to be effective. Other emission control or reduction techniques can be introduced with DHC systems, including optimization of combustion efficiency (i.e., reduces CO*, CO and hydrocarbon emissions) through the use of modern computerized combustion control systems, and utilization of higher quality, lower emission producing fuels.

Decentralized Energy Production
Energy production at conventional or non-district heating facilities differs from DHC plants in several respects.

With the exception of some large boiler plants, most conventional facilities are usually too small to permit staged energy production (through use of multiple units or different energy sources). For systems having multiple boiler and/or chiller units, staged energy production can be utilized to meet base, intermediate and peak loads, allowing the energy production equipment to operate at or near maximum efficiency. Such capability is of course typical of DHC systems. Conventional systems that utilize a single piece of equipment (must be rated for peak loads) operate most of the time at partial loads. Depending on the class of equipment used, this may result in dramatic reductions in operating efficiency.

Conventional systems are faced with high costs if pollution control equipment is utilized or required, due to a general lack of suitable low cost pollution control technologies being available for smaller applications. This creates disincentives to incorporate such equipment. Indeed, in the case of households and small commercial establishments, it is completely impractical to incorporate pollution control equipment that could achieve the low emission levels experienced by DHC systems.

The potential environmental benefits of DHC systems attributable In part to the above differences between district systems and conventional systems, as well as to other features of district systems, are discussed in further detail in Section 3.3.

3.2.3 Thermal Energy Distribution

In district systems, the thermal energy medium, whether it be hot water, steam or cold water, is delivered to customers via a system of arterial and branch supply pipelines. Having exhausted its energy transfer potential to the user, the medium is then normally returned to the production plant via a return pipeline system. While hot or chilled water is pumped to the users and back to the generation plant(s) through the distribution piping network, steam is delivered to the users under its own pressure. Steam, having given up the usable portion of heat at the user's location, is typically pumped back to the thermal energy production source as condensate. (Note: When cooled, steam condenses to hot water.) In some cases, such as when steam is supplied to a plant to meet process needs, the user's process may dictate that the steam be discharged directly into the process, in which case condensate is not returned to the production source. In other instances, where pipeline installation and maintenance costs are excessive, condensate is not returned to the production facility for reuse, but is wasted. In either case, additional energy and chemicals are required to replace the heat energy that is lost when the condensate is not returned.

The piping used in the distribution network is typically buried although it can be supported above ground for industrial applications or run within building basements when owners and costs permit. Depending on the pipe size various common materials of construction can be used. To minimize thermal losses the pipes are normally insulated.

Four types of distribution piping systems are generally in use today:

There are no direct environmental benefits associated with DHC distribution systems when compared to a conventional or non-district system. In fact, because of the extensive burying of pipe that is required with a district system (a mostly non-existent requirement of a conventional system), there are disadvantages. These disadvantages include, during excavation for burying or pipe repair and maintenance when leaks develop in the distribution piping, potential for localized traffic congestion and tie-ups, and general inconvenience to pedestrians and motorists. These factors in most instances are outweighed by the potential benefits of a district heating and cooling system.

3.2.4 System Components at the User's Location

Basically, with DHC systems, the integration of the generation and end use thermal energy transfer functions can utilize indirect and/or direct connected distribution systems.

Direct systems do not have isolated subsystems. Rather, hot or chilled water, or steam from the production source, is distributed directly through the customer's radiators or air handling equipment.

Indirect systems, on the other hand, incorporate heat exchangers at both the energy production location and at the user's location, thus the generation, distribution, and energy utilization subsystems are effectively isolated from each other. Another indirect system option utilizes heat exchangers at the user end only, thereby isolating the generation and distribution systems from the user subsystem. This arrangement is common for steam generation and distribution systems.

In the case of a fully isolated indirect district heating system, hot water (or steam) can be produced by suitable means and circulated through a heat exchanger at the production facility where the hot water (or steam) transfers its heat to the hot water in the distribution network. The water in the distribution network, which has now been heated, is in turn circulated through the end user's heat exchangers where the hot water transfers its heat to the user's distribution system at the rate required to meet the various heating needs of the user. The water in the distribution network is then circulated back to the heat exchanger at the thermal energy production source where it is re-heated for continuing use. In fairly small systems (less than, say, 15 MW), or as noted previously, with steam generation and distribution, the heat exchanger between the production source and the distribution network is omitted. In the case of the latter, this enables users that require steam to be serviced directly from the district heating system while users that have hot water heating systems can utilize isolating heat exchangers.

The same basic principles described above apply for a district cooling applications.

Domestic Hot Water (DHW) is either generated independently from the district heating system, on the user's site, or is passed through a heat exchanger to acquire its heat in both direct and indirect DHC systems.

Although direct systems were at one time the more prevalent of the two systems, indirect systems are now becoming the preferred approach. This is due primarily to several inherent advantages associated with indirect systems including:

The thermal energy users in a DHC system can vary from individual householders to large complexes such as hospitals, hotels, blocks of offices, high rise buildings, manufacturing facilities, universities, etc. The equipment requirements for these various users, if they are considering retrofitting from a conventional system to a DHC system, are not substantially different than that for the conventional system, assuming the user's climate control systems are compatible with hot water or steam heating and cold water cooling (i.e. a conventional system utilizing electric heating or direct gas fired heating, and decentralized air conditioning units, for example, would require significant equipment upgrades if connected to a DHC system).

Two references which discuss the possibilities of retrofitting existing heating and cooling systems are the IEA publication, "Guidelines For Converting Building Heating Systems For Hot Water District Heating", publication No. 1990 R8 and the Washington State Energy Office's "District Heating Development Guide - Legal, Institutional and Marketing Issues".

Typically, facilities utilizing hot water radiators and/or fan coil units (suitable for hot and cold water) for conventional space heating and cooling are ideal candidates for DHC systems. Larger facilities utilizing direct gas make-tip air heaters can also be converted to DHC, although distribution system piping and heat exchangers have to be installed. Ideally, in retrofitting a conventional system to a DHC system, the only appreciable equipment changes required are in the boiler room. Here the "conventional" hot water or steam source, the boilers, is replaced with heat exchangers which tie-in the customer's piping network with the DHC distribution piping. The "conventional" cooling source, normally centrally located chillers producing cold water, is also replaced with heat exchangers which may or may not be the same units as exchanged for the boilers. The potential for utilizing a common heat exchanger depends on the system operating temperatures and the DHC pipe system used (two or four pipe). Cooling towers commonly used to affect heat rejection in the condenser loop of the chillers can be eliminated with conversion to district cooling.

Other important equipment items at the customer's location such as circulation pumps, control valves, the water treatment package, DHW storage tanks, metering devices, etc, are essentially common to both conventional and DHC systems, and are not associated with significant environmental impacts, either positive or negative. Thus, although their importance should not be underestimated, since they are critical to the proper operation of any heating and cooling system, these items will not be considered further with regard to environmental benefits.

The potential environmental benefits associated with this subsystem are closely tied to those of the thermal energy production subsystem described earlier. The benefits relate primarily to the limiting of the number of emission sources (boilers, make-up air heaters) and refrigerant use (chillers) installations in the community to a few efficiently run, well monitored thermal energy production plants. A detailed review of the potential environmental benefits associated with this subsystem compared to conventional heating and cooling of individual buildings is presented in Section 3.3.

| Potential Environmental Benefits Associated with DHC Systems |