District Energy - 2012 1st Quarter

•;hot;or;chilled-water;coils;in;air-handling;units,;fan;coil;units,;unit heaters, reheat coils or other terminal units;

•;baseboard;fin-tube;radiant;heating;;and •;radiant;floor;heating.

Water loop heat pumps have limited use because the heat pumps are electrically driven and would only require additional heating or cooling (from the district systems) during peak periods. Other examples of equipment that is not compatible include •;direct;gas-fired;rooftop;air-handling;units; •;electric;baseboard;heating; •;instantaneous;direct-fired;gas;domestic;water;heaters;;and •;direct-expansion;cooling,;window;air;conditioners,;mini-splits,;etc.

The ETS room or area should be located at the ground or basement level and have adequate lighting, ventilation, floor drains, a domestic water connection and dedicated power. An ETS may be direct (in which the district water is piped directly into the building system) or indirect (in which heat exchangers are used to transfer thermal energy between the district loop and the building loop, as illustrated in figure 1). Determination of the ETS type is based on a range of considerations but it is prudent to allocate adequate space for indirect connections should that be the system of choice in the future. Most district heating systems use indirect connections. In district cooling systems, a direct connection is most common, but this can vary depending on hydraulic considerations.

Design Criteria

Supply and return temperatures are key design elements that must be addressed in DE-R guidelines. The district energy provider is responsible for producing the supply water temperature on the customer side of the ETS. The customer is generally responsible for achieving the design return water temperature or better in the building at all load conditions without compromising building comfort or equipment capacity.

Commercially, district energy tariffs are likely to include a rate incentive for achieving appropriate return water temperatures – thereby encouraging proper design selections and building system operation – as poor return water temperatures may lead to higher capital and operating costs. Therefore, anticipating these incentives, developers should look to passing return water temperature performance requirements through to their architects, engineers, contractors and commissioning agents.

Supply temperature is important to sizing HVAC terminal devices such as air-handling units, fan coils, etc. Return temperature is likewise important to equipment sizing, but it also defines the system delta T (the difference between supply and return temperature). A higher delta T means less water must be pumped to deliver the same energy. Obtaining appropriate return temperatures is the key to maximizing energy efficiency and minimizing capital costs.

The hydronic systems should be designed for variable-volume
flow operation, preferably with variable-speed pumps to minimize
pumping energy. Control valves (terminal units and zone valves)
should be two-way modulating type (or on-off for fan coil units).
Three-way valves must not be used because they allow flow to
bypass the heating elements, thereby degrading delta T. Pressure-
independent control valves are very beneficial in achieving high
delta T because they are properly sized with the design flow rate
alone, independent of the differential pressure in the location
where it is applied. Real-time fluctuations in pressure have no
impact on the flow rate or valve performance, and there are no
balancing requirements.

How Can Local Governments Encourage
DE-R Design?

The ability of a local government to encourage (or even require) DE-R design varies widely based on legal and policy constraints. In some European and Asian countries (e.g., Denmark and South Korea), the local governments have the ability to mandate certain types of energy utility service (natural gas or district heat) in a given zone.

In Canada, both law and cultural factors make it easier for local governments to actively facilitate the use of district energy. For example, the city of North Vancouver, B.C., established a Hydronic Heat Energy Service Bylaw to create a district heating service area for a neighborhood called Lower Lonsdale, and it created a city-owned corporation, Lonsdale Energy Corp. (LEC), to develop a hot water district heating system there. Further, the bylaw required that all new or retrofitted buildings greater than 1,000 sq m (about 10,800 sq ft) be connected to and use a district heating system. LEC has provided district heating services since 2004. Hot water is produced in multiple small plants.

The key performance requirements for buildings connected
to LEC include the following:
•;Hydronic;or;hot;water;heat;must;be;used;in;the;building;heating
systems.

•;Both;the;space;heating;and;domestic;hot;water;systems;must;be heated by the LEC system.

•;The;system;must;be;designed;with;a;high;temperature;differen-tial to ensure that all building heating and domestic hot water needs can be met and that high efficiencies are realized.

•;There;is;to;be;a;maximum;return-water;temperature;of;43;C;(110;F) under all load conditions on the building side of the system. (The LEC system is designed to provide 77 C [170 F] to the customer side of the heat exchanger for building heating purposes.)