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By Gerald Robinson
Best practice with district energy systems is quickly changing and evolving well past what was commonly thought of as an underground system of hot and chilled water lines feeding campus buildings from a centrally located plant of pumps, chillers and boilers. Goals of high reliability, resilience after major disasters, managing energy price volatility, "decarbonizing" energy supplies, reducing capital and operating costs, and high levels of energy and water efficiency are pushing the district energy system to new levels of performance and functionality. It is the district energy system that provides the integration platform and allows planners great flexibility in thinking holistically which make possible the achievement of these multifaceted enterprise goals in a campus setting. Often undervalued and overlooked, it is district energy systems that liberates the architectural design and planning process to achieve ground breaking visions as the often highly constraining (and costly) challenges around siting utilities at each building are resolved.
Applications
The new Stanford University district energy system (under construction) provides an excellent large-scale example of how multiple benefits can be achieved by exploiting the synergies between interrelated systems (Figure 1). The strategy for reducing greenhouse-gas emissions was to move from natural gas to electricity generated by 25% to 33% renewable energy. The green oval in Figure 1 highlights two scenarios (one with 25% and the other with 33% renewable energy) that achieve the simultaneous benefits of reduced emissions, increased efficiency, low life cycle cost, reasonable capital and operating costs and very low water use. This new plant has many of the features depicted in Figure 2, such as ground source heatpumps, onsite renewable energy, and high levels of waste heat capture.
Figure 1. Stanford Energy Systems Innovation - March 2015 Presentation "Systems Overview"
(source: Joseph Stagner)
In addition to reducing the capital costs of new construction projects, district energy infrastructure allows for flexibility in long-range planning and operations. District energy systems provide the flexibility in site planning needed to attain architectural and space planning objectives as each building no longer needs a cooling and heating plant and can be located remotely from energy sources needed. The infrastructure allows sources of energy (electricity, gas, waste heat) to be remote from the loads thereby allowing great flexibility in planning building locations and managing site environmental, noise and sight-line factors. District systems can be established as a campus-wide system or in a grouping of a smaller clusters of buildings which provides for flexibility in investing in the infrastructure.
Harnessing waste heat and capturing the full benefits of heat pumps (in this case used to raise the temperature of waste heat) are examples of strategies enabled by district systems. Waste heat recovered from combined heat and power systems, chillers, food service equipment and data centers offsets the use of hydrocarbons, eliminates water use in cooling towers (and associated chemicals and O&M costs), while increasing reliability and energy efficiency.
The ability to utilize sources such as biogas and reclaimed water located outside the property boundary becomes possible as the district system provides convenient points where sources can be injected into the system. This is important in interfacing with the region's electrical, natural gas, and waste water systems.
Figure 2. Depicting Interrelated Benefits in an Advanced District Energy System.
“Quick payback” present-day technologies
Designing low-temperature (less than 170F) and low-head pressure systems with advanced controls that enable setbacks on temperatures according to loads required to meet space conditioning needs is a very effective investment. Such systems minimize the use of expensive control valves with variable speed pumps placed strategically to ensure a balanced energy delivery to all locations where the supply is closely matched to the need/demand.
Such systems allow for the use of materials (piping and insulation) that are much more durable and less expensive than traditionally used copper, steel and iron piping. Examples of lower cost yet durable piping materials are polyethylene cross-linked (PEX), high density polyethylene (HDPE) and fiber piping. These new piping materials are flexible allowing for high survivability during earthquakes and are used with trenchless installations that have proven to be very valuable when running services through environmentally sensitive areas and or where existing buildings and hardscape make traditional digging, disruptive and expensive.
Lifecycle-Cost-Effective technology
Establishing an advanced microgrid as part of a district energy system provides high value of reduced downtime, disaster resilience along with savings from demand response, efficiency and onsite/adjacent renewable systems. A microgrid should be viewed as an essential element. From an operational and broader societal perspective, a large customer that can clearly demonstrate the value of an advanced microgrid.
Use of energy storage as a means to enable arbitrage between on- and off-peak rates and provide valuable flexibility in load management; compensating for intermittent renewables and demand response (DR) programs. Thermal storage can act in a manner analogous to electrical batteries; charging with excess or low cost power and discharging to offset expensive or scarce electricity. Thermal storage can be used to relieve the timing constraints associated with combined heat and power systems around electrical and waste heat production and use.
Emerging technologies and practices
While still in the early- to mid-development phases, vehicle to grid infrastructure (V2G) is a very germane and exciting technology (Tomic et al., 2007; Wang et al., 2014). The V2G technology value come from enhanced resiliency (a source of backup power in disasters) and from grid services such as micro-second load smoothing of large onsite solar PV systems and other power quality services (voltage, VAR and power factor). V2G technologies can also offer DR value in the form of curtailing charging. However, using vehicles as storage sources for the utility or microgrids is complicated by battery life considerations.
A growing portion of buildings loads can be classified as "natively direct current" meaning the end use components operate on direct current (DC). Lighting, data centers, desktop computers, monitors, telecom equipment and networks are all native DC appliances. Given that onsite renewable and storage systems produce and store DC power, at least a hybrid AC/DC microgrid is important to consider as power supplies can be eliminated (or simplified) thereby saving energy, eliminating most power quality issues, reducing points of failure. and lowering first costs. The move towards power over Ethernet (POE) and over new USB standards is illustrative of this trend towards DC power sources proliferating in buildings to meet DC loads. A main DC electrical bus may also be an ideal integrating (simple, low cost, robust) for onsite renewables, battery storage and native DC loads.
Economics
A financial analysis of district system investments should take into account the avoided cost savings accrued from not installing heating and chilling plants in each building and from the reduction in floor space dedicated to multiple utility rooms. Square footage and infrastructure (boilers, chillers, pumps, controls, fire life safety systems shop space) all represent very large first costs that can be saved altogether. In the presence of a district system, a building can "plug and play" with minimal investment in utility systems. With life-cycle cost analysis, two factors increase the financial attractiveness as these systems as efficiency improves with square footage served. System losses are spread out and greater diversity in loads increases the ability to operate equipment at peak efficiency.
Other considerations
With low- or no-cost design features built into district systems, the distribution of thermal and electrical energy can be attained with very high levels of reliability. For example, with "looped" distribution systems, repairs can be performed without the loss of services to individual buildings.
The need for redundant fire/life safety issues are reduced as generator sets and fuel storage, combustion appliances and fuel, pressure vessels, chillers, cooling towers and water treatment chemicals are located centrally verses at each building. This reduces the need for specialized fire suppression systems and alarms and exhaust systems to manage refrigeration leaks. The number of remote locations where facility engineers must work in is greatly reduced, decreasing labor costs and the opportunity for workplace accidents.
Building occupant comfort is improved as there is a reduction downtime, in noise and unsightly utility equipment located at each building. Compatibility with radiant heating and cooling within the building envelope is also aligned with enhanced comfort.
Institutional requirements & capacity
The costs to maintain the utility infrastructure are greatly reduced. Proactive and intensive preventative maintenance program are also easier to sustain when equipment is located centrally. Cost savings come from several sources. The number of trips taken by facilities engineers with the associated fleet vehicles (plus fuel and wear and tear) is greatly reduced as the majority of maintenance work shifts to central utility buildings. The costs and administrative effort to maintain repair parts and supplies inventories results in real savings with district systems. Each large building with its own heat, cooling and power plants represent a large commitment in man power, specialized service contractors and repair parts inventories.
References
Tomic, J., and W. Kempton 2007 "Using fleets of electric-drive vehicles grid support" Elsevier Press - Journal of Power Sources http://www.rmi.org/Content/Files/Fleetsforgridsupport.pdf
Wang, Y., O. Sheikh, B. Hu, C.C. Chu, R. Gadh 2014 "Integration of V2H/V2G Hybrid System for Demand Response in Distribution Network" Department of Mechanical and Aerospace Engineering - University of California, Los Angeles, pp. 5-6
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