Commissioning combined heat and power (CHP) systems in commercial and institutional buildings requires broad experience.
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Commissioning combined heat and power (CHP) systems in commercial and institutional buildings requires broad experience. This includes small power plant systems (less than 25 MW), heat recovery, thermally activated technologies, electric switchgear, grid interconnection operation and safety, sound and vibration, emissions control; and building, mechanical, and electrical systems integration. While CHP systems are common throughout industrial sites, accounting for about 84 GW of electric capacity in America, less than 1 GW of power is currently operating in the built environment.
The use of CHP systems in buildings is likely to increase as the need to reduce carbon emissions grows and public policy moves to monetize carbon emissions.
What is really different about CHP systems? Let’s break it down first by major components and then examine the integrated system.
Continuous duty drivers: CHP power systems consist of reciprocating engines generally less than 10 MW, microturbines of 65 to 250 kW, and combustion turbines of 1 to 15 MW. Fuel cells also are in use between 5 kW and 1.5 MW, but are generally quite expensive at this time.
Emissions: On-site combustion requires a firm understanding of the federal, state, and local air permit requirements. Air permits are essential before construction starts. Generally speaking, all of these power technologies are capable of being sited anywhere in the United States, with perhaps a few counties in California limiting the use of state-of-the-art reciprocating engines even with after-treatment. The principal question for emissions is generally a matter of cost, and not feasibility.
Generators: There are two principal classes of generators: induction and synchronous. An induction generator produces electrical power when its shaft is rotated faster than the synchronous frequency of the equivalent induction motor. An induction generator is not self-exciting, meaning it requires an external supply to produce a rotating magnetic flux. The external supply can be from the electrical grid or from the generator itself once it starts producing power. A synchronous generator is a machine that generates an alternating voltage when its armature or field is rotated by an engine or other means. The output frequency is exactly proportional to the speed at which the generator is driven. The functional purpose and interconnection issues will determine the generator design.
Interconnection: Grid interconnection requirements have certain common characteristics with respect to operations and safety, like compliance with IEEE Standard 1547. However, state and local utility requirements and grid characteristics (radial or network) will vary dramatically. This is an area where minimum commissioning usually is dictated by the utility and site commissioning issues are sometimes missed.
Waste heat recovery schemes: This covers the wide variety of means to recover waste heat from generators and/or processes for delivery to thermally activated technologies. The range of systems covers ducting and reclaiming heat from hot air sources (process, engine, turbine, microturbine exhausts) and recovering heat in the form of hot liquids (engine jacket water, oil cooling and exhaust, process streams).
Thermal technologies: The most common technologies are heat recovery heat exchangers, heat recovery steam generators, absorption chillers, desiccant dehumidifiers, steam turbines, and organic Rankine cycle (ORC) generators. Turning waste heat into hot water or steam generally is the simplest and most cost-effective. Absorption chillers can convert the waste heat to chilled water, but add another level of complexity and cost to the project. Absorbers are either provided as low-temperature single-stage or high-temperature two-stage machines. Desiccant dehumidifiers can be coupled to hot air streams in the 250 F range or hot water in the 190 F range. ORCs can absorb 400 to 600 F heat and provide electricity at 10% to 15% cycle efficiency.
CHP integration: CHP integration focuses on successfully integrating the power generation with the thermal heat recovery and thermally activated technologies. The effectiveness of this effort varies widely depending on the degree of pre-engineering and packaging. Retrofit systems require more flexibility and ability to balance the system in the field.
Building system integration: Integrating a CHP system to building loads and systems is critical and requires knowledge of the buildings’ operation (retrofits) or design intent (new buildings). Here, too, flexibility and ability to balance systems are essential.
There are more elements that need to be considered in applying and commissioning today’s CHP in buildings. Understanding the components and their integration requirements, along with having the flexibility and means to balance systems, is essential. Finally, having a commissioning plan that tests the system’s capabilities is essential.
Commissioning a CHP plant
A CHP system was installed at a deluxe 336-room hotel in downtown San Francisco. The CHP system is a predesigned standard product that contains four microturbines each rated at 60 kW of electrical power at 59 F at sea level conditions. Rated NOx emissions are less than 9 ppm at 15% exhaust oxygen, which met local emission requirements in force at the time of the installation. The exhaust from each microturbine is manifolded together to deliver input energy to a double-effect absorption chiller.
The lithium bromide/water chiller consists of an evaporator, absorber, condenser, high- and low-temperature generators, solution heat exchangers, refrigerant and solution pumps, purge, controls, and auxiliaries. The chiller is an adaptation of a direct-fired chiller that increases the heat transfer area of the first stage generator to compensate for the lower temperature inlet energy (microturbine exhaust gas).
Because it is a double-effect device, the chiller effectively converts the input thermal energy to chilled water and achieves a coefficient of performance of approximately 1.3. The double-effect feature also permits a manual changeover of the chiller to operate as either a chiller or heater. Thus, the CHP system can provide either space chilling or space heating.
The control system includes a diverter valve in the duct between the microturbines and the chiller. If the chilling demand is zero, this valve diverts the microturbine exhaust to the atmosphere. If a chilling demand exists, the diverter is positioned to deliver the energy required for the chiller to meet the demand. The ability to isolate the chiller under no-load situations is important to avoid excessive concentrations within the chiller and possible solution crystallization.
Fuel gas boosters (FGB) elevate the pressure of the natural gas fuel supplied by the gas utility to the level required by the microturbine. Each CHP system uses one FGB for a pair of microturbines. The FGB is powered by the dc power produced within one microturbine pair, and therefore that microturbine experiences a parasitic electrical load that diminishes its ac output.
Table 1 details the performance specifications of the CHP system at 95 F and at 59 F. The net power levels include power for the two FGB. As indicated, the combined electrical and chilling capability results in CHP efficiency greater than 80%. To achieve this level in an application, the full system output capacity must be used productively by the building.
Site and thermal integration
Based on historical data and analyses, the hotel energy demand averages 670 kW of electrical power and 1,200 kW of combined thermal energy use and power. The electrical demand during the year rarely dropped below 500 kW. Because of the hotel’s significant and persistent air conditioning demand throughout the year, the CHP system was integrated only with the chilled water loop (see Figure 1). The absorption chiller operates in parallel with two existing 300 refrigeration ton (RT) electric chillers (a primary unit and a spare).
However, the design chilled water flow rate was much higher for the electric chiller than for the absorption chiller. To accommodate the different flow rates and pressure drops, a bypass loop with motorized isolation valves was required to balance flow rates during different operating modes.
The “absorption chiller” mode (see Figure 1A) required that the motorized valves were positioned to allow returning chilled water to flow only through the absorber and the bypass loop. The chilled water flow rate setpoint through the absorber was 270 gpm measured by a flow meter at the absorber exit. The bypass loop had a similar flow rate. The “simultaneous chiller” mode (see Figure 1B) required the valve settings to allow flow through both chillers, but not through the bypass. When this occurred, the lower flow resistance of the electric chiller reduced the chilled water flow through the absorber to 170 gpm. The “electric chiller” mode (see Figure 1C) required the valve positions to isolate the absorption chiller and bypass loop. The chilled water flow rate through the active electric chiller was roughly 500 gpm.
The hotel connects to Pacific Gas & Electric Co.’s (PG&E) San Francisco network through multiple feeders to the site. The multiple supplies provide redundancy in the electricity supply, enhancing power reliability. However, they also require “network protectors” on each utility feeder on the customer side of the transformer. A network protector is a combination of a breaker and a reverse current protection relay to prevent the reverse flow of current onto a feeder that experiences a fault. The protectors’ purpose is to prevent the flow of electrical energy from one feeder back onto another feeder. The protectors are set to instantaneously detect the reversal and open the contactor, but that opening takes 5 to 25 sec and requires a manual reset.
When on-site power generation is installed at a site with a network supply, it may be possible for the site load to momentarily drop below the generator output, resulting in an export of electricity unless other preventive devices are used. This possibility is minimized by requiring a buffer between the generator and the normal load.
However, this measure does not guarantee that an export will never occur. If an export does occur, the network protector senses a reverse current and instantaneously begins to open. PG&E expressed concern that all network detectors might sense the reversal and begin to open, rendering the site without any grid-supplied electrical power and requiring time and cost to reset them. To avoid this situation, significant interconnection upgrades were required on this site (see Figure 2). The cost of the interconnect upgrades required by the utility totaled approximately $140,000.
Each microturbine uses advanced natural gas combustion technology to constrain NOx emissions &9 ppm at 15% exhaust oxygen; it is CARB 2003 certified. The exhaust from each microturbine is manifolded together and delivered as the input energy to a double-effect absorption chiller.
On Nov. 15, 2001, the California Air Resources Board adopted a regulation that established a distributed generation (DG) certification program as required by Senate Bill 1298 (chaptered September 2000). The DG certification program requires manufacturers of electrical generation technologies that are exempt from district permit requirements to certify their technologies to specific emission standards before they can be sold in California.
Commissioning of the CHP system was completed by Carrier under contract to UTC Power. The commissioning was performed according to a written protocol that provided guidelines for startup of both the microturbines and the absorption chiller. Following the protocol, a safety inspection, site evaluation, mechanical inspection, electrical inspection, and communications inspection were completed prior to startup. The effectiveness of the grid protection circuitry was confirmed. The microturbines were started and performance was verified. The chiller was evacuated of the nitrogen blanket that had been applied for shipping. The chiller was then started and the charge level was verified. Once commissioning was completed, the system was put into service. No formal report was generated with respect to the exact extent of the commissioning process.
Overall, the CHP system achieved an extremely high level of utility with minimal outages. The system produced at least 60 kW of net electrical power for 8,231 hours, or 94% of the year. Table 2 presents the monthly breakdown of operating, nonoperating, and data gap hours. For the year, data gaps represented 2.8% of the available run hours.
The CHP system was installed with the chiller in the hotel chilled water loop and parallel to existing electric chillers as described previously. Engineers observed that the output from the absorption chiller was very interactive with the electric chiller, particularly from May through mid-November. This interaction resulted in a shift between the CHP cooling-only mode (see Figure 1A) and the simultaneous cooling mode (Figure 1B) for approximately half of the days during this period, as evidenced by a high absorber chilled water flow rate for CHP cooling only mode and a lower flow rate for simultaneous mode. This binary situation is shown in Figure 3, with a reduced flow rate once every day from July 6 through July 18.
The switch from CHP cooling only mode to simultaneous cooling mode occurred whenever the absorber output alone could not satisfy the hotel demand for chilling. In this case, the absorber capacity could not suppress the chilled water temperature returning from the hotel (returning temperature) to the desired setpoint for the chilled water temperature required to cool the hotel. The chilled water temperature leaving the absorber (leaving temperature) could be used for mode control if a parallel chiller was not present. However, with the electric chiller, the returning temperature was a proper indicator that the absorber was not keeping up with the demand and that simultaneous mode should be initiated.
The general sequence when switching from CHP mode to simultaneous mode was:
The absorption chiller output satisfied the hotel demand as indicated by stable and acceptably low returning temperature.
As the hotel demand grew, the diverter valve closed to deliver increasing energy to the absorber and to try to maintain absorber leaving temperature. However, both return and leaving temperatures increased.
When the returning temperature exceeded a “high” setpoint, the motorized valves of Figure 1 activated and the electric chiller started to achieve the simultaneous mode.
The absorber chilled water flow rate dropped suddenly by 100 gpm. The lower demand on the absorber required the diverter valve to open to maintain the absorber leaving temperature setpoint even though the return temperature was high.
The hotel demand was not satisfied until the electric chiller output and the reduced absorber output stabilized the return temperature.
CHP cooling only mode was re-established only after the hotel demand reduced sufficiently to allow the returning temperature to drop below a “low” setpoint (5 F lower than the “high” setpoint to reduce mode cycling).
The commissioning process for this cooling and power system took place in the winter, which precluded meaningful interaction between the CHP absorption chiller and the hotel’s existing electric chillers as the cooling load was low. Thus, two critical design deficiencies were not uncovered.
The anticipated chilling level was based on both predictions and experience, which concluded that high levels of chilling were required every hour of the year. While electrical load was easily determined from utility electricity bills, it was not as easy to determine thermal loads, particularly chilling loads. Perhaps this design flaw suggests the need for a new assessment approach to measuring thermal system performance prior to building retrofits.
The second design flaw stems from the integration of the absorption chiller into the existing chilled water circuit, which led to a 100 gpm absorption chilled water drop when the electric chillers were engaged. This untimely reduction in flow dramatically reduces the CHP performance. This critical design/operating flaw provides an important planning lesson, that if timing and/or weather precludes certain testing, then allowance must be made to perform critical testing when the timing/weather is right.
The U.S. Dept. of Energy and Oak Ridge National Laboratory have provided continuing support for energy-efficient CHP systems, including the demonstration project described in this article and support for the CHP commissioning assessment. Timothy Wagner of United Technologies Research Center provided data and review for the case study.
|Rated performance at 95 F|
|Table 1: The performance specifications for the CHP system installed were provided by the National Accounts Energy Alliance report to Oak Ridge National Laboratory, Oak Ridge, Tenn.|
|Rated performance at 59 F|
|Table 2: This table provides monthly operating hours for the CHP system, non-operating hours, and missing data segments. This early generation CHP system demonstrated high system operation (94%) during its first year.|
Editor’s Note: This article is derived from a paper presented by the author at the 16th National Conference on Building Commissioning (NCBC), April 21 to 24. NCBC is owned and managed by PECI. For the original paper and others, visit www.peci.org/ncbc . Consulting-Specifying Engineer is the media sponsor for NCBC.
|Sweetser is president of EXERGY Partners Group, Herndon, Va., which he founded in 1998 as a consulting firm designed to capitalize on opportunities arising out of utility deregulation and global climate change in the energy and construction industry. Sweetser has spent 36 years commercializing advanced energy, refrigeration, and HVAC technology.|
Combined heat and power (CHP) systems applied to the built environment have created a series of new entrants supplying predesigned and engineered energy solutions, which are likely to grow in their usage. The CHP plants are more complex and require systematic approaches to commissioning. Current commissioning practices appear to be intuitive and not systematic. General themes arising out of this review are:
• A written commissioning report is essential to determine exactly what was tested and how the tests were accomplished
• All essential elements must be tested to ensure functional performance
• If timing and weather precludes performance testing of certain systems, arrangements should be made to perform these tests at a later date
• Site engineering is a vital ingredient particularly for retrofit CHP installations because as-built drawings of complete building systems are often inaccurate or incomplete
• In retrofit situations, series flow should be used on all CHP thermal loops to balance flow and eliminate pumping problems
• Balancing valves are essential to assure flows are correct.