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transcript
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MID-SIZED NEW GENERATION: RECIPROCATING INTERNAL
COMBUSTION ENGINES OR COMBUSTION TURBINE?
Presented to Power-Gen International 2017
Melanie J. Schmeida, P.E., Louis Perry Group, a CDM Smith Company
165 Smokerise Dr., Wadsworth, OH 44281,
mschmeida@louisperry.com, 330-760-0645
Abstract For any new natural gas-fired power generation project, a developer or owner must wrestle with
the question “what is the right technology?”. For very small projects, the answer often defaults
to reciprocating engines. For very large projects, it is combustion turbines in a combined cycle
configuration. But for the facilities in between, the right answer is not always so clear. This
paper compares reciprocating engines to simple cycle combustion turbines for a nominal 50 MW
gas-fired plant in the Midwest, connected to the electric grid. It evaluates capital costs, operating
costs, reliability, operational flexibility, system responsiveness to dispatch requirements, and site
considerations.
Background Inexpensive shale gas has resulted in an increased interest in natural gas-fired power generation
in many parts of the nation. The profusion of this new generation, its implications on the utility
and distributed generation markets, and project viability are topics of many publications. For the
purposes of this paper, it is sufficient to say new natural gas-fired electricity generation is
attractive for an owner in the Midwest, and evaluation of their needs indicates approximately 50-
MW electric generating capacity is the appropriate size. Additionally, the purpose of the facility
is electric generation only; no thermal energy in the form of steam or hot water is being
considered.
For all generating facilities, the best-fit technology needs to be evaluated carefully. Developers
and owners are making large investments, and need to consider many factors to ensure
appropriate returns on that investment. However, conventional wisdom would dictate that a
“small” natural gas-fired generating facility is best served by reciprocating internal combustion
engines (RICE), as it would be expected to operate intermittently, and that a “large” generating
facility is best served by a combined cycle system(s) as it would be expected to operate nearly
continuously. But what about this 50-MW facility, which is “mid-sized”? What is the
appropriate technology for this installation?
When this study was first contemplated, the primary technology options were intended to be
RICE, a simple cycle combustion turbine (CT), and a combined cycle system. However, we
quickly determined that the combined cycle arrangement was not going to be cost effective. It is
conceivable that a combined cycle plant might be the right choice for a mid-sized facility if the
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thermal energy can be used and/or the facility will run continuously, but with our premise that
the thermal energy has no value beyond additional electric generating capacity, the payback for
the additional capital expense was not reasonable. Therefore, this paper focuses on a comparison
between RICE and simple cycle CT for this application, contemplating the major questions of:
• How much should it cost?
• How will it be used?
• Where will it be located?
• How much will it actually cost?
It is also worth noting that, while this study utilizes a specific example site, the items evaluated
can be applied to any project.
How Much Should It Cost?
As a starting point in the evaluation, typical engineering, procurement, and construction (EPC)
costs for the technologies were evaluated to establish viability. Property costs were excluded, as
the site was already owned, as were permitting and other owner costs since those would be
similar regardless of the technology selected.
Figure 1: Published Estimated RICE and CT EPC Costs (see References)
Based on a sampling of published cost information, average EPC costs for RICE technology is
approximately $1100/kW, and $800/kW for CT. The sample selected was based on installations
in the 20-100MW size range, where such delineation was possible, and data points that appeared
to be outliers were discounted.
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Estimated EPC Costs
RICE
CT
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Similarly, typical O&M costs were evaluated for the two technologies. Fuel costs, which
represent the largest portion of overall operating costs, were excluded, as differences in those
costs can be accounted for in the differing efficiencies of the equipment. Apples-to-apples data
comparison for these costs proved more difficult, since the data can be represented in a variety of
ways.
The non-fuel O&M costs in Figure 2 address both fixed and variable costs for a typical
installation. For the most comparable data, over the expected unit life, the average annual O&M
cost for RICE was approximately $0.016/kWh and $0.007/kWh for CT.
Figure 2: Published Estimated RICE and CT Non-Fuel O&M Costs (see References)
Operating and maintenance costs for RICE include maintenance labor, engine parts and materials
such as oil filters, air filters, spark plugs, gaskets, valves, piston rings, and electronic
components, and consumables. The recommended service includes inspections/adjustments and
periodic replacement of engine oil and filters, coolant, and spark plugs every 500 to 2,000 hours.
A top-end overhaul is recommended between 8,000 and 30,000 operating hours, which includes
a cylinder head and turbocharger rebuild, and a major overhaul is performed after 30,000 to
72,000 operating hours, which involves piston/liner replacement, crankshaft inspection, bearings,
and seals.
For CTs, the maintenance requirements are less than RICE, and include labor for routine
inspections and procedures, and major overhauls. Generally, routine inspections are required
every 4,000 operating hours to ensure that the turbine vibration is within tolerance. A gas
turbine overhaul is needed every 50,000 to 60,000 operating hours, which includes a complete
inspection and rebuild of components to restore the gas turbine to nearly original performance.
Note that operating hours for CTs are not directly comparable to RICE operating hours, as virtual
hours are added to CTs for starts/stops and excessive load changes.
0.000
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Estimated O&M Costs
RICE
CT
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As shown, typical installed and non-fuel O&M costs are lower for CTs than RICE. The potential
advantage of a RICE facility comes into play when operating characteristics and usage
considerations are evaluated. Since maintenance costs for RICE installations do not increase
with cycling and multiple starts and stops of the equipment, effective O&M costs begin to
levelize between the technologies when employed in facilities that will experience this type of
operation.
How Will It Be Used?
As engineers, we often seek an optimized solution, a “best fit”. With this mindset, the intended
purpose of the generating facility can often drive the technology selection, since the technical
characteristics of the equipment inherently lend themselves to different applications. However,
careful consideration is still needed, and final selections are, of course, still rooted in economics.
These technologies can be used for a variety of purposes in generating facilities, such as peaking
generation, frequency stabilization and renewable generation support, to address reliability and
resiliency concerns, and for capacity sales. As part of the comparison for these uses, some of the
key differing technical features are shown in Table 1 below.
Table 1: Basic Technical Comparison RICE CT
Heat Rate (Btu/kWh) 7400-8200 8100-9200
Max Efficiency (%)
Full Load 48-50 40-43
50% Load 48-50 30-33
Footprint (ft2/kw) 0.28-0.38 0.02-0.08
Time to Full Load (min) 5 15
Ramp Rate (%/min) 100-130 20-50
Turndown (singe unit) 25% 30%
CHP Applications Hot Water/Steam Steam
Dual Fuel / Fuel Range Low BTU Low and High BTU
RICE heat rates are lower and efficiencies higher than CT, which results in lower fuel costs for
the same output. Since fuel is the single largest operating expense for a generating facility, this
is an important factor. Additionally, RICE efficiency remains steady throughout the load range,
whereas CT efficiency decreases at reduced loads. The load range is broader for RICE than CT,
both for a single unit, as well as for the total facility due to multiple smaller machines instead of
one larger machine.
Reciprocating engines are also able to start-up and reach full load capacity more quickly, and can
withstand dramatic changes in load and many starts and stops with minimal impacts to the
equipment and maintenance cycles. The ramp rate, both up and down, is substantially higher for
RICE than for CT. Although CTs can be cycled, excessive load changes and starts and stops
effectively adds operating hours, dramatically increasing maintenance costs.
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Based on these characteristics, either RICE or CT appears to be the better fit for certain
operational scenarios. When the hours of operation and load range are closer to intermediate
load than to a high-cycling type of operation, the lower capital and O&M costs for the CT
typically result in a higher return on investment, despite the lower efficiency. When the load
profile is more volatile, the lower fuel and O&M costs for the RICE typically results in a higher
return on investment, despite the higher installed cost.
Table 2: Operating Characteristics – Best Fit RICE CT
Load Profile Peaking to Intermediate Intermediate to Base
Starts/Stops Many Few
Capacity Factor Low High
Hours of Operation Low High
Operating Range Low Load Mid Load
Peaking Generation
For peaking applications, both RICE and CT can be viable options. Most of the literature
advocates RICE for its fast start capabilities and broader load range as a better match to changing
grid needs. Reciprocating engine facilities can reach full load within 3-5 minutes, and depending
on the number of units, can operate from 10-100% of total plant load, or even lower. As stated
above, they do not decrease in efficiency at reduced load operation, and can withstand many load
changes and starts and stops without penalizing maintenance costs.
When evaluating the cost implications of these attributes (reduced fuel and maintenance costs),
RICE may very well be superior. However, CTs can still be an attractive option for peaking
applications depending on the specific conditions. For example, many regional organizations
have excellent peak prediction tools. This information allows operators to make informed
decisions regarding start-up and run time for their CT plants, reducing concerns about response
time and cycling operation, as they can choose to respond only to longer duration peaks.
Frequency Stabilization and Renewable Support
Different from peaking applications, the use of generating facilities for frequency stabilization
requires fast response. This is most often needed to support the grid as a result of the increased
use of renewable generation, due to the non-synchronous generation of wind and solar power.
Wind and solar may account for 20% of installed power capacity by 2035, but only contributes
about 2% of firm capacity that can be relied on to generate at any given time. Other factors that
can lead to grid instability include fast variations in consumption, errors in forecasting, and
unexpected disturbances in capacity or loads. As a RICE facility can ramp quickly, it is the
rational choice if this is the goal of the facility.
Reliability and Resiliency
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Recent natural and man-made disasters have placed reliability and resiliency of our electric
power supply at the forefront of national discussion. Both RICE and CT facilities are highly
reliable, with up to 98% availability with proper maintenance; this equipment can be counted on
to operate when called upon. However, RICE does have some advantages in this area. For our
50-MW plant, a single CT would be employed, as that would be the most economical
installation. Since there is only a single unit, versus multiple RICE, the RICE installation has
inherent redundancy that the CT could not match. In the event one engine was out of service, the
remainder could still produce power. In the unlikely event emergency power was required
during a turbine rebuild/replacement, there would be no option for generation. Additionally,
RICE facilities can be used for black-start support, as they can be started without auxiliary
power. Combustion turbines require auxiliary power to start system components.
Capacity Sales
Some facilities exist for electricity sales to wholesale capacity markets. In this case, either
technology is well-suited for the application. Both technologies are completely dispatchable, so
they can be utilized when the price of electricity is advantageous for them to do so or when
called upon by a grid operator. However, some operators attempt to capture very short-term
price spikes, in which case RICE may have an advantage due to its faster response time.
Where Will It Be Located?
Every site is unique, and specific site attributes can have a major impact on the financial viability
of a project in general, and on the selection of the appropriate technology. In many cases, these
will override the well-established rules discussed above.
Footprint
As illustrated in Table 1, CT systems utilize approximately one-third to one-quarter of the area
needed for equivalent RICE generation. Additionally, CTs are relatively lighter weight and do
not require substantial support foundations, resulting in less site work overall. This difference in
footprint is accounted for in the installation cost of the project, including the typical EPC costs
referenced in this paper. However, beyond the common installation costs, this difference in
footprint can result in additional costs to the project. For a brownfield site, this may mean
additional demolition or remediation services are required. Or for a landlocked area, the expense
to purchase additional land could make selection of RICE prohibitively expensive.
Ambient Conditions
Reciprocating engine performance is impacted very little by changes to the incoming air
conditions, therefore air pressure reductions at high altitude (up to 3,000-ft above sea level or
more) and large ambient temperature ranges (up to 100 °F) do not significantly affect operations.
Conversely, CT performance may degrade as much as 10-15% from ISO conditions for the same
range due to incoming air properties. High altitude installations need to adjust heat
rate/efficiencies in their performance model to properly represent the expected output. To
combat the degraded performance for CT at high air temperatures, an inlet air cooler is often
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installed. This results in improved efficiency of the CT, but requires additional capital
expenditure, and operating expense in the form of water usage. Either technology can be
effectively utilized, but RICE has the advantage of maintaining base performance.
Natural Gas Pressure
Combustion turbines require much higher inlet gas pressure than RICE, 300-600 psig vs 75-150
psig. If the site has access to a high pressure natural gas line, this may not be of much concern.
However, most owners do not have such luxury, and therefore will need to install gas
compressors for a CT installation. These compressors are noteworthy pieces of equipment in
their own right, with significant capital and O&M expenditures required.
Noise
Both technologies will generate far-field noise when in operation, so proximity to receptors will
be a concern regardless of selection. Typically, specifically engineered sound enclosures and/or
buildings will be sufficient; however, RICE tend to generate higher frequency noise that is more
difficult to control than the lower frequencies produced by CTs. If the site is in an area with
sensitive receptors, additional sound mitigation measures may be required, resulting in increased
capital costs for the RICE.
Emissions
Both technologies are efficient combustors and have low resulting emissions, and both can be
outfitted with selective catalytic reduction (SCR) systems for NOx and CO control. This is an
area where the fast start and response time of RICE can be a detriment, as the emissions control
equipment does not respond as quickly. During start-up or fast ramping, emissions levels may
fluctuate, causing temporary spikes. Average emissions limits are not likely to be a concern in
most parts of the U.S., however, permits need to be reviewed carefully for instantaneous or peak
allowable emissions levels. Restrictions on instantaneous levels may restrict operational
flexibility, resulting in loss of function that impacts the project pro forma.
Water Availability
As noted above, for high ambient temperature installations, CTs will often be outfitted with an
inlet air cooler, which will require high purity water. Some CT models also require water
injection for cooling and emissions controls. If water scarcity, or the cost of demineralized
water, is a concern at a site, the resulting operating costs may favor RICE. Reciprocating
engines require an external cooling circuit, but typically utilize a closed-loop system with
minimal make-up water needs.
Future Expansion
Both CT and RICE equipment can be supplied as modular units, which can reduce installation
costs by shifting labor from the field to the shop. Additional units can be added on site as a path
to expand generation capacity in the future. Due to the smaller size of the RICE units, it is far
more practical to incrementally expand capacity by adding one engine at a time than it is for CT.
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If incremental expansion is a possibility for a facility, RICE will permit that expansion, whereas
additional CTs will result in major step changes.
Unique Site Considerations
The list of potential site considerations is nearly inexhaustible. There are many unique features
to any location that could impact cost and/or technology selection. In many cases, the outcome
will be the same regardless of the technology selected, but consideration is still warranted. Some
items to evaluate include:
• Does the site share utilities with other facilities? What is the impact of installing new
generation capacity on these utilities. For example, will the natural gas consumption
restrict capacity or impact pressure for the other users? Will a substation connection or
upgrade impact operations?
• For a re-development site, are there opportunities to re-use existing infrastructure, such as
electrical distribution equipment, water or compressed air systems, buildings, etc. to
reduce capital costs?
• Is there the potential for unknown subsurface conditions, contaminated soil, hazardous
materials, or other similar brownfield issues? In cases like these, the smaller footprint of
the CT could result in significant savings over the RICE.
• Air permit considerations were noted above, but are there other permitting concerns that
could impact the installation? Siting and connection permits can be just as challenging as
air permits.
• Does the owner or community have aesthetic concerns or preferences to incorporate?
• Does the installation need to consider future development in the area?
Example Facility
How do the criteria above play out for our 50-MW example facility in the Midwest?
How Much Should It Cost?
As noted in the background, new natural gas-fired electricity generation is attractive for an
owner, who intends to generate electricity only; no thermal energy use is being considered. The
rough pro forma indicated a breakeven EPC cost of $1100/kW, dependent on the actual
estimated O&M expenses. This alone leaves either RICE or CT squarely in contention.
How Will It Be Used?
Like most installations, the facility is intended to address many needs. Its primary purpose is
peak shaving, where the owner feels they can save their customers money by avoiding utility
peak rates. It is also viewed as a resiliency addition, as many customers in the area are served by
a single utility feed; if the primary line goes out this generation can serve as back-up for those
users. Also, if electricity prices increase in future PJM capacity auctions, this operator may
choose to sell into the open market and take further advantage of their investment. Again, this
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blend of needs leaves RICE and CT both as viable options, although many would argue that
RICE would be the better option for a peaking application as well as for redundancy.
Where Will It Be Located?
The site is an existing electric generating facility that has been decommissioned, but the building
and some equipment remains. Figure 3 below shows an edited aerial view of the example site.
Some of the typical site considerations discussed above do not heavily influence the technology
selection for this site. The location is in the Midwest, so altitude or extreme ambient temperature
effects are generally negligible. There is an existing gas line to the property at approximately
150-psig operating pressure. Therefore, a gas compressor will be required for CT, which will be
accounted for in the EPC and O&M cost estimates; the owner has no concern with installing or
operating the compressors. Water is available, and in fact an existing demineralized water
system is still functional. There are no specific permitting concerns for either technology.
Again, clear drivers towards one technology or the other have not presented themselves,
although the added expense of the gas compression may slightly favor RICE.
Figure 3: Example Site for New Generation Installation
At this point, the paths start to diverge. The site is large, and has ample clear space. As shown
in Figures 4 and 5 below, it appears that the footprint for CT or RICE can be accommodated.
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Figure 4: Example Site CT Footprint
Figure 5A&B: Example Site RICE Footprint Options
What’s not clear upon first glance are the unique site considerations. As seen in Figure 3, there
are currently residences across the street from this facility, and there are plans to modify the
same area as a recreational/entertainment district in the future. Therefore, the community has
strong preferences to maintain the vintage appearance of the old boiler house, and keep any new
equipment out of view from the road. They are also dictating noise restrictions at the road.
These restrictions rule out the RICE A arrangement without erecting a barrier wall or upgraded
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building walls to create an aesthetically pleasing façade and provide additional sound
attenuation.
This is an old site, that has had equipment added and removed over its lifetime. The potential to
encounter unknown subsurface utilities and structures is high, so a smaller footprint presents less
risk. Specifically, regarding the RICE B arrangement, in the half of the clear area near the
neighboring building, there are groundwater remediation and monitoring wells for a nearby site.
Obtaining approval and relocating these wells to accommodate the RICE B arrangement would
be a costly endeavor.
In addition to the demineralized water system already mentioned, the existing stack shown is in
good condition for re-use, as is the compressed air system, and some electrical distribution gear.
The differentiator is the stack; the single CT could possibly utilize the stack, whereas multiple
RICE cannot.
The owners of the proposed generation facility also prefer to leave space for additional capacity.
There is space for another CT unit, but increasing the size of the RICE facility would only
exacerbate the aesthetic, noise, and subsurface situations.
How Much Will It Actually Cost?
Typical EPC and O&M costs were presented at the beginning of this paper. While average
numbers are good to use for screening purposes, as shown in Figures 1 and 2, the actual figures
can vary widely. EPC costs for RICE varied from $700/kW to $1700/kW, and from $400/kW to
$1100/kW for CT. Non-fuel O&M costs varied from $0.007/kWh to $0.025/kWh for RICE and
$0.004/kWh to $0.015/kWh for CT. Based on the factors presented here regarding facility use
and location, the reader can gain appreciation for why that variation exists.
For our example project, the system’s essential purpose and expected usage would tend to favor
RICE. In a vacuum, that’s likely what the owner would choose to deploy. But footprint, noise,
future expansion, and other unique site considerations favor CT. Fortunately, the financial
implications of those site factors could be evaluated to select the technology best suited for the
project overall.
The EPC cost for the CT installation is approximately $850/kW and the cost for the RICE
installation is approximately $1250/kW. In this case, the financial models showed that CT was
the preferred choice. Over the lifecycle of the facility, the additional capital associated with site
modifications for the RICE installation was costlier than the lower efficiency and O&M penalties
associated with less than ideal operation of the CT.
The owner evaluated changing the plant size to see if the financial model would favor RICE at
another output. As the facility decreased in size, the differential did close. However, concerns
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then arose regarding the ability to meet peak load requirements. As the facility increased in size
up to approximately 100MW, the preferred technology remained CT.
Conclusion
For a mid-sized generating facility, approximately 50-MW, either RICE or CT technology can be
the “right” choice depending on the specific attributes of the project. Conventional wisdom
exists for a reason, and often points to the best fit solution. However, like our example facility,
care needs to be taken to account for many competing factors before making a final selection,
some of which have been discussed in this paper, and others that may be completely unique to an
owner/developer or to a specific site. With proper diligence, the proper selection emerges.
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