Over the past few years, the momentum behind renewable energy has accelerated considerably. To ensure supply and demand match, system operators need controllable resources with ramping flexibility and the ability to start and stop multiple times per day as dictated by real-time grid conditions. Both over supply and under supply must be mitigated by making sufficient flexible controllable capacity available.

Over supply happens when all anticipated generation, including renewables, exceeds the real-time demand. It requires manual intervention of the market to maintain reliability. During oversupply times, wholesale price can be very low and even go negative (i.e., generators paying utilities to take the energy). Under supply results in grid frequency drop, system failures and potential black-outs or services interruptions. Certain grids are already facing these problems (Figure).

Natural gas-fired generation has an important role to play in supporting a flexible, cost-effective, environmentally friendly power grid. Aeroderivative GTs and reciprocating internal combustion engines (RICE) are the two most commonly employed simple cycle power generation options available to system operators to cope with the challenges posed by the high content of renewable energy.

Larger RICE units have power outputs ranging from about 10 MW to 20 MW. Therefore, large power plant specifications must be met via multiple units with consequences, such as economies of scale and site infrastructure costs.

As an example, GTs up to 100 MW typically operate within their own weather proof enclosure. Larger RICE units, on the other hand, require a building. In addition, an array of fin-fan coolers is needed to cool RICE jacket water. This adds to the cost of buying and operating the equipment as well as on-site infrastructure work. The size of the required building and infrastructure depends on the power density of the generating equipment; both from a MW/kg viewpoint driving the type of foundation and MW/m2 driving the footprint and land cost of the plant.

Aeroderivatives come out ahead on power density. Note also that recip pistons generate low frequency vibrations that can necessitate anti-vibration mounts, isolating pads, and sometimes floating concrete foundations. A 20 MW RICE will typically require a 1.5m concrete raft per engine. An aeroderivative will need around 4.5 times less concrete. For a RICE plant, however, the engine hall only represents about one third of the equipment footprint. Another third is occupied by the silencer and emission treatment system and the last third by the secondary water system array of finfan coolers.

A modern natural gas-fired plant might have a minimum of one daily start for the afternoon demand surge and one shutdown either at the end of the day when this peak disappears (peaking application, approx. 1,700 h/ year), or the next day when by the end of the morning the renewable surge drives the grid towards an over generation risk (mid merit, approx. 5,300 h/year). The number of daily starts could be increased further if the plant is used to balance the slight increase in demand in early morning before renewables capacity fully kicks in (2 starts); or if it is used to offset the sharp variations that could occur during the day and not because of the scale. Under these conditions, wear and tear of equipment will mostly be caused by fired hours for RICE units and aeroderivative gas turbines.

Thermal stress levels are more likely to impact ramp rates and cycling capabilities. The architecture and materials used in aeroderivatives are intended for multiple starts, stops, and sharp power spikes without maintenance or life penalty. RICE units show the same resilience to cycling operations. Time to start and reach full power also needs to be considered within the start cost through fuel burnt cost: starting time could range from two or three minutes for the 10 MW class of RICE under “readiness to start” conditions. For 20 MW-class RICE, this rises from seven to 18 minutes. This compares to five- to 10-minute start times for large aeroderivatives.

The cost of keeping units and plants in “starting mode” also should be considered. GTs usually start from cold iron. Only trace heating is employed to maintain fluids above freezing temperature as well as mandatory enclosure ventilation. To meet fast start conditions, RICE jacket cooling water and lubricating oil are preheated and maintained above a minimum temperature (typically 70°C for the cooling water), engine bearings are continuously pre-lubricated, and the engine is slow turning (typically driven by an electrical motor).

Preheat to start

Large RICE units typically require some form of preheating to start after 12 hours of down time, but it could be required as early as 1 hours after shutdown to retain faststart capability. In an environment, such as California, RICE units could require 16 hours or more per day of heating, lubricating and turning to retain their fast start capability with energy consumption exceeding 5 MW for a 300 MW plant. For an aeroderivative plant, the same conditions could require less than 0.9 MW of energy consumption.

Siemens SGT-A65

Overhaul and maintenance

Overhaul and maintenance strategies are different for RICE and aeroderivative gas turbines. Aeroderivatives have long intervals between maintenance inspections based on running hours, typically around 25,000 fired hours before the “hot section” inspection and basic maintenance. At 50,000 fired hours, a more complete overhaul is typically performed. This pattern generally repeats at 75,000 and 100,000 hours. The latest equipment tends to extend these period intervals toward 36,000 hours. In addition, borescope inspections are typically performed every 12-to-18 months without the need for disassembly.

Downtime can be decreased through a core exchange program offered by the OEM, or through the installation of a leased engine into the package to restore full operations within 48 hours. RICE units will have more frequent and longer maintenance events. Piston rings, cylinder liner and head, and inlet and exhaust valves will typically have repair intervals ranging between 16,000 to 24,000 hours. Some of these components have similar replacement intervals, depending on the fuel and operating conditions, or up to 60,000 to 100,000 hours for pistons and cylinder liner and heads.

The sheer size of large RICE unit (over 250 tonnes for the engine, 1.2 tonnes for a cylinder head) will require regular in situ maintenance operations to be planned and staffed accordingly, with cranage within the building and free space above and around the units for maintenance access. As regular maintenance is based on fired hours, RICE overhaul costs are quoted per MWh; around $8.5/MWh for 10 MW class engine and $6.5/ MWh for the 20 MW one. This is based on number of operating hours at full load. Part load operation means higher maintenance costs per MWh for RICE. Gas turbines will typically benefit from extended operations hours or reduction of maintenance for part load operation.

Efficiency is an area where large RICE units have an advantage. RICE range from 45% to 48% for simple cycle efficiency for large units (at the generator terminals). Large aeroderivative turbines have a simple cycle efficiency of around 43% in some cases. However, RICE units require higher auxiliary loads than aeroderivatives to drive lubricating oil pumps, heating ventilation and air conditioning (HVAC) for the engine hall, as well as cooling water pumps and fin-fan cooler motors. These loads result in about a 2% efficiency penalty.

In addition, the combustion tuning of large RICE units to combat emissions has an efficiency penalty of around 1.5%. Lube oil consumption RICE units generally consume lube oil at a rate of 0.5kg/ MWh. Regular oil changes or continuous conditioning of the oil is required. Oil consumption in aeroderivatives is negligible and the oil can sometimes last for the life of the machine.

These factors should be taken into consideration in any analysis of running costs of RICE versus aeroderivatives, as RICE high maintenance cost and oil consumption and disposal costs can cancel-out high efficiency fuel cost savings. Operational flexibility The capacity for the plant to generate revenue in different market environments should always be included in technology selection. These can range from energy-only markets with real-time, day-ahead, and ancillary service markets to capacity markets or operating reserve markets.

Flexibility demands characteristics, such as the ability to rapidly ramp up and down, fast start, low load operation, reliability, accurate forecast operating capability and high availability. Based on the factors noted above, the demands of the marketplace largely favor the aeroderivative. The ramp rates, load acceptance and shedding characteristics of aeroderivatives allow operators to balance the grid with up to 4 MW/s per unit when under-supplied grid sees frequency drop and a fast supply is required to prevent system collapse.

This also allows operators to switch on/off large equipment as up to 20 MW instantaneous load can be accepted by a unit idling at 0 MW. Some 35 MW can be accepted by a unit at 25 MW (i.e., over 2,100 MW/min). Alternatively, a unit at 50 MW can instantly shed the full load (i.e., over 3,000 MW/min) and stabilize at synchronous speed at 0 MW without tripping, ready to respond to new demand.

The starting flexibility of aeroderivatives also allows operators to optimize dispatch at the plant level. They can either maximize part load efficiency by switching units on/off to match demand or share load between several units to maximize the spinning reserve in a capacity market. As a result, large aeroderivative gas turbines can participate in all revenue schemes for which large RICE units qualify. They can outperform RICE units in terms of start capability, time to baseload, ramp rate, and load acceptance and shedding. The inertia of the power train is another factor to be taken into account when considering grid frequency stabilization.

The rotary movement of GTs and the mass of the larger electrical generator provides greater grid stabilization benefits than a battery or smaller RICE unit. This is even more important when considering that renewable energy with low or zero inertia displaces traditional power generation equipment with large inertia.Take the case of a 50 MW step in a 280 MW small grid. This might be due to a single point failure, such as an electrical transformer failure (corresponding to a single GT or three large RICE units connected to a large transformer and breaker) in island mode. The large frequency excursion in the RICE supported grid risks damaging the driven electrical equipment. It may also cause trips and upset grid stability.

Large aeroderivatives retain more grid stabilizing inertia while displaying higher ramp rates per unit and similar cycling capability than RICE units. Environmental impact Although RICE units and GTs may often be used for the same power generation application, due to the different technologies, different regulations typically apply. Despite being compliant with mandatory applicable regulations, RICE units tend to be more polluting. They also require the purchase and upkeep of exhaust catalysts and scrubbers (Figure).

Additionally, RICE units will burn lube oil on the cylinder walls (about 0.5g/kWh) which will result in emissions of approximately 9mg/Nm3 SOx on gas fuel, typically not treated by emission abatement processes. Beyond NOx and CO, the most striking difference is unburned hydrocarbons, which would include what is referred to as “methane slip”.

Due to an explosive combustion cycle, RICE units will have unburned hydrocarbons escaping through the exhaust. This amounts to between 3 g to 6 g/kWh at 100% load and from 13g up to 40 g/kWh at 25% load. A GT will typically emit UHC over two orders of magnitude less than a full load RICE unit. Methane, which degrades slowly over time when released into the atmosphere, is 84 times more potent a Green House Gas (GHG) compared to CO2 over a 20-year horizon, and as a result ‘methane slip’ negates RICE environmental benefit of higher efficiency and less fossil fuel burn.

Aeroderivatives, then, offer a flexible, fast-responding, and economical solution to the challenges facing the electricity markets faced with a growing penetration of renewable power generation. They offer superior power density, transient response to grid balance requirements and economics, availability, in addition to far lower impact from exhaust emissions.

Author: Vincent Perez is Advanced Projects Manager for gas turbines in Siemens Canada. For more information visit