Hybrid Plant Technology

For

Distributed Power Generation

Dr. Michael Nakhamkin and Boris Potashnik, ESPC, Inc.

Ronald C. Hall and Dale T. Bradshaw, Tennessee Valley Authority

Dr. Robert Schainker, Electric Power Research Institute

Robert Moritz, Rolls Royce Allison

Ronald Wolk, Wolk Integrated Technical Services

Introduction

Based on number prognostications by leading authorities, electric power will be the fastest growing source of end-use energy throughout the world over the next two decades. Worldwide electricity usage is projected to grow to 23 trillion kWh in 2020, nearly doubling present demand. Bechtel Power estimates that the annual worldwide demand for distributed power generation (DG), including the combined heat and power generation (CHP) systems, will be on the order of 30 to 40 gigawatts over the next five years. Westinghouse estimates that 40 percent of all worldwide capacity addition from 1998 to 2008 will be DG of some form (Reference 1).

In the bulk of the power market, distributed energy technologies will find it difficult to compete with the nearly 60-percent-efficient combined-cycle plants as the design of choice for a power generation company, at least in the near or medium term. It should be expected, that the cost of electricity ($/kWh) produced at the place where power is generated by a small capacity DG plant, (which is less efficient and more expensive), should be higher compared to that produced by large power plants. DG plants are helped in this competition with large central station plants by various charges associated with the delivery of power from those plants such as a capacity charge ($/kW) for the power availability, transmission losses, distribution fees, etc. Also, sometimes DG economics are improved by the deferral of the construction of transmission lines.

DG plant economics are affected by the fact that a single power plant must typically meet a whole range of specific operating requirements, associated with power supply to a limited number of local customers. Specifics of a DG plant operation could be summarized as follows:

A single power generation unit performs :

• base and intermediate load continuous operation
• peak load operation of a short duration.

Due to a singular consumer or limited number of consumers:

• peak/base load ratio is significantly higher (and could be over 200%), than for large utility plants with a variety of customers (typically 130%)
• peak load is concentrated during fewer hours/day (1-2 hrs/day)
• typical operation is load-following with a significant emphasis on part loads

As a result, a typical DG plant is be sized to meet maximum (peak) power demands, but operates most of the time at 50% loads or lower with significantly reduced efficiency.

Currently available alternatives for small capacity DG plants are as follows:

• combustion turbines (4MW - 10 MW) with approximate efficiency of 29-35%, specific capital costs of approximately $550- $600/kW and prohibitively inefficient part-load performance
• internal combustion engines with approximately 40% efficiency, high O&M costs and inefficient part load operations
• combined cycle plants, which typically have capacities greater than 30 MW, efficiency of approximately 40%, but very high costs due to small sizes (approximately $900/kW)

This paper presents the novel Hybrid Concept (HC), which allows a single power plant to meet, in the most cost-effective manner, power supply requirements of a DG plant. This paper will concentrate on the explanations of the HC principles, description and availability of major components- CT-derivative plants and the compressed air storage vessels, typical HC operations, installed costs and economics.

The Hybrid Concept

The Hybrid Concept (the U.S. Patent #5,778,675) was invented by Dr. M. Nakhamkin of ESPC, with patent rights assigned to EPRI in accordance with a separate patent rights sharing agreement.

Figure 1a. Hybrid Concept Schematic

The basic HC plant consists of four major components and systems as shown in Figure 1a:

• A combustion turbine (CT-) derivative power plant (simple cycle combustion turbine in this example)
• Booster compressor with intercooler and aftercooler
• Compressed Air Storage (CAS) -either above ground in pressure vessels or pipelines or in underground geological formations
• Piping connections and control valves to integrate the components

The major HC principles and the equipment selection specifics are as follows:

• The CT-derivative power plant shall be sized to meet the base load power requirements and, therefore, it operates most of the time with close to design point efficiency. The selection of the CT-derivative plant type should be based on the DG plant operations and economics. In remote areas, where simplicity and full automation are dominant considerations, a simple cycle CT is the natural choice. But for locations with high projected fuel costs and significant base-load operating hours, consideration should be given to more sophisticated and efficient CT–derivative plants, such as a combined cycle (CC) plant, Cascaded Humidified Advanced Turbine (CHAT) plant (Reference 2), CT with steam injection, etc.

• When a power output above base load is required, the compressor discharge airflow is supplemented by the compressed air flow from the storage. The maximum flow through the expander of the CT (typically achieved during operations at the lowest ambient temperatures) for the HC application could be limited by a number of considerations, with the compressor surge being the most restrictive one. For the Rolls Royce Allison (RRA) CT described later in this paper, the use of the supplemental air flow allows increasing the CT power by approximately 65%. This power increase is even higher for CTs with inlet guide vanes (IGV). The closed IGVs reduce the power consumption by the compressor and more power of the expander will be transferred for electricity generation.

• A CT-derivative plant could generate maximum power to the grid, if a provision could be made to separate the compressor from the expander. For a typical CT, the expander power is approximately triple the combustion turbine net power. As it was mentioned above, peak/base load power ratio is particularly high for DG plants.

• The compressed air storage volume and parameters are determined based on the storage of sufficient compressed air mass to support operations requiring supplementary compressed air - the intermediate-peak and maximum load operations during specified hours/day. For small capacities (up to 15-30 MW) the air could be economically stored in pressure vessels or high-pressure pipes. For larger capacities, the compressed air could be stored in underground geological formations (if available), similar to the typical storage of NG and the compressed air for CAES plants.

The alternative basic HC plant shown in Figure 1b differentiates from Figure 1a concept by the fact that the stored air is humidified before being injected into the CT. This concept (denoted as HCH) has two new components: the saturator for the air humidification with the hot water and the heat recovery unit for the water heating for the humidification process. The major idea behind this concept is that for the same peak power, the mass of the air stored could be significantly reduced due to humidification, thus reducing the storage size and costs, which is particularly critical for the above-ground storage. Also, the boost compressor size and costs are reduced. This concept is not particularly cost-sensitive to long hours of the storage.

Combustion Turbine (or CT - derivative plants) for the Hybrid Concept

The combustion turbine for the HC (HCH) application differs from any standard turbine by the following:

• It should allow an injection of the full or a fraction of the supplementary compressed air flow from the storage into the turbine, upstream of combustors to produce the peak power higher than the base load;
• It should allow for, the storage recharging, the extracting of the compressor discharge flow (full flow or a fraction of it) and sending it to the storage;
• For the maximum peak power generation, it should have a provision to separate the compressor from the expander with use of some kind of automatic clutch.

The Rolls Royce Allison Company’s (RRA) KM7 combustion turbine model could be used practically without changes for the HC plants. RRA converted the KB7 into the KM7 model for a biomass application project, sponsored by DOE. The KM7 has provisions to take out the compressor discharge flow for external heating in a biomass burning FBC plant, and to inject the heated compressed air flow into the turbine. RRA’s personnel, jointly with ESPC, established that, while KM7 base load power is 4.77 MW, with the supplementary air from the storage, it could be increased to 7.9 MW for peak power generation.

As mentioned before, for maximum peak power the compressor shall be separated from the expander with the installation of an automatic clutch. The separation of the compressor, to provide for the separately operating combustor/expander assembly, was also motivated by its application for a small capacity CAES plant – another power plant that is very promising for DG applications.

The newly developed by RRA combustor/expander package is shown in Figure 2. It is comprised of a 501-KB7 turbine/combustor/diffuser configured with a 501 KC5S power turbine output shaft and exhaust system. The expander has the hot end drive and allows for direct inflow of air into the diffuser, eliminating a new radial inlet scroll design required for a front-end drive design. RRA performed required engineering work to establish the feasibility of this development and to estimate performance and costs.

Compressed Air Energy Storage System

The compressed air storage is a major component of the HC plant. In order to meet a variety of site locations for DG plants, ESPC concentrated on storage in man-made buried vessels. These vessels are feasible and economical for the HC application, because only limited amounts of the supplementary air should be stored. An analysis of various potential compressed air storage containers, including steam-drum type pressure vessels, reinforced concrete pipes and various high pressure piping systems, resulted in the selection of the latter. The system consists of buried pipe, designed and fabricated to the U.S. DOT Transportation Pipeline Safety Standards. The precedence for pipeline air storage is the natural gas transmission lines throughout the U.S. Pipeline, with a 48-inch diameter and maximum pressure of approximately 1550 p.s.i.a was selected as the storage facility for the hybrid plant. Due to higher allowable stresses, based on the aforementioned standards, engineering and cost estimation efforts resulted in the conclusion that the air storage system can be installed for approximately $30/kW per hour of storage. For the HCH plant with 60% of the injected air humidification (which is easily achieved), this cost could be reduced to $10/kW per hour of storage.

Hybrid Concept Operations

Flow path diagrams and performance characteristics for various HC plant operations are presented in Figures 3 through 8. For simplicity reasons, the operations are explained for the HC plant based on RRA’s KC5S simple cycle CT, which includes the aforementioned combustor/expander package and the compressor connected via a clutch.

There are five modes of HC plant operations:

• Base load operation (Figure 3). In this mode the combustion turbine is separated from the boost compressor and the storage, and operates as a conventional CT.

• Intermediate-peak load operation (Figure 4). The HC components are connected as shown and the compressor discharge flow is supplemented by the compressed air flow from the storage, preheated in the recuperator (optional) before being injected into the CT upstream of combustors.

• Maximum peak load operation (Figure 5). In order to operate in this mode the compressor shall be mechanically separated from the expander (via disengaged automatic coupling) and disconnected by flow from the boost compressor and the storage. The expander is fed by the compressed air flow from the storage, preheated in the recuperator (optional), and its full power goes for the electric power production.

• Compressed air storage recharging using off-peak energy from the grid (Figure 6). This operation uses typically inexpensive off-peak power from the grid for driving both the CT compressor and the booster. It does not require a mechanical separation of the compressor from the expander. In order to prevent the expander overheating, calculations show that very small airflow from the compressor discharge is delivered to the turbine to provide cooling during its rotation. The boost compressor delivers the bulk of the compressor discharge flow to the storage at the required pressure. The storage pressure is a subject of optimization as it affects the storage volume and costs, the boost compressor cost and power consumption.

• Compressed air storage recharging using self-generated power (Figure 7). This operation could be typical for remote areas not connected to the grid. During this operation, called the self-charging mode, the compressor discharge air is divided into two flows. The air and fuel flows to the turbine are regulated to drive the compressor and to develop net power from the generator sufficient to electrically drive the booster compressor. The remaining compressed air flow goes to the boost compressor and ultimately to the storage system. In this mode, only fuel is used to charge the storage. To run in this mode, a control valve between the compressor and turbine must be added to distribute the flow.

• The HC pant operation as a synchronous condenser (Figure 8). With the addition of a clutch between the expander and generator, both expander and compressors can be disengaged from the generator, and the unit can operate as a synchronous condenser, providing power factor correction.

Hybrid Concepts with CHAT and Combined Cycle Plants

As it was mentioned above, the hybrid concept could be based on any combustion turbine derivative plant, which is able to utilize the hybrid concept’s major feature – supplementing the compressed air flow during peak hours with the compressed air stored during off-peak hours. Figures 9 and 10 show the flow path and performance characteristics of HC plants based on the CHAT and CC plants, respectively. The net power and heat rates at base and peak loads are indicated, as well as the required off-peak power during recharging, and the time needed to recharge after 3 hours of maximum peaking operation.

As it is evident from Figures 9 and 10, these two concepts are much more efficient, and, though more complicated in operations and maintenance are preferable for locations with high projected fuel costs and significant base-load operating hours.

Hybrid Concept Plant Costs

ESPC and its contractors conducted rather detailed cost estimates for various HC and HCH configurations. Capital costs are based upon a combination of manufacturer quotations, adjustments to previous equipment quotes for other projects, and in-house installation costs for projects of similar size and scope. Total plant specific capital costs ($/kW) are based upon the maximum peak output rating of HC plants, all based on the RRA’s combustor expander package and on three hours of the storage or three continuous peak generation hours. Estimated costs are as follows:

• $430/kW for both the HC and HCH plant based on the CT with recuperator
• $590/kW for the HC plant based on the CHAT plant
• $562/kW for the HC plant based on the CC plant

As it was stated above the HCH plant will not be significantly affected by additional storage hours. As anticipated, the simplest Hybrid CT (though less base-load efficient) has the lowest specific capital cost due to both the simplicity of the cycle and the largest peak/base ratio. Conversely, the most efficient CHAT hybrid has the highest specific cost primarily due to its lowest peak/base ratio.

Comparative Analysis of Economics of Hybrid Concepts with Other DG Alternatives

Table 1 presents a comparative economics of three potential alternative DG plants:

• Column 1 represents a HC plant with maximum peak power of 15.94 MW and the base load power of 4.8 MW
• Column 2 represents a single CT with the same maximum peak power of 15.94 MW and with the 4.8 MW base-load power being provided as the turbine operation at approximately 33% part load
• Column 3 represents two 7.89 MW combustion turbines (total peak power of 15.78 MW), with 4.8 MW base load provided by one CT operating at 61% part-load.

The comparative analysis was performed based on the following assumptions:

• Same peak and base load power and operating hours

• Cost of Fuel = $3.0 per Million Btu
• Off-peak Energy Cost = 1.0 Cents per kWh
• Fixed Charge Rate = 12%
• Peak Load Operation - 3 hrs/day

Table 1

Comparative Analysis of Hybrid-CT vs. Two CT Alternatives

Table 1 is self-explanatory. It shows that for assumed DG plant operations and economic factors, the HC has both the lowest specific costs ($/kW) and the lowest cost of electricity ($/kWh) as compared to two alternative CT plants (selected for the same power generation requirements). Key factors contributing to the HC plant better economics are built up in the very essence of the HC:

• The CT is selected to meet base load requirements and therefore operates most of the time with the best efficiency

• The method to arrive at the peak capacity (the storage and compression equipment) has lower incremental costs ($/kW) than that for a CT

• The operating expenses to recharge the storage are based on the least expensive off-peak power.

It was demonstrated that for assumed DG plant operations and economic factors, the HC based on the CHAT plant has both the lowest specific costs ($/kW) and the lowest cost of electricity ($/kWh) as compared to the CHAT plant selected for the same duties.

Similar results could be demonstrated for HC plants with the utilization of a CC plant as a CT-derivative plant.

Summary of Hybrid Plant Operation and Benefits

Extensive engineering and economic studies resulted in the following conclusions:

1. The Hybrid Concept for DG is expected to be more economical, as compared to any other evaluated alternative.
2. The advantages of the Hybrid Concept are embedded in its essential principles and could be explained as follows;

• It has the lowest specific costs ($/kW) because a CT-derivative plant is sized to meet long duration base load requirements of a customer (approximately 50% of peak load requirements) and not peak requirements.

• It is more efficient because the CT derivative plant (CT, CHAT, CC, CT with steam injection, etc.) operates most of the time at the best efficiency.
• Specific incremental costs ($/kW) of the conversion of a CT-derivative plant into the HC are significantly lower than the cost of additional CT capacity, particularly as it relates to typically small CT sizes for DG.

1. Rolls Royce Allison developed the following combustion turbine models which could be easily applicable for the HC plants:

• The commercially available 501 KM 7 model with 4.8 MW base load and approximately 7.9 MW peak load, when it is integrated into HC;
• The combustor/expander package K7S5, which can provide the same 4.8 MW base load and, 15.9 MW peak power when it is integrated into HC.

1. The compressed air storage capacity is a relatively small, due to small plant unit sizes (5 MW – 30 MW) and very short peak power duration (1-2 hrs/day). Therefore, it could utilize man-made storage vessels, vs. conventionally used underground formations for CAES plants. ESPC developed the compressed air storage concept allowing placement of the HC without any site restrictions. The concept is based on the storage facility based on buried HP with specific storage costs of approximately $30/kW per hour of storage for HC plant and $10/kW for the HCH concept.
2. For underground storage (if available) specific costs of storage could be significantly lower.

References

1. "Small Generators Fuel Big Expectations," Power Engineering. February 1999.
2. "CHAT Technology at 54.7% Efficiency, Ready for Commercial Demo," Gas Turbine World. May-June 1996.
3. "12-MW demo plant proposed to prove out CHAT technology", Gas turbine World, May-June, 1999.
4. US Patent # 5,778,675 "Method of Power Generation and Load Management with Hybrid Mode of Operation of a Combustion Turbine Derivative Power Plant, Dr. Michael Nakhamkin, Inventor. July 14, 1998.