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Publication Title | Organic Rankine Cycle Configurations

Organic Rankine Cycle

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Proceedings European Geothermal Congress 2007 Unterhaching, Germany, 30 May-1 June 2007

Organic Rankine Cycle Configurations

Uri Kaplan

Ormat Technologies, Inc., 6225 Neil Road, Suite 300 - Reno, NV 89511-1136, USA ormat@ormat.com

Keywords: Binary Power Plant, Organic Rankine Cycle, Matching and Optimization, Two-phase Geothermal Resources, High Enthalpy, Recuperator, Geothermal Combined Cycle.

ABSTRACT

In the last two decades the binary power plant, utilizing the Organic Rankine Cycle (ORC), has become a preferred means of exploiting low to moderate enthalpy geothermal resources. It has been widely used to utilize the brine in existing single flash plants and in many other applications as an efficient and reliable way of employing a geothermal resource, in the form of brine only or brine and low pressure steam coming from a separator. Over the years the basic ORC has been improved and modified to better adapt the cycle to various conditions of the heat source.

In this paper we will describe some advanced versions of the Organic Rankine Cycle and will demonstrate its means of providing an efficient conversion cycle adapted to specific thermal and chemical properties of geothermal fluid sources.

Examples of implementation in different power plants include the power plants in the Azores, Iceland and the plant being constructed in Landau by Geo X GmbH of Ludwigshafen.

1. INTRODUCTION

The process of designing a geothermal power plant can be considered as one of matching and optimization of the entire system. The matching process must take into consideration the characteristics of the geothermal fluid and selection of the optimum power conversion cycle, as well as other factors such as system simplicity, low maintenance requirements, and reservoir and environmental considerations. The availability and plant factor of the operating power plant is at least as important as the plant efficiency A very high efficiency conversion cycle will not do its job if the power plant is too complicated to maintain, too expensive to construct or too harmful to the environment. A conversion cycle that prevents injection of all or most of the geofluid along with the concomitant pressure support may negatively impact the sustainability of the reservoir and as a result will not be economically viable in the long term. The advantages and benefits of Organic Rankine Cycle power plants in terms of their high reliability operation, reservoir sustainability and environmental friendliness has been well demonstrated during more than twenty years of successful operation around the world. The power conversion cycles described in this paper are some examples of optimizing and maximizing the power output from different geothermal resources, while maintaining the simplicity and high reliability of the ORC equipment [1] [3].

The cycles described in this paper utilize geothermal heat sources containing steam and brine, where the enthalpy is relatively low. No thermodynamic cycle provides a “total” solution to all low enthalpy cases, but rather can provide a working tool to the plant designer to enable selection of the proper answer for optimization for the specific site conditions.

The intention of this paper is to describe several innovative processes in geothermal power plants using ORC, some of which have recently been developed and patented by Ormat, and which provide good solutions for the utilization of geothermal resources with certain characteristics.

2. COMPARISON BETWEEN THERMODYNAMIC CYCLES [2]

The second law of thermodynamics determines the limitations of performance of a power generation process. Exergy is a useful tool to define the maximum theoretical power output for a given heat source, and at a given environmental temperature. The exergy and the exergetic efficiency provide a useful tool for comparison between cycles. The exergetic efficiency is the ratio of the plant output to the maximum theoretical output at the plant conditions.

The exergy is defined by the following expression:

o: marcom/papers/EGC/4547 1

April 16, 2007

e=h–ho -To(S–So) where:

e is the specific exergy h is the enthalpy

T is the temperature

and S is the specific entropy.

(DiPippo–1984)

The subscript o refers to the ambient (dead state) temperature.

For a fluid flowing at a certain mass flow rate, multiplying the specific exergy by the mass flow rate results in the maximum power output theoretically obtainable from the given fluid for the given surroundings.

The real power generated by the power plant is always lower than the maximum theoretical value as defined above as a result of losses or irreversibilities in the cycle and the power plant. The main losses are due to the fact that the input heat to the system is limited in temperature, i.e. the heating fluid cannot be cooled down to the ambient temperature. One more major irreversibility in a binary power plant process is the difference in the temperature and enthalpy between the heating fluid and the secondary (working) fluid. An efficient process is one with a minimum such enthalpy difference. The enthalpy difference

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