ADVANCED ORGANIC RANKINE CYCLES IN BINARY GEOTHERMAL POWER PLANTS

Uri Kaplan
ORMAT Technologies, Inc.
(November 2007)

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

Abstract
In the last two decades the binary geothermal 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. More then 800 MW of such plants are in operation, about 90% of them manufactured by Ormat.

In this paper we describe some advanced versions of the Organic Rankine Cycle and their application for an efficient utilization of geothermal fluids having specific thermal  and chemical properties.

Re'sume'
Au cours des deux derni'eres de'cennies les centrales ge'othermiques binaires utilisant le cycle de Rankine a vapeur organique (ORC) out e'te' le proce'de' pre'fe're' pour l'utilisation de sources ge'othermiques 'a basse ou de moyenne enthalpie.

Le Cycle de Rankine a e'te' utilise' pour la re'cupe'ration de la chaleur de l'eau chaude se'pare'e non utilise'e dans les centrales 'a vapeur et d'autres applications utilisant des sources ge'othermiques produisant de l'eau chaude seulement ou de l'eau chaude et de la vapeur 'a basse pression. Au cours des anne'es le cycle de Rankine 'a vapeur organique a e'te' modifie' et perfectionne' pour une meilleure adaptation aux conditions varie'es des ressources. Plus de 800 MW de telles centrales sont en fonctionnement, dont environ 90% construites par Ormat. Cet article pre'sente diffe'rentes versions de ce cycle qui permettent d'atteindre des rendements plus e'leve's ainsi qu'une adaptation aux proprie'te's thermiques et chimiques des fluides ge'othermiques.

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. 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 represent an approach for 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 Organic Rankine Cycle, 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. A comparison between Organic Rankine Cycle and other cycles, (such as single flash, dual flash, or the Kalina cycle), is only given in a referenced table.

Overall Power Plant Efficiency [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 is defined by the following expression:

 e = h - ho - To(S - So)  (DiPippo - 1984)

where:
e is the specific exergy
h is the enthalpy
T is the temperature
     and S is the specific entropy.

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 between the cooling media and the working fluid in the cold section of the process (condenser) is another form of loss.

The overall exergetic efficiency of a power plant is defined as the ratio between the plant net power and the exergy of the hot source, as follows:
 
 
  Where:

    = overall exergetic efficiency
 m    = the heat source fluid mass flow
 Wnet  = Net power generated by the plant

In addition to the above, there are mechanical and electrical losses which reduce the net generated output, as compared to maximum available exergy.

The irreversibility of a binary process on the hot side, namely the temperature difference between the heating fluid and the working fluid, is very nicely demonstrated on a Q/T (Heat rejected from the heating fluid vs. Temperature) diagram. Figure 1 is a typical Q/T diagram showing a liquid-type heat source heating the working fluid in a simple Organic Rankine Cycle containing a preheater and vaporizer. The marked parts between the two curves represents the irreversibility (losses) of the conversion process. It is clear from this figure that the similarity in shape of the two curves and the proximity between them are good indications of the process efficiency. [4]

 

 

Figure 1

Two-Phase Geothermal Power Plant
In the majority of geothermal fields worldwide, the geothermal fluid is separated in an above ground separator into a stream of steam and a stream of brine.

In a low to moderate enthalpy resource the steam quality is 10 to 30% as a function of fluid enthalpy and separation pressure. The two streams can very efficiently be utilized in a "Two-Phase Organic Rankine Cycle Unit", as shown in Figure 2. Separated steam (usually with some percentage of Non-Condensible Gases or NCGs) is introduced in the vaporizer to vaporize the organic fluid.

 

 

 

Figure 2
 

 

The geothermal condensate is mixed with the separated brine to provide the preheating medium of the organic fluid. In the ideal case, as presented in the Q/T diagram (Figure 3), the steam latent heat would be equal to the heat of vaporization of the organic fluid and the sensible heat of the brine plus condensate would be equal to the heat required to preheat the organic fluid. This "perfect" match of heat transfer between the geothermal fluid and the working fluid represents maximum thermodynamic efficiency with minimum losses.

 

 

 

 
Figure 3

Recuperated Cycle [6]
In most of the actual cases, the perfect match as above is not feasible, mainly because of limitation in the cooling temperature of the brine and condensate mixture. The limiting factor in most of the cases is the silica scaling risk, which is increased as the brine temperature drops. A method to partially overcoming the cooling temperature limit is to add a recuperator which provides some of the preheating heat from the vapor exiting the turbine.

The recuperator is applicable when the organic fluid is of the "dry expansion" type, namely a fluid where the expansion in the turbine is done in the dry superheated zone and the expanded vapor contains heat that has to be extracted prior to the condensing stage (Figure 4). The recuperated Organic Rankine cycle is typically 10-15% more efficient than the simple Organic Rankine cycle (Figure 5). This applies also to the two-phase geothermal power plant.

 

Figure 4

 

 

 

 

 

 

 

Figure 5

 

Figure 4 is the process flow diagram of the recuperated two-phase cycle.

The recuperated two-phase process is used by Ormat in many geothermal projects all over the world, such as 20 MW Zunil in Guatemala geothermal power plant (Figure 6), 14 MW Ribeira Grande I and II geothermal power plants in San Miguel in the Azores, 1.8 MW Oserian and 13 MW Olkaria III geothermal power plants in Kenya, 6.5 MW Rotokawa Extension and 12 MW Ngawha geothermal power plants in New Zealand, and 2.2 MW Hatchobaru geothermal power plant in Japan (Figure 7).

 

 

Figure 6 
Figure 7

Higher Enthalpy Two-Phase Geothermal Power Plant

When the resource enthalpy is higher, and as a result the proportion of steam in the total fluid increases, the "perfect match" between the heat source and the working fluid is not maintained, and thus some of the available heat or the available exergy is not used for power generation.

Advanced Organic Rankine Cycle Using a Secondary Organic Loop
To utilize the two-phase heat source in a more efficient manner, one can use a secondary organic loop, which uses the extra steam available. The cycle is shown in Figure 8 and is feasible when vapor extraction is possible within the expansion phase of the organic cycle. The simplest way to perform the extraction is with two turbines in series. In this case, some vapor is extracted between the high pressure and the low pressure turbines and is condensed at an intermediate pressure (and temperature). The condensed vapor preheats the main organic fluid stream as it exits the recuperator. The extracted organic fluid forms a secondary cycle which generates an additional 5 to 8% electrical power. When there is extra steam compared to brine (higher enthalpy) the above cycle is effective and the cooling temperature of the brine plus condensate is limited.

 

Figure 8

Figure 9 is a Q/T diagram of the higher enthalpy cases. Line A is the simple two-phase cycle preheating phase. The significant irreversibility is represented by the large space between the steam and brine lines and line A. Line B shows the preheating phase in a recuperated two-phase cycle; the irreversibility is reduced and the cycle efficiency is increased accordingly. The third line - C - demonstrates the additional gain in efficiency by using the two-phase/extraction cycle. The line moves further to the right, thus decreasing the gap between the heating line and the working fluid line. Another indication of the increase in efficiency from cycle A to B and to C, is the increasing heat quantity for heating the working fluid, as presented by points QA, QB, and QC.

Figure 9

Applying the exergy equations on the two-phase/extraction cycle proposed for a moderate enthalpy resource in New Zealand results in the following:

 
e = 353.2 kJ/kg
 m = 86.53 kg/sec
 Wnet = 15,000 kW

 and the exergetic efficiency is

which is very high compared to any alternative power conversion cycle for similar heating fluid conditions.

Operating conditions of the above New Zealand resource are as follows:

Steam inlet temperature ('C) : 187
Steam flow rate (t/h) : 476.46
Brine inlet temperature ('C) : 187
Brine flow rate (t/h) : 1392.6
Geothermal fluid outlet temperature ('C) : 85
Plant net power (MW) : 90
Dead state temperature ('C) : 14 

Use of a back pressure steam turbine

Another approach for the higher enthalpy two-phase heat source is the use of a back pressure steam turbine which generates extra power from excess steam not required for the vaporizer of the Organic Rankine Cycle.

Part of the preheating of the organic fluid is now done with low pressure steam exiting the back pressure steam turbine (Figure 10).
 
                                                               Figure 10

The gap between the steam and the preheating line of the organic fluid could be filled even more efficiently by a multi-stage (two or more) back pressure steam turbine, with extraction of steam between the stages, but the decision on the number of stages is based on the consideration of the trade off in the process optimization between higher efficiency and the complication (and cost) of the system.

A system based on the above cycle is now operating in the 20 MW Amatitlan geothermal power plant in Guatemala. (Figure 11).

Figure 11

 

Figure 12

 

Geothermal Combined Cycle
For high enthalpy fluids with very high steam content a solution is the geothermal combined cycle configuration where the steam flows through the back pressure turbine to the vaporizer, while the separated brine is used for preheating or in a separated Organic Rankine Cycle (Figure 13).

Figure 13

This configuration is used in the 30 MW Puna plant in Hawaii (Figure 14), as well as in the following plants: 125 MW Upper Mahiao geothermal power plant in the Philippines (Fugure 15), 100 MW Mokai 1 and II geothermal power plants in New Zealand.

 

Figure 14

 

Figure 15

Comparison of Geothermal Power Plants Technologies [5]
Table 1 shows a comparison of processes and geothermal power plants prepare by Prof. DiPippo to which the advanced Organic Rankine Cycle cycle was added in italic.

Table 1

Geothermal Power Plant Exergetic Efficiencies (in order of increasing efficiency)
by R. DiPippo / Geothermics 33 (2004) 565-586
Science Direct, April 24, 2004

Technology Plant Name Specific exergy input (kJ kg) Exergetic efficiency (%)
Binary Brady bottoming (p.m. data) 36.70 16.3
Binary Brady bottoming (a.m. data) 49.86 17.9
Binary: recuperated Rotokawa 227.96 18.7
Binary Nigorikawa (Mori) pilot 92.77 21.6
Binary Kalina KCS-34, Husavik 81.49 23.1
Double - flash Beowawe 205.14 26.0
Binary: simple Rotokawa 646.71 27.8
Single - flash Blundell 278.67 35.6
Hybrid flash - binary Rotokawa 461.45 42.0
Binary: dual - level Heber SIGC 125.84 43.0
Binary: flash evaporator Otaka pilot 126.65 53.9

Advanced Organic Rankine Cycle New Zealand Proposal 353.2 49.0

Conclusions
Improvement of the efficiency of an energy conversion process can be carried out in many ways, including the selection of a suitable motive fluid or working with a mixture of more than one fluid. In this paper we have described improvement of the conversion efficiency by using advanced thermodynamic cycles, which can be applied to specific conditions of a given heat resource to enable adjustment of process and cycle parameters to different the geofluid parameters. Such improved processes and thermodynamic cycles result in high efficiency while maintaining the high reliability, simple construction and operation as well as the high resource sustainability.

References
1. Geothermal Power Station, Lucien Bronicki - Encyclopedia of Physical Science and Technology, Academic Press, 2002.

2. Second law assessment of binary plants generating power from low-temperature geothermal fluid - Ronald DiPippo - Published by Elsevier Ltd - Science Direct, April 24, 2004.

3. Innovative Geothermal Power Plants, Fifteen Years Experience - Lucien Bronicki - World Geothermal Conference, Florence, Italy, January 1995.

4. Advances in binary organic Rankine cycle technology - A. Elovic - Published on Geothermal Resources Council Transactions, Vol. 18, October 1994.
 
5. R. DiPippo - Geothermics 33 (2004) 565 - 586.

6. U.S. Patent No. 3,040,528 - Vapor Turbines, issued on June 26, 1962.