US20110100003A1

US20110100003A1 - System and method to reduce the temperature of geothermal water to increase the capacity and efficiency while decreasing the costs associated with a geothermal power plant construction

Info

Publication number: US20110100003A1
Application number: US12/611,501
Authority: US
Inventors: Todd McLeod; Robert Ricard
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-11-03
Filing date: 2009-11-03
Publication date: 2011-05-05

Abstract

The system and method to reduce the temperature of geothermal water to increase the capacity and efficiency while decreasing the costs associated with a geothermal power plant construction uses a high efficiency steam turbine. The steam turbine lowers the high temperature geothermal resource so it can be used in parallel with an innovative low temperature Organic Rankine Cycle (ORC) geothermal power plant to increase the efficiency and capacity while at the same time to reduce the costs associated with constructing the power plant because of logistics, labor and material.

Description

FIELD OF THE INVENTION

The present invention relates generally to geothermal power generation, and more particularly to a system and method to reduce increase the capacity and efficiency while decreasing costs associated with geothermal power generation.

BACKGROUND OF THE INVENTION

There are two kinds of basic geothermal power plants in operation today. Organic Rankine Cycle (ORC), also known as Binary cycle, and Single Flash Steam power plants are very famous in geothermal power generation applications. Producing more electricity and more thermal energy are the main goals from the geothermal power plants.
The best way to currently increase the overall output from a single resource is to “bottom cycle” the primary technology with additional equipment that can operate within the geothermal conditions to produce additional power from a single resource.
Currently there are geothermal resources within the earth's soil that are at temperatures that range from 100° F. to upwards of 750° F. These temperatures are classified into two areas of resource. The first is the low to moderate temperature (condition 1) which is typically described as 100° F. to 325° F., and the moderate to high temperature which is in the range of 325° F. to the high end of the spectrum that is roughly greater than 650° F. (condition 2). Depending on the resource temperature, a geothermal developer has only two equipment options—one being Organic Rankine Cycle technology for condition 1 and the other being a single or dual steam flash system for condition 2. Typical ORC systems achieve 8%-11% efficiencies and steam flash systems range from 18%-25% depending on what type is used.
Prior art methods have been developed which address improvements to geothermal power plant efficiencies and combining geothermal technologies to bottom and top cycle which produces additional power from a single resource (well).
All of the prior inventions address similar topics but none of them address a method of reducing high temperature geothermal resources to enable low temperature ORC technology to be used, nor do they address the cost savings associated with generating additional electrical power from a single resource (well) as opposed to constructing multiple small power plants to generate the same amount of power. Additionally, the prior art efficiencies are no more than 25%.
With the foregoing problems and concerns in mind, it is the general object of the present invention to provide a system and method of using geothermal water to produce electrical power which overcomes the above-mentioned drawbacks.

SUMMARY OF THE INVENTION

The present invention operates in the 40% system electrical efficiency range from a single geothermal resource, reduces the number of geothermal wells needed to produce the same electrical power output as current conventional systems do, and allows low temperature ORC equipment to operate in a high temperature environment. As technology in the low temperature ORC process is improved, these efficiencies will increase as well which will help make geothermal power production a viable alternative to other power producing technologies.
The present invention employs a high temperature (>350° F.) thermal resource from a geothermal well to produce electricity through a single flash steam turbine generator combined with a low temperature (<300° F.) Organic Rankine Cycle power plant. Steam under pressure is what is known as saturated steam and can be transported through piping systems as a liquid at temperatures from 212° F. to greater than 750° F. The temperature and pressure relationship are represented in the Properties of Saturated Steam tables, which can be found in any engineering publication on the subject.
By reducing the pressure of the high temperature liquid, it is cooled down to a lower temperature and a certain known percentage of the liquid is converted into vapor in the form of steam. This is done through use of a steam flash tank which is a pressure vessel that allows steam to flow out of the top of the vessel while the remaining reduced temperature resource continues to be in a liquid state and is drained out of the bottom of the vessel to be used in the ORC part of the process.
The steam from the flash tank is transported through piping to the inlet of the steam turbine where the energy available from the steam is extracted from the turbine and converted into electrical energy through means of expansion. After the energy is extracted and converted, the remaining steam vapor is sent to a condensing tank where it is converted into liquid by lowering the pressure even further and is then transported by piping to the geothermal reservoir by a method known as re-injection.
On the liquid side of the flash tank, the reduced temperature hot water is transported by piping to the inlet of the evaporator of the ORC equipment where it is then used to expand or “boil” a working fluid inside the ORC machine that expands the liquid across the turbine section and produces electrical energy. This fluid is then cooled down in the condensing side of the ORC machine and re-injected back into the geothermal reservoir to complete the cycle. By operating a geothermal power plant using this method many benefits can be realized over current operational methods.
ORC technology does not allow electrical power generation through thermal conversion at temperatures above 325° F. and therefore limits the application to areas that only contain low temperature resources. This method allows high and low temperature resources to be developed from an ORC geothermal power plant which ensures the maximum amount of energy that can be extracted from the geothermal reservoir is employed for power generation.
Current bottom cycling systems are limited by the efficiencies associated with the equipment and technology available which currently are at 25-30% at best. The method of the present invention employs an industrial high efficiency steam turbine generator that is typically used as a pressure reducing system in commercial buildings to lower the process steam pressure and also to produce electrical power at electrical efficiencies greater than 70%. When combined with the lower efficiency ORC equipment, the system's overall efficiency is at 40% or greater.
By maximizing the energy extraction using high efficiency methods such as this, the need to install additional piping from many geothermal wells is eliminated. A single well can now produce as much electrical power as four wells of the exact same temperature and pressure. This method greatly reduces construction costs associated with well drilling and installation of long runs of piping and electrical distribution wiring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a geothermal system embodying the present invention.

FIG. 2A is a flow diagram of a process of generating geothermal power in accordance with the present invention.

FIG. 2B is a continuation of the flow diagram of FIG. 2A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a geothermal system embodying the present invention is indicated generally by the reference number 10. The system 10 comprises a geothermal well pump 12, a steam flash tank 14, a steam turbine 16, a steam turbine generator 18, a condensing tank 20, a low temperature ORC system 22, and an electrical generator 24.
An inlet 26 of the geothermal well pump 12 is to be in fluid communication with a geothermal reservoir 28. An outlet 30 of the geothermal well pump 12 is coupled to an inlet 32 of the steam flash tank 14. A steam outlet 34 of the steam flash tank 14 is coupled to an inlet 36 of the steam turbine 16. A power outlet 38 of the steam turbine 16 is coupled to an inlet 40 of the steam turbine generator 18. A steam outlet 42 of the steam turbine 16 is coupled to an inlet 44 of the condensing tank 20. An outlet 46 of the condensing tank 20 is to be coupled to the geothermal reservoir 28 for heating and recycling.
A liquid outlet 48 of the steam flash tank 14 is coupled to an inlet 50 of the low temperature ORC system 22. A power outlet 52 of the low temperature ORC system 22 is coupled to an inlet 54 of the electrical generator 24. A liquid outlet 56 of the low temperature ORC system 22 is to be coupled to the geothermal reservoir 28 for heating and recycling.
FIG. 1 illustrates by way of example a single high temperature geothermal well with an estimated 1,000 kWe production potential that is capable of producing 4,250 kWe. Implementing the system and method of the present invention reduces the temperature of geothermal water to increase the capacity and efficiency while decreasing the costs associated with a geothermal power plant construction. Efficiency of this example system is 39% while current technology efficiencies are at best in the 25% range for dual flash systems.
The inventors have found that a system and method embodying the present invention enables a geothermal power plant to operate in the 40% system electrical efficiency range from a single geothermal resource, reduces the number of geothermal wells needed to produce the same electrical power output as current conventional systems do, and allows a low temperature ORC system to operate in a high temperature environment.
With reference to FIGS. 2A and 2B, the operation of the system 10 will now be explained through process steps. The geothermal reservoir 28 is selected and contains a large body of liquid that is produced from ground water that is heated and pressurized by the earth's core. These reservoirs are typically created by natural fault zones and cracks within the earth's crust at depths ranging from 100 feet to over 10,000 feet. By drilling through the soil and installing geothermal well piping the hot fluid can then be extracted by means of the geothermal well pump 12. The hot liquid enters the inlet side of the pump (step 100) and is discharged at generally the same temperature and pressure that the hot liquid was extracted at from the geothermal reservoir 28 (step 102). At an elevated pressure, the amount of liquid that is present will determine the flow rate at which the liquid can be transported through the system 10 which varies by each geothermal reservoir's characteristics. This hot fluid is then pumped into a vessel known as the steam flash tank 14 (step 104) where the process of separating steam from the liquid is done by lowering the pressure of the fluid in order to achieve the thermal reaction needed to produce a known percentage of vapor and liquid to be used in the process. This temperature and pressure relationship can be found in engineering steam tables that also describe the heat content (enthalpy) of the vapor and liquid at various conditions. The % of flashed steam is represented by the following equation:
$% of Flash = \frac{High pressure liquid enthalpy - low pressure liquid enthalpy}{Enthalpy of evaporation at the lower pressure}$
In order to determine the specific volume of the saturated geothermal liquid that will be used as the ORC resource and the saturated vapor that will be used for the steam turbine generator the following equation is used in conjunction with the steam tables:

Where:

v_f=specific volume of saturated liquid
v_fg=increase in specific volume when state changes from liquid to vapor
v_g=specific volume of saturated vapor
The relationship between v_f, v_fgand v_gis given by the equation
V _g =V _f +V _fg
The flashed steam vapor is then transported by piping from the top of the steam flash tank 14 to the inlet 40 of the steam turbine generator 18 (step 106) where it is expanded across a turbine rotor which spins the steam turbine 16 and the electrical generator 24 to produce electric power (step 108) that is transported to the utility grid or consumed by the site power demands. When the expansion process is completed there is still steam vapor remaining that is transported by piping from the outlet of the steam turbine generator 18 to the condensing tank 20 (step 110) where the pressure and temperature are lowered once more in order to convert the vapor to a liquid state so it can be re-injected back into the geothermal reservoir 28 to be heated and recycled (step 112). During the process of converting the geothermal liquid to steam in the steam flash tank 14, the pressure is lowered to a level where the ORC system 22 can be used to convert the remaining hot liquid into electrical power (step 114).
The majority of the resource in the steam flash tank 14 will remain a liquid at the same temperature and pressure the vessel is controlled to which is then transported from the bottom of the steam flash tank 14 by piping to the inlet 50 of the low temperature ORC system 22 (step 116). Once the liquid enters the low temperature ORC system 22, the Organic Rankine Cycle takes place where the geothermal liquid heats a working fluid by means of non-contact shell and tube heat exchanger. This process causes the ORC working fluid to expand into a vapor across a turbine inside the system that is connected to the electrical generator 24 to produce electrical power that can be either transported to the utility grid or consumed locally (step 118). The low temperature ORC system 22 also has its own internal condensing unit that through means of an additional non-contact shell and tube heat exchanger with cooling water lowers the temperature and pressure of the working fluid so it can be pumped back into the evaporator section or the inlet 50 of the ORC system 22 to be recycled (step 120). Once the energy is extracted from the ORC system 22, the liquid exits and is transported by piping to a re-injection well 60 and returned back to the geothermal reservoir 28 where it is reheated (step 122).
The present invention has many advantages and one of the greatest is the ability to use a geothermal resource that would typically not be able to be employed for a low temperature ORC system over a range of high temperatures from 325° F. to over 600° F. In order to maintain a controlled temperature to the inlet of the ORC system, the present invention uses the physical properties of saturated steam at known conditions in which a commercial flash tank is deployed with a variable pressure setting. Geothermal water can be delivered at temperatures much higher than the ORC system is rated for and by reducing the pressure to 50 psig in the flash tank the liquid temperature of the geothermal fluid remains at a constant 298° F. which is the highest temperature for which the ORC inlet conditions are designed. Lower temperatures can be used using this method, but the ORC system is at its optimal design point when supplying the inlet with the highest temperature fluid for which it will operate. In doing this the ORC system requires the least amount of geothermal liquid flow to achieve the same electrical power output that it would produce using lower temperature geothermal hot water inlet conditions.
Other advantages the present invention delivers are cost savings in the construction and well drilling areas. By implementing this method of delivering low temperature geothermal water from a high temperature well, the amount of wells that would be required is reduced by a factor of three (3). For every mega watt of electricity produced by the steam turbine generator an additional three (3) mega watts of electrical power can be generated by the low temperature ORC process. Current well prices range from $500,000 per 1,000 feet to upwards of $800,000 per 1,000 feet, and the typical well depth for a geothermal resource at 350° F. is 4,000 feet. Although this varies based on geographical locations the cost savings is applicable to all drilling activities based on the 3-1 ratio of power being generated from a single resource. Using the depth and costs detailed in this disclosure, the system and method of the present invention generates an average cost savings of $7,800,000.00 in well drilling costs alone.
The well drilling costs have been published to be 40%-50% of the entire construction costs for a geothermal power plant, and at an industry average of $5,000.00 per kilo watt (kW) for plant construction, this method reduces the average cost per kilo watt to $2,500.00 which equates to a 37.5% savings in well drilling costs.
This is represented by the following formula:
$\begin{matrix} 50 % well drilling costs, 1, 000 kw = \underline{$5, 000, 000} \\ = $2,500,000 per 4, 000 foot well 2 \end{matrix}$ $\begin{matrix} 4, 000 kw = \frac{$20,000,000}{2 (50 %)} \\ $10, 000, 000 Four wells at 4, 000 feet each \\ \underline{- $2, 500, 000} cost for 1 well \\ $7, 500, 000 cost avoidance of 3 additional wells \\ $20, 000, 000 Total Cost for 4, 000 kw project \\ \underline{- $7, 500, 000} Cost avoidance of 3 wells \\ $ 12,500,000 Adjusted project costs \end{matrix}$ $Cost per kw to install = $3, 125.00 or 37.5 % savings$
In the financial climate of geothermal power production this method poses a significant step change in the construction costs that will enable geothermal power production to become more cost effective and competitive with other technologies. As technology in the ORC area progresses, this method will also play a significant role in those advancements and aid as a tool that ORC and steam turbine equipment manufacturers will use to direct their focus on equipment design and applications.
Another factor that has hindered geothermal development is the relative location of the power plant to the electrical utility company's transmission lines. In order to develop multiple geothermal power plants they must be separated by a considerable distance that is dictated by the production capacity of the geothermal resource. Moreover, each power plant must install its own individual electrical power transmission wiring to the utility source. It is difficult to quantify the cost savings this method will achieve in the area of electrical transmission distance because of the site variables that exist from one to another, but it is the professional opinion of the inventors that there will be some cost savings associated with the electrical transmission wiring, but it cannot be calculated accurately enough to be conveyed at this time.

Example

Utilizing a 350° F. geothermal resource at a 1,750 gallon per minute flow rate the system can generate 1000 kWe net power from a resource that using current technology would only generate 1,000 kwe.
Saturated steam at a temperature of 350° F. will operate at 120 psig with a mass flow rate of 908,877 lb. per hour or converted to a liquid flow rate of 1,750 gallons per minute (gpm). By using a steam flash tank upstream of the geothermal power generation equipment the saturated steam liquid pressure can be reduced to produce a lower temperature geothermal liquid based on engineering steam tables. By reducing the 120 psig geothermal liquid to a pressure of 50 psig, 7% of the mass in the tank will flash into steam at 298° F. which can be used to operate the steam turbine. Under these conditions the steam mass flow will be 74,246 lbs per hour and the turbine generator's electrical out based on this will be 1,000 kWe. The remaining 93% of the initial mass will remain in a liquid state in the flash tank at 298° F. and be pumped into the low temperature ORC system to generate additional power.
This liquefied mass converts to a flow rate of 1,628 gpm at 298° F. which will produce an additional 3,260 kWe net power for a total system output of 4,260 kWe net power production from the geothermal resource. This process will increase the geothermal well's power production capability by 325% compared to a typical single flash turbine generator system.
In sum, the system and method of the present invention uses a high efficiency steam turbine to reduce the temperature of geothermal water to increase the capacity and efficiency while decreasing the costs associated with a geothermal power plant construction. The steam turbine lowers the high temperature geothermal resource so it can be used in parallel with an innovative low temperature Organic Rankine Cycle (ORC) geothermal power plant to increase the efficiency and capacity while at the same time to reduce the costs associated with constructing the power plant due to logistics, labor and material.
As will be recognized by those of ordinary skill in the pertinent art, numerous modifications and substitutions can be made to the above-described embodiments of the present invention without departing from the scope of the invention. Accordingly, the preceding portion of this specification is to be taken in an illustrative, as opposed to a limiting sense.

Claims

1. A system to reduce the temperature of geothermal water to increase the capacity and efficiency of a geothermal power plant, comprising:

a geothermal well pump;

a steam flash tank;

a steam turbine;

a steam turbine generator; and

a condensing tank;

the inlet of the geothermal well pump configured to be in fluid communication with a geothermal reservoir;

an outlet of the geothermal well pump being coupled to an inlet of the steam flash tank;

a steam outlet of the steam flash tank being coupled to an inlet of the steam turbine;

a power outlet of the steam turbine being coupled to an inlet of the steam turbine generator;

a steam outlet of the steam turbine being coupled to an inlet of the condensing tank; and

an outlet of the condensing tank configured to being coupled to the geothermal reservoir for heating and recycling.

2. A system as defined in claim 1, further comprising:

an ORC system; and

an electrical generator;

a liquid outlet of the steam flash tank being coupled to an inlet of the ORC system;

a power outlet of the ORC system being coupled to an inlet of the electrical generator; and

a liquid outlet of the ORC system configured to be coupled to a geothermal reservoir for heating and recycling.

3. A method of reducing the temperature of geothermal water to increase the capacity and efficiency of a geothermal power plant, comprising the steps of:

extracting hot liquid from a geothermal reservoir via a geothermal well pump;

discharging the hot liquid from the geothermal well pump at generally the same temperature and pressure that the hot liquid was extracted from the geothermal reservoir;

pumping the hot liquid into a steam flash tank;

transporting flashed steam vapor from the steam flash tank to a steam turbine generator; and

expanding the flashed steam vapor across a turbine rotor for spinning a steam turbine and an electrical generator to produce electric power.

4. A method as defined in claim 3, further comprising the steps of:

transporting remaining steam vapor from the steam turbine generator to a condensing tank; and

lowering temperature and pressure of the steam vapor in the condensing tank to convert the steam vapor to a liquid for reinjection back into a geothermal reservoir.

5. A method as defined in claim 3, further comprising the step of lowering the pressure of remaining hot liquid in the steam flash tank to a level where an ORC system can be used to convert the remaining hot liquid into electrical power.

6. A method as defined in claim 4, further comprising the steps of:

transporting the remaining hot liquid from the steam flash tank to an ORC system; and

generating an ORC cycle where the remaining hot liquid heats working fluid causing the working fluid to expand into vapor across a turbine connected to an electrical generator to produce electrical power.

7. A method as defined in claim 6, further comprising the step of lowering temperature and pressure of the working fluid for reinjection into the ORC system.

8. A method as defined in claim 6, further comprising the step of returning the remaining hot liquid to a geothermal reservoir for reheating and recycling.