A Project To Estimate The Power Capacity Of Geothermal Power Plants Based On The Enthalpy Of Geothermal Fluid And Plant Design

Nigeria has some surface phenomena that indicate the presence of viable geothermal energy. None of these locations have been explored extensively to determine the feasibility of sustainable geothermal energy development for electricity generation or direct heating. In this context, the present study aims to provide insight into the energy potential of such development based on the enthalpy estimation of geothermal reservoirs. This particular project was conducted to determine the amount of energy that can be gotten from a geothermal reservoir for electricity generation and direct heating based on the estimated enthalpy of the geothermal fluid. The process route chosen for this project is the single-flash geothermal power plant because of the temperature (180℃) and unique property of the geothermal fluid (a mixture of hot water and steam that exists as a liquid under high pressure). The Ikogosi warm spring in Ekiti State, Nigeria was chosen as the site location for this power plant. To support food security efforts in Africa, this project proposes the cascading of a hot water stream from the flash tank to serve direct heat purposes in agriculture for food preservation, before re-injection to the reservoir. The flowrate of the geothermal fluid to the flash separator was chosen as 3125 𝑡𝑜𝑛𝑛𝑒𝑠/ℎ𝑟 . The power output from a single well using a single flash geothermal plant was evaluated to be 11.3 𝑀𝑊 *. This result was obtained by applying basic thermodynamic principles, including material balance, energy balance, and enthalpy calculations. This particular project is a prelude to a robust model that will accurately determine the power capacity of geothermal power plants based on the enthalpy of fluid and different plant designs.


INTRODUCTION
Many experts have noted a direct correlation between the economic development of a nation and its power generation capacity; a nation cannot have one without the other; lack of access to adequate energy contributes to poverty and economic decline (Olaniyan et al, 2018). Africa has abundant renewable energy resources which if safely harnessed could rescue the continent from the present energy challenge and sustain her budding economic growth transformation.
The Africa Development Bank (AfDB) estimates the renewable energy potential in the continent to be 110 GW for wind energy, 15 GW for geothermal energy, 1000 GW for solar energy, and 350 GW for hydroelectricity (Hafner M, et al, 2018). Nigeria has an estimated 93,950 MW (approx. 94 GW) as untapped renewable energy (UN ECA, 2017).
Nigeria, the most populated country in Africa has less than 40% of its populace connected to the electricity grid; those that are connected (less than 40% of the population) hardly enjoy long hours of uninterrupted power supply (Abubakar S et al, 2013). The rest of the population either generate electricity for themselves using generators or abandon electricity use completely.
The country has an estimated population of 206 million (National Population Commission, 2021) with an installed energy capacity of 12, 522 MW; however, only about 30% of this installed capacity (4,000 MW) is functional on most days (USAID, 2021). A bulk of this capacity is contributed by coal, hydroelectric power, and natural gas sources (figure below).
Nigeria requires an additional 50,000 MW to adequately light up the nation (World Energy Outlook, 2014).

Figure 1 Energy Sources in Nigeria (Energy Commission of Nigeria, 2015)
Geothermal energy is a renewable and clean energy resource with infinite potential to adequately bridge the energy gap in Nigeria. Compared to other renewable energy sources, geothermal energy is relatively cheaper and readily available, showing no intermittency like solar or wind energy (ThinkGeoEnergy, 2020). The advancement of geothermal energy development in Nigeria (despite various surface manifestations) has been limited by a lack of adequate knowledge among policymakers, entrepreneurs, and local communities in Nigeria about the immense potential and benefits of geothermal energy (IRENA, 2015).
The situation is different in some other parts of Africa. In the 1950s, Kenya began exploration for geothermal energy development and built its first geothermal power plant -Olkaria in 1981 (ThinkGeoEnergy, 2020). Due to this early start, (barely five decades after the first geothermal plant was installed in 1904, Italy) Kenya is currently the eighth largest geothermal energy producer in the world, with an installed capacity of 707+ Mwe, only behind nations like the United States, Indonesia, Philippines. Geothermal energy is the second-largest contributor to Kenya's energy grid. (BBC Future, 2021 Currently, the only major source of renewable energy in Nigeria is hydroelectric power (HEP).
Of all viable hydroelectric power resources available in Nigeria, only the Kanji dam, Jebba hydropower station, and Shiroro Power Station is functional and represents about 10 % of installed energy capacity in Nigeria (ICE, 2018). The rest serve merely as tourist centers (Premium Times, 2017).
In this context, the present study offers insight into the potential of geothermal energy in Nigeria to contribute in no small way to electricity generation thereby reducing over-reliance on fossil fuels and mitigating environmental pollution due to carbon emission. This study aids the sustainable development of geothermal energy by providing insight into the feasibility of pursuing a geothermal reservoir when the enthalpy of geothermal reservoirs is known or can be accurately estimated.

Objectives of Design project
This project work was carried out:

•
To calculate the power capacity of a single flash geothermal power plant based on enthalpy/temperature of fluid from an unexplored geothermal reservoir.

Significance of work
The economic development of any nation is proportional to the amount of energy that nation produces (Olaniyan K et al, 2018). Nigeria has the highest number of people without access to electricity in Africa. According to a division of the United Nations, Nigeria is estimated to reach a population of over 250 million by 2030, and 400 million by 2050; at this rate, Nigeria will become the third-largest nation in the world (National Population Council, 2021). If nothing is done, the number of people without access to energy will significantly increase leading to further economic decline.
Nigeria's energy demand is barely met, especially in the densely populated northern part of the nation; the future looks bleak because, without proper exploration of other abundant (renewable) energy resources, a large percentage of her populace (over 100 million) will not be connected to the electricity grid. (Olaniyan K et al, 2018).
A lot of studies have been done in the past to discuss the potential for geothermal energy in Nigeria and recommend further exploration to determine the feasibility of sustainable development. The present study evaluates the power capacity of a single flash geothermal power plant based on the estimated enthalpy of a geothermal reservoir. This is a timely contribution because currently, geothermal energy is gaining more support from governments.
Although the power capacity of the single flash geothermal power plant was done applying basic thermodynamic principles (material balance, energy balance, and enthalpy calculations), the project is only a prelude to a robust model that will accurately estimate the power capacity of geothermal power plants based on enthalpy and design type.
The fluid temperature from an unexplored geothermal reservoir in Ikogosi warm spring (Ekiti State, Nigeria) was estimated to be 188°C. Using a single flash geothermal power plant design, and applying basic thermodynamic principles, the power capacity of the geothermal power plant was calculated to be 11.3 MW*.
5MW can power up to 50,000 Kenyan homes (BBC Future, 2021)

Process route
The process routes available for this project include the different technologies available for utilizing geothermal energy for electricity generation: Dry steam process, Flash steam process (single flash and double flash processes), and Binary process or Organic Rankine Cycle (ORC).
The process route selected for this design is the single flash steam process.

Process route justification
The single-flash process route is suitable because of the properties of the geothermal reservoir located around the Ikogosi warm spring. The geothermal fluid is assessed to be a high-pressure mixture of brine and steam, existing as liquid because of its high pressure. Because of this, it cannot be used directly in a simpler dry steam power plant, hence it is introduced to a flash separator where the pressure is lowered and a percentage of the mixture flashes rapidly (with enough pressure) to steam. The steam is then used to drive a turbine for power generation.
The majority of geothermal sources do not produce temperatures that are close to the critical point of water. A temperature between 150 °C and 200 °C is suitable for a single flash process (Dincer & Ezzat, 2018).
Our geothermal fluid falls in-between at 188 °C.

Process route description
A single flash geothermal power plant operation involves a single stage where the superheated mixture of steam and liquid water is passed through a low-pressure flash separator. When the pressure is lowered, part of the fluid vaporizes and flashes into steam. The steam and liquid are separated; the steam is sent to drive the turbine and the liquid is re-injected underground.
After the steam is used to drive the turbine, it is condensed back to a liquid in a cooling tower before reinjection into the reservoir (Ronald Dippo, 2016).
For this particular project, the hot liquid is sent to a nearby food preservation plant for further heat to be extracted before re-injection.

Site Location Justification
The area around Ikogosi was chosen as the site location because of the presence of warm spring in that location. The temperature of the warm spring reaches as high as 70°C at the source of the spring (Ayodele, et al, 2019). Going by subsurface geothermal gradient data from oil drilling and borehole temperature in the southwestern region of Nigeria, 4.0 -4.5 °C /100m (Akpabio, et al, 2003), the well-depth will be about 3km -4km to get a temperature of 188°C which is typical of geothermal wells.

Limitations of the project
The limitations of the project work include: • No single geothermal power plant exists in Nigeria • No exploration or drilling has been carried in promising locations to determine the feasibility of geothermal energy development.
• Geothermal fluid conditions (temperature, pressure, flowrate) used for calculation are based on estimation and secondary sources. No site visit was done.

Classification of Geothermal Reservoirs
Enthalpy and chemistry are key factors that determine the efficient utilization of a geothermal resource. Both factors depend on the geothermal system from which the fluid originated.

Enthalpy of geothermal fluid
Some authors classify geothermal resources based on temperatures while others have used enthalpy (Dickson and Fanelli, 1990). Enthalpy gives us an idea of the thermal energy or 'value' of fluids in terms of energy. Geothermal resources are classified into low, medium, and high enthalpy (or temperature) resources, according to criteria that are generally based on the heat content of the fluids and their potential forms of utilization.
The following classification by Muffler andCataldi, 1978 (Dickson andFanelli, 1990) was used for the purpose of this project..
The uses of geothermal resources depend on their heat content. High-temperature geothermal resources are mainly used for electricity production; medium-temperature resources are also used for electricity production in binary units and direct heating, while low-temperature resources are mainly for direct heating (M. Mburu, 2009).
Going by the classification by Muffler and Cataldi, our geothermal fluid temperature, 188°C is a high-temperature geothermal resource.

Geothermal power plants
This refers to plants built around the geothermal reservoirs for electricity generation.
The process generally involves drilling a well into the geothermal reservoir and allowing the geothermal fluid to either flow naturally to the surface or use a pump to cause the upward flow.
There are different ways of utilizing this steam for power generation, and each process depends on the energy content of the fluid.

Types of Geothermal Plants
There are three main designs that a geothermal power plant can take, these are The dry steam power plant is suitable where the geothermal steam is not mixed with water.
Our geothermal fluid for this project is assessed to be a mixture of hot water and steam.

Figure2.3 Dry steam power plant
Interesting facts about Dry steam plants: • About 6 percent of the energy used in northern California is produced at 28 dry steam reservoir plants found at The Geysers dry steam fields in northern California.
• At peak production, these dry steam geothermal power plants are the world's largest single source of geothermal power producing up to 2,000 megawatts of electricity per hour.
• That is about twice the amount of electricity a large nuclear power plant can produce.

Flash steam power plants
The i.

Single flash steam power plant
This design was chosen for this project. In a single flash steam power plant, the superheated mixture of steam and liquid water is passed through only one low-pressure flash separator. As the pressure is lowered in the flash drum, part of the fluid vaporizes and flashes into steam.
The steam is sent to the turbine for power generation and the liquid is re-injected into the geothermal reservoir (R. Dippo, 2011).
In this study, since the liquid from the flash process has a lot of heat content, this project proposes that further heat energy should be extracted for food preservation before re-injection.
The steam that drives the turbine is cooled and condensed to liquid in a cooling tower before re-injection into the geothermal reservoir.

Figure2.4 Schematic of a single flash power plant
ii.

Double flash steam power plant
The Double-flash steam power plant is a more efficient but complicated design than the single flash plant. For the same geothermal fluid conditions, the double flash process may produce 15-25% more power than a single flash system. The increase in efficiency, however, comes at a higher initial capital cost since the geothermal system is far more complex (R. Dippo, 2011).
The double flash process and the single flash process operate under the same principle; the difference lies in how the hot water from the flash is treated. In a double flash process, the hot water is sent to a second flash drum to generate additional steam at a lower pressure before reinjection. The additional steam from the second flash drum is also sent to the turbine for power generation. For more efficient operation, the turbine in this case should be able to effectively utilize the lower pressure steam at an appropriate stage. Another option is to use two separate turbines.

Binary cycle power plants
Compared to the other two technologies we've discussed; binary cycle power plants are the latest. This process effectively utilizes a lower geothermal resource. In a binary power plant, moderate to low-temperature geothermal fluid is passed through a heat exchanger to heat a secondary working fluid that generally has a lower boiling point than water.
The geothermal fluid is re-injected into the geothermal reservoir and the heated working fluid is then passed through a turbine for power generation (R. Dippo, 2011).
Examples of working fluids commonly used in binary cycles are ammonia/water mixtures and hydrocarbons. A schematic illustration of the binary power plant is shown below.

Advantages of the binary cycle process
The binary cycle process offers more flexibility to manage the unique thermodynamics and chemical properties of geothermal resources. Some advantages associated with the binary cycle process include: • The working fluid is carefully selected to minimize the degradation of the system's machinery. The less equipment that a geo-fluid comes into contact with will minimize the amount of equipment that will be damaged by scaling, corrosion and abrasion over the lifetime of a geothermal power plant. A working fluid that is compatible with the wetted surface of the metal surface extends the lifetime of the plant equipment.

•
The lower boiling points of most applicable working fluids allow for the effective utilization of low-temperature geothermal resources.

•
There is zero to low emission of harmful gases to the surrounding environment as the used geothermal fluid from the heat exchanger is reinjected to the geothermal reservoir in a closed loop.
• After the working fluid drives the turbine, it is cooled and re-circulated to do its job over and over again.

MATERIAL BALANCE
The following section is the material balance of unit operations in a single flash geothermal power plant to be sited in Ikogosi warm spring, Ekiti State, Nigeria. The thermodynamic properties (temperature and pressure) of the geothermal fluid gotten at a depth of approximately 2km -3km is estimated at 187.96°C and 12 respectively.

Material balance for Flash Tank:
Key assumptions: i.
That there was about 70% pressure reduction in the flash tank from 12bar to 4bar.
ii. A basis of 75,000 ℎ ( . 3,125 ℎ ) was chosen as inlet flow rate to the flash tank. iii.
Operating conditions at flash tank inlet: Temperature and pressure of geothermal fluid are 187.96°C and 12 respectively.
iv. All flowrates are in mass.

v. Enthalpy equivalents are obtained using s thermodynamic steam table
More details about the mathematical workings are shown in the appendix section.

Steam quality
The percentage of the fluid that separates or flashes into steam in a flash tank when pressure is lowered can be calculated as thus: Where: • : the enthalpy of the inlet hot water and steam mixture from the geothermal well that is delivered to the flash separator. • The feed stream is designated F, • The steam mass flowrate as S • And the bottom stream as W.
The bottom streamflow can be calculated by: Our flash steam tank balance now looks like this.

Material balance for Turbine
The turbine inlet enthalpy is a function of steam inlet pressure i. Operating conditions at inlet: ℎ1 = 2737.63 / specific enthalpy of steam @ 4 . ii.
Operating conditions at outlet: specific enthalpy of steam @ 0.1

Where:
• ℎ1 = the enthalpy of the steam at inlet conditions

ENERGY BALANCE:
In addition to the assumptions made during the material balance, we can add; i. That the loss of heat in the transfer lines from flash tank to turbine is negligible. ii.
The enthalpy from the reservoir to the flash tank remained constant Note: Mass flowrates are represented in kg/s by multiplying flowrates in / / /

Energy balance at the flash tank:
The pressure and temperature operating conditions are maintained.
Note: Energy flow is a function of enthalpy. i.

Energy flow at the inlet is calculated by:
= ℎ 1 ×

Where:
• ℎ 1 is the inlet enthalpy to the flash tank Convert flow rate at the inlet from tonnes/hr to kilogram/second:

= ×
Where: • is referred to as latent heat of vaporization. It was obtained at the flash tank pressure (4 bar).
Convert flow rate at the inlet from tonnes/hr to kilogram/second: : .
= . . = / If we perform our balance, we should expect the energies in the output streams to be equal to the energy in the input stream i.e = + but that is usually not the case for energy systems.
In reality, no energy system has 100% efficiency. And because energy systems are irreversible, part of the energy is lost during the process. The energy lost is referred to as lost work.
Performing our balance to get the lost work: First, let us get the work that should have been done in an ideal situation.
Ideally That is because heat cannot fully be converted to work and will always be lost to some degree.
161 / is referred to actual work done by the system Therefore, the lost work can be obtained by: Brine 476

Turbine Output Work
The turbine output energy is a function of inlet pressure to the turbine Assuming that: i. There was no enthalpy change from the flash separator to the turbine inlet ii.
The steam was delivered to the turbine at a pressure of exactly 4 r Work 11.5 The turbine output energy can be calculated by: Where: • ℎ1 ℎ2 are inlet and outlet enthalpies respectively ~7.14 % The above efficiency is typical of a power generation systems from steam plants.
This means up to 90% of the energy of the geothermal steam is discarded as waste heat. This presents a strong argument here for the use of geothermal resources for direct applications such as district heating (for example food preservation by drying) instead of power generation, when economically feasible. (Radmehr, 2005)

Generator Power Output
Using a generator with 98% efficiency, the power output can be calculated. P = E × G 1 = 1 P capacity of our geothermal plant = 11.3 11.27 / 11.5 /

Uses of Geothermal Energy
Geothermal energy can be utilized for electricity generation, and direct heat purposes in form of geothermal heat pumps (for cooling and heating buildings), in agriculture for food preservation, and so on.
Our focus in this project is to emphasize how geothermal energy can be used for safe power generation as well as effective food preservation. The numbers are alarming on the amount of food that is being wasted due to poor preservation methods. What makes it worse is that a large population of the world faces food shortages and scarcity (FAO, 2015). Geothermal energy development can contribute to the preservation of food and mitigation of food waste.

Countries
For four decades, Iceland has shown the world how to enhance food security by food preservation utilizing the heat stored in abundance in our earth (FAO, 2015). As the number of people to feed continues to increase in leaps and bounds globally, it's time to seek guidance from the pioneering developments and technology in Iceland to secure food that is currently wasted due to a lack of effective storage or preservation methods. When applied on a global scale, drying technology has the potential to increase the availability of food by up to 20 percent. No other single method holds such potential. (FAO, 2015).

Figure: Tomatoes loaded on drying racks
Low to moderate temperature geothermal resources including waste heat and cascading from power plants are the sources of geothermal energy for agricultural and agro-industrial uses.

Geothermal Energy in Food Drying
The thermal drying process is popular among many agricultural and food industries as an effective food preservation method (Senadeera et al. 2005). Drying is an important food preservation method that has the potential to avoid wastage and ensure the availability of nutritious food all year round, and during droughts.
Examples of food preserved by drying include (FAO, 2015):

•
Tomatoes are dried using geothermal hot water at 59 °C.
• Chilli and garlic drying in Thailand

Sterilization processes
In a wide range of industries such as meat and fish canning, sterilization is an important step to eliminate the growth of bacteria, particularly Clostridium botulinum.
The recommended temperature for killing C. botulinum bacteria is 121 ºC for three (3) minutes.
Geothermal steam (or hot water at 105-120 ºC) is commonly used to sterilize equipment in the food processing, canning, and bottling industries (Lund, 1996).

Milk pasteurization
When milk is collected, enzyme activity and growth of microorganisms especially under unhygienic production and storage conditions at ambient temperature cause the milk quality to deteriorate rapidly.
To avoid microbial growth and enzyme activity, milk production and processing must be done by pasteurization process or ultra-high temperature (UHT) process (Perko, 2011;Torkar and Golc Teger, 2008).
The temperature of the inlet geothermal hot water used in the pasteurization process is about 87 ºC and the outlet is 77 ºC (Lund, 1997). This is a low-temperature geothermal resource that can be obtained directly from simple production wells or wastewater and cascading from geothermal power plants.

Greenhouses
For the past two decades and a half, the most common use of geothermal energy in agriculture has been in greenhouses. Geothermal heat is currently used especially in many European countries to produce vegetables, fruits, and flowers on a commercial scale in all seasons.
The use of geothermal energy to heat greenhouses has several benefits (Popovski and Vasilevska, 2003): • Geothermal energy is relatively cheaper than energy from other available sources.
• Geothermal heating systems are relatively simple to install and maintain.
• Greenhouses account for a large share of agriculture's total consumption of low enthalpy energy.
• Greenhouse production areas are often sited close to low-enthalpy geothermal reservoirs.
• The use of geothermal energy improves the efficiency of food production by making use of locally available energy sources.
From the food preservation examples above, low enthalpy geothermal energy that cannot be used to efficiently generate electricity can be channeled to nearby food preservation plants.
This is crucial given the alarming rate of population growth and the threat to food security in many regions of the world. Geothermal energy can be safely harnessed to combat this challenge. When applied on a global scale, drying technology has the potential to increase the availability of food by up to 20 percent. No other single method holds such potential. (Olafur, 2015).

Challenges Facing Sustainable Geothermal Energy Development
The main constraints and challenges associated with the use of geothermal energy for electricity generation, in agriculture for food preservation are policy and regulatory barriers; technical barriers; and financial barriers.

i.) Policy and regulatory barriers
Government policies and legislation are key factors in creating an enabling environment for the development of geothermal energy.
It wasn't until recently in nations like Japan that government policies were relaxed to promote the development of geothermal energy. For example, the feed-in-tariff for geothermal energy was made higher than other sources of renewable energy, and the 10 years exploratory phase was reduced to 8 years. Also, in Japan, the budget for the upcoming FY 2022 was increased for the exploration and development of geothermal in the nation (thinkgeoenergy, 2021). This was done to aid Japan's goal of having up to 42% of renewable energy by 2030.
Notwithstanding, few governments in favorable geothermal locations still lack clear policies that promote the development of geothermal energy. Also, budgetary allocations to geothermal energy research and development tend to be low in developing countries.

Nigerians don't involve Geothermal Energy Professionals when carrying out Renewable
Energy Plans and Policies hence little support exists on the subject in Nigeria.

ii.) Technical barriers
Although geothermal energy development has been on for over a decade now, there is still a relatively low amount of technical expertise in the industry. Being a somewhat new and unknown technology, technical expertise is crucial for developing geothermal systems safely.
A critical mass of policy analysts, economic managers, engineers, qualified personnel, and other professionals is required for safe geothermal energy development. This is lacking in most developing countries (Kombe & Muguthu, 2018).

iii.) Financial barriers
The most popular constraint to geothermal energy investment and development especially in resource-constrained economies is the high upfront cost of geothermal energy technologies.
As earlier noted, most developing countries with abundant geothermal resources lack the financial resources to even explore the viability of the development of geothermal systems (Kombe & Muguthu, 2018). However, geothermal energy is a clean energy source that reduces carbon emissions significantly. To support the global clean energy efforts in the face of climate change, investment in geothermal energy will be a small cost compared to the cost of dealing with the consequences of climate change.
Governments can play a very important role in initiating geothermal projects by financing the early phases (i.e., exploration and appraisal). However, this requires the right policy environment, which is lacking in most cases.

CONCLUSION
The result from our enthalpy evaluation shows that a single flash power plant with a capacity of approximately 11 MW can be installed in Ikogosi, Ekiti State. A geothermal power plant with a capacity of 5 MW can power up to 50,000 homes in Kenya (Future Planet, 2021).
The scope of this project does not cover the cost analysis from exploration and development to installation of the power plant, therefore we cannot assess the economic viability of the project. The depth of drill, enthalpy of fluid, and amount of geothermal resource were based on estimation and secondary sources. This result was gotten based on estimation. No site visits were done to measure temperature and information was based on secondary sources. The evaluation result does not take into account heat energy losses at various stages during the process.
The present study argues that if an 11 MW power capacity can be obtained from a single drilled well in Ikogosi, then there is immense potential for the sustainable development of geothermal energy in Nigeria.
This particular project is only a prelude to a robust model that will be developed based on data from primary sources of enthalpy of geothermal fluid and plant design.
Armed with this information, key stakeholders involved in the development of geothermal energy across the world will have the insight to project feasibility studies.