Materials and Technologies for a Green Environment

Renewable Energy Generation Using a Novel Geothermal-Solar Hybrid Power Plant Using RORC

K. C. Ramya¹, *, S. Sheeba Rani¹, S. Sivaranjani¹, R. Vinoth Kumar²

¹ Department of Electrical and Electronics Engineering, Sri Krishna College of Engineering and Technology, Coimbatore, India

² Department of Electrical and Electronics Engineering, New Horizon College of Engineering, Bengaluru, India

Abstract

A recent survey of energy consumption indicates that there has been exponential growth in the need for renewable energy and also for curbing the growth of fossil fuel reserves. To meet this future need, renewable energy sources are being explored. In this paper, we have proposed a Recuperative Organic Rankine Cycle that operates in conjunction with air-cooled condensers. Solar energy is said to be an energy source that varies periodically, unlike geothermal energy which is available round the clock, to generate electricity continuously. Hence it is a highly recommended source to meet the growing demands for electricity globally. A major contribution to geothermal power development is the progress in Organic Rankine Cycles. These plants are best known for their ability to curb harmful gas emissions, especially that of non-condensable gases. There is a significant growth in geothermal power owing to the ORC (Organic Ranking Cycle) power units that are implemented. In this methodology, the working fluid of ORC is made to go through an evaporator where a hot turbine is used to heat the liquid. In this process, the temperature of the preheated liquid is further increased with the aid of solar energy. This heat generated thus is further converted into electricity when the turbine unit causes the expansion of the fluid. Finally, an air-cooled condenser is used to condense the final exhaust of the turbine. Combining the two powerful forms of renewable energy (solar and geothermal), it is possible to generate power in such a way that the need for power begins to drop from its peak that it has achieved already. The simulated results define the decline in energy consumption of condensers based on the minimum heat transfer area of the condenser as well as the minimum power consumption of the fans.

Keywords: Duct curve, Geothermal-solar power generation, Hybrid power plant, Renewable energy, Recuperative Organic Ranking Cycle.

* Corresponding author K.C. Ramya: Department of Electrical and Electronics Engineering, Sri Krishna College of Engineering and Technology, Coimbatore, India; Email: [email protected]

1. Introduction

1.1. Renewable Energy

As the planet becomes more populated, the demand for energy also increases subsequently at a very fast pace. In recent times, this demand for energy is met with the help of fuels based on fossil. However, the use of these carbon-based fuels resulted in air quality deterioration, increased pollution and global warming. In May 2018, a report by WHO stated that about 90% of people all over the world are breathing polluted air. Because of this, there is a need for renewable energy sources and governments across the globe are investing heavily in this aspect. The Clean Energy future is made possible through the invention of renewable energy. In the beginning, energy was highly dependent on fossil fuels. However, due to the emission of carbon and other impurities that had harmful effects on the environment, there has been much need for cleaner energy usage. The introduction of wind and solar generation was a ground breaking invention that paved the way to a better and healthier atmosphere. This was further improved by the introduction of renewable energy. Renewable energy is commonly referred to as clean energy as it is generated from natural processes or sources. Though the use of natural sources as energy is thought of to be a novel concept, we have been using them for various aspects like transportation, heating, and so on. The sun has been used since ancient times to keep us warm during the day and further to kindle fire and provide warmth during the night. Similarly, windmills have been used to grind grain and wind has been used for a long period of time to sail boats.

However, during the past few centuries, the invention of many energy sources such as fracking gas and coal has led to a more polluted environment. These types of energy sources are known as non-renewable sources. These types of energy take a longer time to replenish and are available for only a limited period of time. Most non-renewable energy sources will have a harmful impact on human health and will also cause harm to the environment. Some known impacts are: drilling of oil performed using fracking might result in water pollution and cause earthquake, while a coal power plant will make the air smell foul.

The following are some of the renewable energy sources that are being used by us:

• Wind Energy: One of the most ancient ways of producing electricity is the use of wind as a source of energy. The turbines are built high and as the blades of the turbine turn, electricity is produced using an electric generator. It is also one of the cheapest forms of energy and accounts for about 23% of the total energy produced.

• Solar Energy: Solar energy has been used for a long period of time by us for various purposes like drying fruits, staying warm, and growing crops. Energy from the sun is used in many ways to power devices, and to warm water. Solar cells, also known as photovoltaic cells are built of a base metal plate of either steel or aluminum. These cells convert sunlight into electricity directly.

• Geothermal Energy: As the radioactive particles begin to decay slowly, in rocks at the earth’s centre, they will be as hot as the surface of the sun, resulting in a natural hot spring. An underground well is dug up that acts as a hydrothermal resource, which can be used to create electricity.

• Biomass Energy: Trees, waste wood, crops, carcasses of animals, and withered plants are used to make biomass. Chemical energy is liberated in the form of heat from burning biomass. This in turn can be used to generate electricity. Though this form of energy is considered to be a cleaner and greener alternative, it still produces a large amount of carbon emissions.

• Hydroelectric Power: The source of hydroelectric power is water which descends rapidly from a higher end to the foot of the river. This force of water can be converted into electricity.

• Ocean: Though the production of tidal energy through the waves of the ocean is currently in an experimental phase, it still remains a good option to harness power.

According to energy statistics 2020, a report released by the Ministry of Statistics and Programme Implementation, Government of India, there is an abrupt increase in the use of renewable resources of energy in India. The report states that about 28.18 GW of electricity is generated using solar power, which is a 12.23% increase from the previous year. This is also reflected in decreasing the cost of solar electricity, thereby attracting more people to use the same.

Estimated potential of renewable energy sources is tabulated in Table 1. Here Jammu & Kashmir, Maharashtra and Rajasthan are the states that contribute highly to using renewable energy in India.

Table 1 Usage of Renewable Sources in India.

A survey of the renewable energy potential across the different states of India indicates that the major source of renewable energy used is solar energy followed closely by wind power as shown in Fig. (1). However, there are many more sources of energy that are still progressing and the use of hybrid geothermal and solar energy is still in the experimentation process.

Fig. (1))

Renewable Energy Potential.

Solar energy is said to be an energy source that varies periodically unlike geothermal energy which is available round the year to generate electricity continuously. Hence it is a highly recommended source to meet the growing demand for electricity globally. Moreover, the cost of generating electricity from the geothermal resource is reasonable, making it a good choice for the production mix. Except in certain volcanic countries, the use of high enthalpy, and high-temperature geothermal resources have been explored vastly all over the world. A major contribution to geothermal power development is the progress in Organic Rankine Cycles. These plants are best known for their ability to control harmful gas emissions, especially that of non-condensable gases. There is a significant growth in geothermal power owing to the ORC (Organic Ranking Cycle) power units that are being implemented.

As the accumulation of atmospheric carbon dioxide increases, there is much need to initiate the production of electricity using non-fossil energy sources. Hence renewable and nuclear energy sources are experimented with to provide a large part of global electric power in the days to come. However, solar and wind sources which are renewable are not available on a regular continuous basis and hence will not be able to meet the demand for fluctuating electric power. Fig (2) represents the duck curves or U-shaped demand curves which indicate the imbalance in the supply and demand of electricity because of renewable energy sources’ higher penetration. When this occurs, there is a possibility for time-shift and augmentation of electric power demand to increase such that it causes an imbalance. This figure represents the power demand during a peak summer afternoon in India. The demand for electricity is currently being met by electricity sources like wind, nuclear, coal, hydro and natural gas power plants along with a small input from the solar power plant.

Fig. (2). also represents the amount of electricity required when the electricity generated from solar energy contributes to 15%, 25% and 35% of the total energy. This indicates that during the day time, solar energy can be used to the maximum while energy from other sources can be minimized so that during the night when there is no solar power available, the electricity demand can be met by other sources of energy. This effect will cause a huge impact on the already existing coal and nuclear power plant units which will find it difficult to adjust their supply of electricity based on the requirement, making it almost impossible and inefficient.

Fig. (2))

Duct Curve.

To address this duck curve issue, the optimal solution would be to take advantage of geothermal energy which is available continuously and solar energy which is available during the daytime when the demand for electric power is high. Moreover, the use of hybrid geothermal and solar power plants will prove to be an excellent method to generate high electricity to meet the peak electric demand. Recently a patent was submitted introducing a system that uses solar power during the time that it was available and further uses geothermal energy during the other time to balance the demand. However, the demand for electricity in the morning hours is not as high as at other times, and this results in duck curves.

Ever since the invention of geothermal power generation, it has been experimented in hybrid combination with a number of other power plants because of the many advantages that it has to offer.

Geothermal energy has a number of advantages over other types of renewable energy sources. They can operate at a lower cost when compared with other energy systems. Soil is said to have better heating capacity when compared to air which is proven by the fact that the temperature of soil that is deep inside the ground is quite low. As the temperature of the soil at a depth of 20m or more is constant, it results in the earth being cool during summer and warm during winter. Hence in order to control the temperature of air, 3 different types of ground heat exchangers are designed. The types of configuration will vary depending on the available soil and space considerations (Fig. 3).

Fig. (3))

(a) Vertical configuration (b) Horizontal configuration.

• Spiral Configuration: This type of configuration is a combination of both vertical and horizontal types for optimal thermal performance.

• Horizontal Configuration: The depth of this configuration is less than 1.5 m.

• Vertical Configuration: The depth of this configuration is greater than 50 m.

In general, a grout material made up of a mixture of bentonite and sand or cement surrounds the ground heat exchanger. This is done in order to improve heat transfer and protect groundwater. The major reason for using geothermal energy is because of its low operating cost as well as its minimal effect on the environment in comparison to the other operating plants.

This chapter is categorized into the following sections:

• Section 2 gives a brief outline of all power plants and their implementation and usage since their introduction.

• Section 3 describes the proposed unit that uses Organic Rankine Cycle that operates in conjunction with air-cooled condensers.

• The results are recorded and the comparative charts are drawn explaining the implementation of the system along with appropriate graphical support.

• A conclusion is drawn and a brief suggestion for future work is also recommended.

2. Literature Survey

All energy sources have their own weaknesses. Because of this, a proper combination of renewable energy sources will go a long way in rectifying this problem and further serve to be a good source. One of the recent hybrid technologies is combining geothermal and solar energy to provide a new source of energy which is more advantageous than the two used individually. Geothermal energy is important as it is not restricted by constraints like time, season, and condition and can be used without any interruption. But, since the temperature of the liquids released is very low, it will result in decreasing the plant’s efficiency while making it a more expensive option. On the other hand, solar energy has been used by us for a very long time and is a clean and free source of energy. The drawback is the cost price at which solar energy devices are sold in the market. This has boosted the usage of energy sources like natural gas and crude oil which are available at cheaper rates.

Another major drawback of using solar energy is that the sun is not always shining brightly and is greatly dependent on factors like the position in the sky, climatic changes and time. This will have a great impact on the output of energy which will change regularly. Moreover, the use of a single energy source will require high investment in constructing the plant as well as an increase in the requirement of the area. Taking these aspects into consideration, it is found that a combination of geothermal energy and solar energy will prove to be an excellent source of renewable energy in terms of efficiency and economy. Moreover, it is also possible to get rid of the disadvantages of solar and geothermal energy when they are operated separately, and also this combination can convert the heat produced by means of a thermodynamic cycle. Some of the investigations made in studies are outlined in this literature survey.

A study [1] experimented with geothermal fluid by applying it to double and single evaporator configurations. He observed that there were both advantages and disadvantages while heating geothermal fluids directly. The advantage is that it resulted in reducing the heat exchangers and also increased the rate at which steam was flowing in the solar collectors. This also decreased the cost price dramatically. However, the disadvantage is that there is high possibility that salt could not be deposited in the fluid. Hence, in another study [2], Tempesti et al. used the simulation of geothermal and solar energy simultaneously and observed their output in work production and micro heat environment. In this work, the organic fluid that is used is initially pre-heated before it is sent to the evaporator. To superheat this fluid, solar fluid is given for evaporation.

Both geothermal and solar resources are available in places where water is limited. This is especially the case when the level of groundwater in a particular area is less due to excessive usage for agricultural and urban applications. This is also the major reason why air conditioners are preferred as they do not require water and take the humidity from the air. In a study [3] Greenhut analyzed the ORC configuration