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Design of a Standalone Energy System

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Published: Mon, 26 Feb 2018

Design of a Standalone Energy System and its Comparison with Grid Connected System in Pakistan

Abstract

During last few decades the investment and interest in the development of solar energy is being increased. PV is becoming more and more popular in those countries which can bear large investment cost and can offer economic incentives to the investors and almost in all those areas of world which are looking for a clean environment and working hard for green energy projects. This thesis work covers the design of a standalone power system and comparison of this designed system with already existing grid connected system in Pakpattan, Pakistan. After preliminary designing, an estimation of system and different components sizing was made and on the basis of this sizing, different energy sources like PV, wind and diesel generator were tested. Also if PV can fulfill the daily load or a backup diesel generator would be needed. For preliminary designing, Sandia sheets and PVSYST were used but for the actual designing, HOMER was used because HOMER is more accurate and precise tool for designing standalone power systems. Simulations were carried out and the cost of energy per kWh was 0.435$. Total initial capital cost was 6517$. Further economic evaluation was carried out and was compared with already existing grid connected system. On pure economic terms, this system does not seem to be economical feasible but when different recommendations were taken into account like government subsidy and carbon credit then this system was economically feasible over the 25 years which is the life of this project.

1. Project background and Introduction

1.1 Introduction

The project with respect to this thesis is to design a standalone energy system and to compare it with already existing grid connected system. This system might be PV alone system, PV wind hybrid system, PV Diesel hybrid system or just diesel generator system. This system is designed for a Jamia Masjid (Islamic center) in Pakpattan, Pakistan. The exact site of the project is a colony of Pakpattan which is in the south-west of the city Pakpattan. The Pakpattan city is situated around 161 Km south west to Lahore. The climate and weather data is almost same for Lahore and Pakpattan. Furthermore it will be first project of its own nature in this area and it will help to design the systems for the residential areas by which a common person can get benefits and get rid of power failures.

“Pakistan is located between latitude 24 and 37 degrees North and longitude 62 and 75 degrees East. Pakistan has Afghanistan in the north-west, Iran on west boarder, India is on the east, China in the north and the Arabian Sea is on the south” [1]. Pakistan is ageographical centre of the Asian Continent because it builds a type of bridge between Far East and Middle East; also it has a continental type of climate which can be characterized by extreme variations of temperature. Generally the climate of Pakistan is arid, means very high temperature in summer and low temperatures in winter. High altitudes change the climate in the freezing northern mountains which are covered by heavy snow.

There is little rainfall. There are some differences exist distinctly in various locations, e.g. the coastal line along Arabian Sea is usually under warm conditions, whereas the Karakoram mountain range and some other mountains of far north are so cold, completely frozen and covered with snow that these are only visible and accessible by some international world-class climbers for a couple of months of May and June of each year. The variation of daily temperature could be 11 0C to 17 0C but in winters the minimum mean temperature is about 4 0C in January.

Pakistan has tremendous recourses of energy but unfortunately due to mismanagement out of 170 million population just 65-70% has access to electricity. Demand is more than supply of energy to residential and industrial sector. Currently Pakistan is facing 3000 MW of power storage and it is expected that in year 2010 the demand will exceed supply by 5500 MW. The current power and electricity demand and supply gap shows that there is a big need to increases the current power generation capacity in Pakistan. [1]

1.2 Aim of the thesis

The main aim of the project is to design an independent and stand alone energy system for an Islamic center in Pakpattan, Pakistan. During the attempt to design this independent energy system, there could be sub aims as well, which are the procedures and details of the design are presented with step by step. There are main following aims which are supposed to be fulfilled by this thesis.

* To go through the core knowledge of the designing process.

* Obtaining data for the boundary conditions such as load, solar radiation wind data, available components, cost for fuel, electricity and components.

* Sizing of the system.

* Optimizing the cost.

* To get familiar with different tools used for designing and make selection between them.

* Beyond from bookish knowledge, get to know some real and practical aspects of different PV systems and projects.

The body and structure of the thesis is mainly divided into four phases.

* Introductory part (Abstract, contents)

* Main part (Introduction, Background, Description, Analysis, Calculation of the primary load for this specific project. Boundary conditions and design parameters. Economical look on existed grid connected system and Comparative study between this newly design system and already existing grid connected system in terms of economics, Results and Conclusion )

* Reference part (Reference list and Appendices)

1.3 Method

As the aim of this thesis project is to design an independent energy system for Islamic center and then compare it with existed grid connected system. The main interest is to provide electricity for Islamic center without shortfall. To do so, first of all literature survey was carried out and was gone through different designing procedures for standalone independent systems. Different tools were also tried and testified, so that an appropriate design could be chosen. In this way Scandia sheets, PVSYST and HOMER were considered especially because these tools were used previously to design such systems. First of all load demand was calculated. Why these three tools were used, the reason is that Scandia sheets are very helpful in sizing different components of standalone energy system while PVSYST has very big database of PV modules, batteries, inverters and diesel generators in itself. After that when the sizing was done then specific components were chosen for this stand alone system. In HOMER there are different options to check weather PV, PV wind hybrid or PV diesel hybrid system is feasible

After designing the system, its economic analysis was carried out using different economic parameters like pay back periods, net present value, benefit to cost ratio and internal rate of return.

1.4 Energy profile of Pakistan

The energy profile of Pakistan is inadequate and there are always short falls of energy (electricity) especially during summer. Pakistan needs around 14,000 to 15,000 MW electricity everyday to meet all residential and industrial demands. But Pakistan can produce around 11,500 MW, so it means there is around 3000 MW to 4000 MW short fall. The reasons for this deficiency are limited fossil fuel resources, weak economy and mismanagement of the available energy sources. In 1.1, an overview of primary energy supplies in Pakistan is presented in MTOE (TOE: ton of oil equivalent. It is an energy unit which is equal to the energy of burning of 1 ton of crude oil which is about 42 GJ) [2]

From 1.1 it is clear that energy supply of Pakistan is highly dependent on Oil and Gas, both contribute more than 79% of total primary energy supplied. The other sources like hydro- electricity, coal, nuclear electricity and imported electricity contribute about 21% of the total share. Pakistan has been growing in agricultural and industrial sector during last decade and that’s why energy demand is being increased. As population and industry is growing, the daily demand will increase up to 20,000 MW in 2010. Natural gas has played an important role to meet the energy needs in recent years. But Pakistan needs to expand its energy resource on permanent bases. In order to overcome this energy shortage, Pakistan needs to develop its indigenous energy resources such as hydropower, wind and solar energy. Pakistan is one of the highest solar insulation areas of the world. [3]

Here are the main sources of energy in Pakistan right now;

* Natural Gas

* LPG

* CNG

* Oil

* Coal

* Nuclear Energy

1.5 Renewable energy sources in Pakistan

Pakistan is situated in such a special geographic location that it is blessed with abundant and inexhaustible Renewable Energy (RE) resources. These resources can play an effective and considerable role for the contribution towards energy security of Pakistan. When we look into current world energy scenario in general and energy scenario of Pakistan in particular, the development and adoption of RE technologies makes better sense. Government policies and objectives to develop RE technology are also matching with this sense so that the share of RE in overall energy mix should be at least 5% by the year 2030. Solar energy has 2.9 Million MW potential and wind energy has around 0.346 Million MW while Mini & Small Hydel has 2,000 MW. [4]

There is a significant potential of wind energy in Pakistan especially in the coastal belt of Baluchistan and Sindh, and also in deserts of Sindh and Punjab. However this renewable energy source has not been utilized. “AEDB collected the wind data of all Pakistan from Pakistan Metrological Department and was analyzed. On the basis of this collected data and analysis, it was stated that the coastal belt of Pakistan has a God gifted 60 km wide (Gharo-Keti Bandar) and 180 km long (up to Hyderabad) wind corridor” [4]. This wind corridor has the potential to generate 50,000 MW of electricity. AEDB has done other different surveys in Gharo and Jhimpir regions and some coastal area of Baluchistan. After these surveys it is concluded that in the south region most of the remote villages can be easily electrified through micro wind turbines. Furthermore it is estimated that in Baluchistan Sindh and Northern areas more than 5000 villages can be electrified through wind energy. [4]. In 1.2a and 1.2b wind and solar maps for Pakistan are shown.

Sincere efforts and aggressive lobbying has been done by AEDB with national and international investors to invest and to make them realize the tremendous potentials of RE. AEBD is in negotiations with international companies to set up their business in Pakistan. However large wind mills have not been installed yet but 30 wind mills for water pumping have been installed on experimental basis in different parts of Baluchistan and Sindh. In southern coastal areas of Pakistan remote villages are currently electrified with energy and so far more than 17 villages have been electrified using micro wind turbines. Around 95% of total electricity generation is from hydropower in Pakistan. But during hotter months of summer, it cannot meet the energy requirements due to less productive. Also 70% of population lives in around 50,000 villages. Many of these villages are very far from the main transmission lines and also it is not economically viable to connect these small villages to the main grid due to their small population. On the other hand solar energy has excellent and significant potential. Pakistan is one of those countries which receive solar radiations at high level throughout the year. Every day it receives an average of about 19 MJ/m2 of solar energy. Studies have been already done and solar systems have been developed and tested. [4]

1.6 Potential of PV in Pakistan

The location of Pakistan is very ideal to take advantage of solar energy as a source of energy because Pakistan is in the Sun Belt region. Solar energy is available abundantly and widely distributed all around the country. Following shows solar insolation map for Pakistan. The map shows around 200-250 W/m2 per day. The Baluchistan province is very rich in solar energy. It receives around 19-20 MJ/m2 per day averagely which is equal to 1.93-2.03 MWh/ m2 per day with annual 8-8.5 mean annual sunshine hours. These conditions are ideal for PV and solar energy applications. [5]

Solar energy is very good option for off-grid villages. There are around 75,000 off-grid villages which contains 4 million homes and every home accommodates around 4-5 people. These off-grid villages are situated in the Baluchistan and Frontier Province. AEDB has set a target to electrify a thousand villages via solar technology by the year 2010. In this respect the first contract has been given to the Sehgal electronics group (Pakistan). Each home which is electrified with PV will have around 400W power supply and lead acid batteries for overnight storage. There are also other plans to have local production using PV modules with and estimation of this production is around 3MW/year. [6]

1.6.1 Possible routes for solar PV in Pakistan

The global demand of PV equipment is increasing day by day and due to this fact the prices for PV systems, equipment and electricity has gone down remarkably. PV could be exploited in Pakistan through following two routes. [6]

1.6.1.1 Off-grid or stand-alone sector

Stand-alone or off grid systems generate electricity independently of the utility grid. Stand alone systems can be a very good option for the remote areas and very deep located villages, where the extension of power transmission lines would be more costly. Also it could be implemented in environmentally sensitive areas as parks, remote homes and cabins. In rural areas, it could be used for solar water pumps and farm lighting. [6]

1.6.1.2 Grid-connected sector

Grid-connected PV systems supply extra power when the home system’s power supply is not sufficient to fulfill the load. These systems remove the need of battery bank. In some situation, utilities allow net metering, by which the owner can sell extra power back to the utility. [6]

1.6.2 Current solar energy applications in Pakistan

Both PV and solar thermal have a wide range of applications in Pakistan. Although the scale of utilization and adoption has been very small but it has been utilized for last 25 years in Pakistan. Different applications mainly PV and solar thermal applications are summarized as. [5]

1.6.2.1 Photovoltaics

Eighteen PV stations were built by the government in the early 1980s to electrify different villages the country. The installed capacity was nearly 440 kW but due to the lack of technical knowledge and follow up, these systems could not perform as required. Currently in Pakistan solar energy is being used for telephone exchanges stand alone rural electrification, cathodic protection, highway emergency telephones and vaccine refrigeration in hospitals. In different parts of Baluchistan, about 20 solar water pumps have been installed for drinking purposes by The Public Health Department. The northern and western area of Pakistan are mostly hilly and mountain areas (Hindu Kush-Himalayas, HKH region), which are blessed with a lot of sunshine with 4-6 kWh/m2 daily average solar radiation. Seven solar stations were installed in this region in the late 1980s for lighting by different companies. The total capacity of these systems was 234 kW. They are not in operation now due maintenance problems.

SIEMENS Pakistan has installed many stand alone solar systems in Pakistan. On the Lahore-Islamabad Motorway, it has installed power supply systems for many microwave-link repeater stations and more than 350 emergency call boxes. [5]

1.6.2.2 Solar thermal applications

There are many applications which utilize the heat characteristics of solar energy directly. These applications are very simple, low price and easily to adoptable. These include heating and cooling of residential and commercial buildings, cooking, water heating for domestic and industrial use and drying agricultural products. A brief description of such applications in Pakistan is given here. [5]

1.6.2.3 Solar water heaters

This technology is quite mature in Pakistan but very limited because of its higher capital cost as compared to conventional water heaters which operate on natural gas. But in last couple of years it has started to gain popularity because a number of public sector organizations are working to develop low cost solar water heaters. The prices of natural gas and electricity are increasing day by day, so people are adopting solar water heaters and also private sector has already started the production of such heaters. [5]

1.6.2.4 Solar cooker

Different public sector organizations have been working to develop low cost and efficient design solar cookers. In HKH region of Pakistan, more than 2000 solar cookers are in use. This number is very small. It needs to be more popularized. Pakistan needs to reduce the use of precious forest resources as fuel wood and to replace it with solar cookers. [5]

1.6.2.5 Solar dryers

Solar energy can be very good option for drying agriculture products. By this, we can get very good quality products at much less cost. Northern mountainous areas like Gilgit and Sakardu are very rich in fruit production like apricots which used to be wasted by tons every year. But now solar dryers are being used to dry large quantities of fruit, which is leaving a positive effect on the economy of this area. Different NGOs are working for the popularizing and the use of such dryers. [5]

1.6.2.6 Solar desalination

Drinkable water is unavailable in many parts of Sindh, Baluchistan and southern Punjab and it is very critical issue. Underground water is available but it is highly saline. This saline water is not fit for drinking at all and causes many dangerous diseases such as hypertension. Solar energy can be utilized to convert this available saline water into drinkable water. Solar desalination is very simple, low cost and easy to use. Also it is very easy to adopt. A successful solar desalination project is in operation and it is working very fine and helping to change the life style of the population of Gawader in the Baluchistan province. It consists of 240 stills and each can clean 6000 gallons of seawater per day. [5]

2. Types of PV and PV Hybrid systems

Generally the classification of PV systems is based on their operational and functional requirements, the configuration of their components and the connectivity of the equipment to power sources and electrical loads. PV systems are designed to supply DC and/or AC power and can operate interconnected with utility grid or independent of it. There are classified as;

· Grid-connected Photovoltaic systems

· Stand alone Photovoltaic systems

2.1 Grid-connected PV systems

Grid-connected PV systems are designed to operate parallel with the interconnection of electric utility grid. Power conditioning unit (PCU) or inverter is very basic component in grid-connected PV systems. PV array produces DC power supply and the PCU converts it in to AC power supply which is consistent with the power and voltage requirements of the utility grid. PCU automatically stops the power supply to the grid when utility grid is not energized. [7]

2.2 Stand alone PV systems

Stand alone PV systems or off grid systems are designed to operate independently. Mainly stand alone PV systems are used in isolated and remote areas where the connection with grid or electricity network is not possible. In this type of systems the storage system (batter bank) is very important component and storage is guaranteed by batteries. The design and sizing of such system should be done in a way that it could supply and meet the required load even in bad weather conditions or during winter months. For this surety these systems could be coupled with diesel generator, wind turbine or hydro generator and the systems after this type of coupling is called PV Hybrid systems.

There could be different arrangements and designing methods of PV systems depending on the requirements and type of load to be fulfilled. In direct coupled system, DC power is supplied directly from PV array to DC load and there is no energy storage, that’s why this type of systems can operate in sunlight hours which make them suitable for common applications like water pumps, ventilation fans and small circulation pumps used in solar thermal heating systems.

In many other type of PV stand alone systems battery bank is used for the storage of energy and power inverters which can fulfill AC/DC loads at the same time. [7]

2.3 PV Hybrid systems

PV hybrid systems are composed of combined solar energy with some other electricity producing sources like wind turbines, diesel generators or small hydro plants. The choice of other source of energy to be combined depends on the needs and the geographic situation and other specifications. The hybrid systems are best for the remote areas like islands and remote villages, also for remote applications like communication stations and military installations.

Before go for designing a hybrid system, the specific energy needs and the available energy sources should be known. It means the potential for all available energy sources like solar energy, wind energy and hydro energy must be studied, so that the best combination could be made which can meet the specific energy requirements in best way of economy and availability. [8]

2.4 PV Diesel hybrid systems

In remote areas the electricity has been produced by engine driven generators in the past. For those applications where we need a reliable and stationary generator is required, diesel generators are preferred. Petrol generators may provide electricity at lower cost due to their less frequent use. Engine driven generators are less efficient when driven at light loads (around 40 to 50% of their rated capacity) which can shorten their operating life and it results in high maintenance cost. When the engine is operated at light loads, the combustion temperature goes down which results incomplete combustion and carbon starts to deposit (glazing) on cylinder walls and this leads to premature engine wear and tear.

In recent years, the cost of renewable energy technology has been declined continuously and also the concept of usage of alternative energy is growing day by day. Due to these two factors, the utilization of renewable energy has been increased for remote areas. Typically PV modules with small to medium size wind turbine are being used, but for some locations small hydro electric generators are suitable. In simple words combination of renewable energy sources and conventional energy sources with energy storage (battery bank) makes a Hybrid system which can give reliable and economic electricity supply. If we compare a system only with PV generator with a PV hybrid system, the second one reduces the batter size and improves the reliability of overall power supply. In hybrid system, the renewable energy source and battery bank try to reduce the run time of diesel generator. There is sufficient storage in these systems which allow the load to be shifted. Generally these type of systems are installed in those locations where the logistics and costs of a reliable supply of fuel are not major contributing factors to overall system operation cost. [9]

The displacement type systems are sized to decrease the fuel consumption of diesel generator by 70 to 90% as compared with a diesel battery system, so it relies mainly on renewable energy sources like solar. The engine driven generator still remains in the system to equalize the battery and it provides a backup for those periods when there is low solar input or high load demand. Such systems are installed in those locations where some attractive incentives for the use of renewable energy exist or fuel supplies are costly and unreliable. [9]

Usually the conventional power supplies with diesel in remote areas are not flexible to react to the changes in load demand and varying operating conditions. This results in the compromises on reliability and efficiency. Significant changes in long term and short term load demand could happen as a result of

* Increase or decrease of population;

* Special community events;

* Seasonal change in environmental conditions ( summer, humidity);

* Change of consumer trends (increased use of home appliances)

But renewable energy sources and batteries are modular in nature and can be upgraded without any problem when in future the load demand is increased with time. It means that we do not need to change the whole system. But as far as other components of the systems are concerned, they are different in their nature. For example inverters, battery chargers and PV charge controllers should be in such a way that the future increased demand should not exceed their rated capacity. Power conditioning devices are also inherently modular and they facilitate convenient system upgrade. [9]

2.5 Hybrid System configurations

PV-Diesel hybrid systems produce AC power supply by the combination of PV array with inverter, which can be operated parallel or alternatively with engine driven generator. PV diesel hybrid systems can be classified as

· Series hybrid energy systems

· Switched energy systems

· Parallel energy systems

2.5.1 Series hybrid energy systems

In this configuration, the power generated by generator is rectified first and then converted back to AC supply to fulfill AC demand which incurs much conversion looses. During low electricity demand periods, the diesel generator is powered off and the demand can be fulfilled from PV and stored energy. AC supply reaching to the load is converted from DC by an inverter. In series configuration the system efficiency is low because most systems pass large fraction of produced energy from battery bank which increases the cycling of the battery. [9]

The SOC (state of charge) of the battery and actual load decide whether the diesel generator will operate or not, which depends on power supply from PV and diesel generator, load demand and the batteries are either charged or discharged. Solar controller is used to control such situations which prevent the overcharging of the batteries, when PV supply is more than the load and also the batteries are fully charged. The gain in energy is marginal for a good sized system but we can add a maximum tracking point which can improve the utilization of available PV energy. The system can be operated either in manual or automatic mode. This can be done by adding some extra components in the system. [9]

There are certain merits and demerits of these configurations, they are as below.

Merits

* It has simplified electrical output interface as no switching of AC supply is required between different energy sources.

* The supplied power to the load is not interrupted when diesel generator starts.

* The inverter can produce a square wave, modified square wave or a sine wave depending on application.

Demerits

* The cycling of the battery bank increases which decreases the life time.

* As diesel cannot supply power directly to the load, that’s why system efficiency is low.

* If there is some problem in inverter or in case of its failure, it results in complete loss of power. In this case diesel generator has to supply power directly for emergency purposes.

* The cycling profile requires the large battery bank to limit the depth of discharge.

2.5.2 Switched configurations

It is one of the most common configurations used, but it has some operational limitations. As the name shows, it operates either with diesel generator or inverter as AC source but no parallel operation of the main power generation source is possible. Switched configuration hybrid systems can be operated in manual mode but it makes the system more complex. In order to get rid of this complexity, it is desirable to add some automatic control unit. “This automatic control unit can work by adding appropriate battery voltage sensor and start/stop control unit of diesel generator” [9]. The advantage of this configuration is that the load can be fulfilled directly from diesel generator, which gives overall higher conversion efficiency. In this configuration both PV array and diesel generator can charge the battery. [9]

This configuration has also certain advantages and disadvantages as

Advantages

* As the generator can fulfill the load directly, it improves the efficiency and reduces the fuel consumption.

* The inverter can make a square wave, modified square wave or a sine wave depending on application.

Disadvantages

* Power supply is interrupted time by time as AC power sources are transferred.

2.5.3 Parallel configuration

In this type of system PV and diesel generator supply the load separately when the load demand is low or medium. But when the load demand reaches at peck point, then PV and diesel generator combine and supply that peak load. In this configuration we use a Bi-directional inverter which has two functions

* It can charge battery bank when excess energy is available from diesel generator (rectifier operation).

* DC/AC converter (inverter operation).

The bi directional inverter can also provide “peak shaving” which is defined as “the ability of parallel hybrid energy systems to supply load that exceed the power rating of the engine driven generator of the inverter from combine sources as part of the control strategy when the engine driven generator is overloaded ”[19]

Parallel configuration hybrid systems have also merits and demerits over other systems, like

Merits

* The efficiency of diesel generator could be maximized.

* The maintenance of diesel generator could be minimized.

* The system load could be fulfilled by optional ways.

Demerits

* It should be controlled by automatic control unit in order to make the operation of the system more reliable.

* Operation of the system is much complex for untrained users

2.6 Power conditioning

In PV diesel hybrid energy systems three types of conversion devices are used to control and conditioning of power flow. They are battery charge regulator, inverter and a rectifier. The rectifier or battery charger is included in the system to convert AC power generated by diesel generator to DC voltage. This is done to recharge the battery bank. Series type hybrid systems have always low efficiency because they use two conversions AC/DC and DC/AC. If we assume that both efficiencies of rectification and subsequent inversion of DC voltage are very high, let’s say 90%, it will result a loss of 19% of total power gained in these conversions. This is the reason why parallel and switched cond systems have always more overall system efficiency. In hybrid energy system operation, usually the generator operates at 80% of its rated capacity. In switched or parallel cond energy systems AC power is supplied directly from diesel generator but the excess power which is more than the required load is used to recharge battery bank. This supply of power to battery bank is according to a defined battery charge strategy which takes the battery to high state of charge.

In some modern parallel hybrid systems, a bi directional inverter unit is used. This bi directional inverter unit consists of solar controller, i


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