Reducing emissions using renewable sources for electricity generation in Stewart Island

The electricity is generated in a traditional way extensively using diesel in off-grid areas. However, it increases the consumption of fossil fuels and carbon emissions and therefore it is essential to find alternative solutions to help reaching sustainability goals. The case study is Stewart Island, where the electricity is provided by a distribution network powered by up to five diesel generators at a central diesel power station. Local residents believe that reducing the consumption of diesel and having a renewable source of electricity generation are two of the island’s highest priorities. HOMER PRO software is used to compare current diesel generators system with a hybrid system merging a tidal energy source (predictable) with wind (unpredictable) and diesel (back-up), through a microgrid. Using two wind and four tidal turbines, plus one diesel generator for back-up, produces the highest renewable fraction (61.1%) and reduces the fuel consumption and green gases emissions (GHGs) 199,176 l and 534,061 kg/yr, respectively, in comparison with using diesel station. Its levelized cost at 21 c/kWh is also more cost-effective rather than 23 c/kWh in the present scenario.


Introduction
The World Energy Outlook 2019 clarifies the effect of current decisions on energy systems in the future. If no policy actions are taken, then the current trend of energy demand is anticipated to increase by 1.30% per year until 2040, leading to an increase in emissions [1]. Nowadays with the increasing global energy demand and global climate change, two important solutions have been considered to tackle this issue: (i) reducing the cost of energy and (ii) finding new sources of energy that are environment-friendly [2]. With the negative climate impact of fossil fuel power generation and the requirement of global policy to shift towards a green mix of energy production, the investment in renewable energy is an opportunity in developing countries. However, a poor economy associated with limited income, funds availability, and regulations governing project funding and development are key factors that challenge investors in the energy sector [3]. The New Zealand government has set the country's goal to reduce greenhouse gas emissions by 30% from 2005 levels by 2030 and 80% and more by 2050 [4]. Between 1990 and 2019, Green House Gases (GHG) emissions increased from 41,114-kilo tonnes carbon of dioxide equivalent (kt CO2-e) to 82,318-kt CO2-e [5].
In New Zealand, the government aims for 90% of electricity production to be from renewable sources by 2025 [6], and 100% by 2035 [7]. Currently, 11,349 km of transmission lines distribute electricity from remote areas, where generators are located, all over New Zealand. New Zealand consumes about 38,800 gigawatt-hours (GWh) of electricity per year. In 2017, renewable sources (hydro, geothermal, wind, and solar) supplied 59%, 17%, 5%, and 0.2% of the country's electricity needs, respectively, and thermal sources (coal, diesel, and gas) supplied the other 18.8% [7].
Wind turbines are an essential source of renewable energy and are used by many countries as part of their strategy to reduce their reliance on fossil fuels [8]. A research assessment carried out by Evans et al. [9] claims that wind energy has the "lowest relative greenhouse gas emissions, the least water consumption demands, and the most favourable social impacts" compared to photovoltaic, tidal, geothermal, coal and gas energy. Wind power systems, as a form of renewable energy source, are often integrated with other forms of power-generating systems to form a small power grid, known as a microgrid. The integration of both wind and tidal turbines using the same foundation reduces the cost of electricity [10] and enables a more predictable power generation from two different sources of energy.
In remote areas, isolated from centralized grids, diesel generators are being extensively utilized for power generation in a relatively expensive and quick way [11]. In microgrid design reaching a higher percentage of power from renewable sources or renewable fraction (RF) is an advantage. So, it is necessary to optimize an offshore site to increase electricity from wind and tidal sources as an alternative to diesel generators to reduce emissions.
From a technical point of view, the contributions of the paper are as follows: 1. Explore the use of optimization software as a method how to apply wind and tidal data into microgrid design; 2. Explore the use of optimization software as a method how to apply diesel generators into microgrid design; 3. Propose an alternative system of power generation to reduce fuel consumption and emissions in an off-grid system.
Homer Pro software used for the integration of energy sources and analysing the resources' potential. The HOMER Pro ® microgrid software is used to optimize the off-grid and on-grid designs in both engineering and economic aspects for residential, commercial, and community purposes [12].
The paper is organized as follows: After the Introduction section, Sects. 2, 3 and 4 present some information about generating electricity using diesel generator and site investigated in this study. Sections 5 and 6 compare both microgrid designs, and finally, the results and conclusions are given in Sects. 7 and 8.

Site location
Resources are external parameters of the microgrid system which are required by Homer. The selection of resources depends on the location and affects the power generation and financial viability of microgrid design and hence a careful choice is necessary for a successful analysis [13]. Stewart Island is selected as an off-grid site isolated from national supply grid located in the south of New Zealand as shown in Fig. 1.
The resource types essential for this microgrid design are wind speed (downloaded by Homer from the National Aeronautics and Space Administration (NASA)), and the water speed (provided by NIWA). Tidal current data have  [14] been obtained from a simulation model that MetOcean Solutions Limited (MSL) conducted on an NZ-wide grid with a 0.06°resolution (5.6 × 6.6 km). The simulation nested highresolution domains over Foveaux Strait, located in the north of Stewart Island, is 0.004°; 340 × 450 m. The Princeton Ocean model (POM) was used to hindcast the tidal current in a vertically integrated two-dimensional mode with boundaries provided from the global TPX07.1 solution [17]. This model is the result of solving a set of partial differential equations of spatial variability tides using the Goring simplified continuity equation, the Navier-Stokes equations by eliminating the vertical dimension (integrating velocities over depth), and a finite-element method [15]. The output power from wind and tidal turbines can be calculated by: where V is wind velocity (m/s) and ρ is the density of air for wind and water for tidal turbine (kg/m 3 ) and A is the turbine swept area (m 2 ). The A is the area swept by the rotating blades and is dependent on rotor radius R (m) and is given in Eq. (2) [16].
Last parameter is the power coefficient C P , according to Betz theory, the maximum theoretical value of which is 0.593 [17].
Stewart Island Electrical Supply Authority (SIESA) provides the required electricity (1,260,332 kWh/year) for 408 customers in Stewart Island by operating a distribution network powered by up to five diesel generators at a central power. The generation plant consists of a 4(+ 1) configuration made up of: • 1 × CAT 3406 320 kW prime output diesel generator • 2 × CAT 3408 208 kW prime output diesel generators • 1 × Detroit Diesel Series 60 360 kW prime output generator • The standby generator (+ 1) is a 550 kW Detroit Diesel generator capable of supplying the entire island load when necessary The distribution network on Stewart Island is made up of: • 14 km of 11 kV overhead lines • 1 km of 11 kV underground cable • 6 km of 400 V and 230 V overhead and underground wiring • 43 11 kV/400-230 V transformers [18].

Diesel generators
A diesel generator is a combination of a diesel engine with an electrical generator to generate electrical energy. These generating sets are mostly used in locations without connection to the national power grid. Typically, two types of diesel generators are in market either two poles with 3000 rpm or four poles with 1500 rpm characteristics. The speeds of 50 Hz synchronous diesel generators with 3000 or 1500 rpm can be matched by the following expression: where n is the speed (rpm), f is the frequency, and P is the number of poles of diesel generator. The 3000 rpm machines are simpler in structure with 2-poles and thus result in lower acquisition cost. These are most suitable for light duty applications and appropriate for operation of less than 400 h per year. The 1500-rpm units are 4-pole machines. These are more common for heavy duty applications and are rather more expensive. 4-poles diesel generators are recommended when more than 400 h of operation per year is anticipated. In general, the higher the rpm, the more wear and tear on the bearings, consequently more recurrent maintenance requirements. The lifetime period of the diesel generator varies from 5000 to 50,000 h with an average of 20,000 h depending on the quality of the engine, its proper installation and execution of regular operation and maintenance [19].
In this case, the shaft is directly coupled to a generator and produces electricity as shown in Fig. 2.
The generator has four-stroke compression-ignited engines. The four cycles (intake, compression, power, and exhaust) are completed in every two rotations of the crankshaft producing a power stroke every other turn of the shaft. Diesel engines do not have sparkplugs nor the associated coils and distributors that contribute to the dependability issues for spark ignited engines, rather they rely on the fuel to ignite itself.
The ignition of the fuel is due to an increase in pressure and temperature within the piston chamber. Engines that are directly coupled to generators to produce electricity are generally run at speeds defined by the frequency of the electricity grid being supplied. They run at 1500 revolutions per minute. Other system components that are typical of diesel generators and the system installed include: air filtration, lubrication system, cooling system and an excess heat recovery system. The excess heat recovery system is a major component of the powerhouse. Approximately one third of the energy content of the fuel in a diesel genset is exhausted or radiated from the engine. The recovery of this energy in order to heat water that is pumped to increase the level of base comfort is  [20] not only useful, but absolutely crucial. The powerhouse contains two oil-fired boilers for back-up in case additional heat is necessary; however, the majority of the thermal energy is recovered from the generators. This is accomplished through marine manifolds that facilitate thermal recovery through heat exchangers [20].
The diesel generators are characterized by their efficiency and rate of specific fuel consumption. The efficiency of diesel generator depends upon the ratio of its rated power to output power. The overall efficiency of diesel generator depends upon its thermal, mechanical and generator efficiency. The thermal efficiency depends upon the quality of diesel oil. The typical mechanical efficiency of diesel engine is around 80-85%, and generator efficiency is around 95-98%. The specific fuel consumption (l/kWh) of a diesel generator is defined as the consumption of fuel required to produce 1kWh of energy at the rate of 1 L/h for supplying a given load during 1 h time [21].

Emissions from diesel generators
The diesel generators are the most widely used as small electrical power-generating units in off-grid locations in the world due to their low capital costs [22]. However, diesel engines release many hazardous air contaminants and greenhouse gases (GHG) including particulate matter (diesel soot and aerosols), carbon monoxide, carbon dioxide and oxides of nitrogen. Particulate matters are largely elemental and organic carbon soot, coated by gaseous organic substances such as formaldehyde and polycyclic aromatic hydrocarbons (PAHs) which are highly toxic [23].
The Global Burden of Diseases, Injuries, and Risk Factors Study 2015 identified ambient air pollution as a leading cause of the global disease burden. Recent estimates of the global burden of disease suggest that exposure to PM2.5 (particulate matter with an aerodynamic diameter < 2.5 µm) causes 4.2 million deaths and 103.1 million disability-adjusted lifeyears (DALYs) in 2015, representing 7.6% of total global deaths and 4.2% of global DALYs [24].
The consumption of one litre diesel emits around 2.7 kg of CO 2 [25]. However, the number of kg of CO 2 produced per litre of fuel consumed by the diesel generator depends upon the characteristics of the diesel generator and of the characteristics of the fuel, and it is usually falls in the range of 2.4-2.8 kg/l [26].

Microgrid design
This paper compares the current model of electricity generation in Stewart Island consisting of five diesel generators with a model integrated of two available offshore renewable energies wind and tidal called microgrid with the aim to avoid the detrimental effects of diesel on the environment and decrease the cost of electricity in a remote off-grid community.
Combining multiple sources and loads in a close geographic proximity with microgrid has been proposed, e.g. Tina et al. [27] wind and solar and Gao et al. [28] wind and wave. Possible advantages of a hybrid system are a more predictable power from two different renewable energy sources and decreasing the generation and supply cost by using shared structure [29].
Optimization software is used to simulate electricity consumption from a set of hourly operating loads and then simulate reductions in the percentage of diesel power output dependent on the percentages of wind and tidal power available each hour and compares it with the case of "diesel only".
To carry out the feasibility study presented in this paper, an industry-recognized simulation software called HOMER Pro ® or HOMER (Hybrid Optimization of Multiple Electric Renewables) is used. This software simplifies the task of evaluating designs for both off-grid and grid-connected power systems. The HOMER Pro ® microgrid software has become a recognized global standard for optimizing microgrid design in all sectors, from off-grid village power and island utilities to grid-connected campuses and military bases [30].
The novelty of the paper, with respect to the state of the art, is proposing an electrical system with lower emissions in comparison with the current diesel system. This issue is of particular interest for diesel-based island microgrids that face who are exposing to detrimental effects of using fossil fuels for their power supply.
The paper extends previous work of the same authors [31] which evaluated connecting different scenarios of wind and tidal turbines to off-grid site of Oban. In contrast, the current work models diesel station system of Stewart Island and evaluates how alternative hybrid system can reduce emission from pollutants.

Methods
The diesel station described in previous section is simulated with HOMER to be compared with the optimized scenario of integrated system consisting of two wind and four tidal turbines as described in Fig. 3. The 2W + 4T scenario is optimal for power generation, because it produces the highest renewable fraction (61.1%), compared with other scenarios; the renewable fractions of 1W + 1T, 1W + 2T, and 2W + 2T are 30.2%, 32.5%, and 57.1%, respectively. Also, 949,280 kWh power can be generated from wind and tidal turbines which is 75.3% of total power demand of Stewart Island (1,260,332 kWh/year). This percentage in 1W + 1T, 1W + 2T, and 2W + 2T is 34.6%, 37.7%, and 69.1%, respectively, which means that the rest of demand must be supplied from diesel generators. The total NPC of 2W + 4T seems affordable at 3.47 million dollars. Its levelized cost at 21 c/kWh appears better than the present scenario (5D = 23 c/kWh) [31].
The detailed of microgrid design using wind and tidal turbines is presented in [31]. For modelling diesel station, below elements are used: 1-Electric load: The daily, monthly and annual load of Stewart is shown in Fig. 4. The average load for one year with a load factor of 0.69 is 1,260,308.5 kWh/year.
2-Storage unit considered is a generic 12-V lead-acid battery with 1 kWh of energy storage due to highly reliable performance and cost-effective operation [32] (see Table 1). The capital, replacement, maintenance, and life of the battery are taken as $154, $154, 15.4$/year, and 10 years respectively [33].
3-Converter: A generic system converter is used to rectify the ac output of the generator to the DC which is much cheaper than a bidirectional converter. Selecting the Homer optimizer allows the Homer grid to optimize the size of the converter. The capital, replacement, maintenance, life, and efficiency of the converter are taken as $154, $154, 15.4$/year, 15 years, and 90%, respectively [34].
4-Controller: The load following (LF) strategy is a dispatch strategy whereby whenever a generator operates, it produces only enough power to meet the primary load. Lower-priority objectives such as charging the storage bank or serving the deferrable load are left to renewable power sources. The generator may still ramp up and sell power to the grid if it is economically advantageous. This controller allows diesel-off operation and generators to operate simultaneously. Also, it allows systems with generator capacity less than peak load [12]. The capital, replacement, maintenance, and life of the controller are taken as $200, $200, 5$/year, and 25 years, respectively. 5-Generators: The generators presented in previous section are selected. The capita cost, replacement cost and O&M cost are calculated by multiplying their nominal capacities at 500,400 and 0.015, respectively [12].

Results
Using hybrid scenario of two winds and four tidal turbines (2W + 4T) yields 61.1% renewable fraction compared to diesel scenario (5D) with 0% renewable fraction. It is also more cost-effective according to financial results shown in Table 2, although needs more money for investment.
Using 2W + 4T reduces fuel consumption about 60% compared to 5D as shown in Table 3. Table 4 shows total production and hours of operation from turbines and generators. Figure 5 compares the amount of pollutants between 2W + 4T and 5D scenarios. From Table 3 and Fig. 6, it can be concluded that 5D produces pollutants and consumes fuel 2-4 times more in comparison with 2W + 4T.   Figure 6 shows that reducing of electricity consumption by 20% reduces the amount of pollutants 33%. This can be used as an approximation to estimate how power consumption can affect pollution. The fuel consumption decreases from 143,548 to 95,451 L.  Figure 7 clarifies the effect of renewable sources in reducing emissions while system is working with 80% load. The

Percentage of Electricity ProducƟon
Actual Load 80% Load SHARE OF TIDAL SHARE OF GENERATOR Fig. 7 Share of electricity production from resources in 100% and 80% load excess electricity in actual load and 80% load is 5.18% and 12.5%, respectively, which increases RF from 61.1 to 68.6%.

Conclusion
This paper simulated a DC microgrid currently is used in Stewart Island for providing electricity of customers and compared it with an optimized scenario of using wind and tidal turbines. In addition of cost-effectiveness of hybrid system, diesel generators issue pollutants and it is essential to think about alternative scenarios to reduce emissions. Based on comparing both microgrid systems, using 2W + 4T reduces the fuel consumption from 342,724 l (in 5D) to 143,548 l. The total amount of greenhouse gases (GHGs) also reduces from 916,551 to 382,490 kg/yr.
While initial capital of 2W + 4T is higher than current 5D, the cost of electricity will be less for customer and 199,176 L less diesel will be consumed which is essential for health of Islander. This issue is crucial and needs to be considered for off-grid systems due the detrimental of fossil fuels on global death and environment of small communities.