Solar & Wind
Over the past two decades, the utilization of wind and solar energy has been promising in both standalone and grid-connected systems.As the demand for electricity is rising, a hybrid renewable energy system combining solar and wind are emerging options to meet these demands. A hybrid system properly utilizes the operating characteristics and obtain higher efficiencies compared to a single power source (Khare, Nema, and Baredar, 2016).
From fig 1 and 2, it is witnessed that countries are opting for solar and wind technologies in the next few years as noticed in the graph. The reliability on the electrical network can be enhanced with a combination of the two energy systems. The generated power can be stored in a battery which is optimized to the system for the energy buffering. In certain periods, when wind velocity is low solar resources are high and vice versa, hence making hybrid system the best solution. As for Scotland, since wind and solar energy have opposite cycles and intensities during the day/season, the output and performance of the system is enhanced.
(An accelerated case from the graph above indicates the policy and market improvements that can unlock further growth of renewable energy in electricity)
The major input for designing a hybrid renewable energy system is the usage of appropriate weather data, depending on the wind energy potential and solar irradiation availability of that location. The magnitude of the potential solar irradiation and wind velocity of that particular location will determine the efficiency and optimization of the design technique for the hybrid wind-solar system.
From the global wind velocity map below, it is evident that wind is the main source of energy for Scotland whereas the wind speed changes for countries lying closer to the equator. On the other hand, the solar irradiance map indicates a shift in the rise of solar energy for the equatorial regions.
As shown in figure 3, the windy areas such as Scotland has an advantage in wind energy. In fig 4, it the electricity supply for Scotland from onshore wind reaches to 66.68 per cent or 17,808 GWh in 2018 (Mitchell, 2019). On the other hand, a country closer to the tropical areas faces the problem with the stochastic nature of wind occurrence or weakly wind speed. Hence, a hybrid system (PV + Wind), mitigates the issue of unsteady energy generating from the stand-alone system (Wind).
From the global solar irradiation map, it is evident that Africa, India or Southern Europe receives much more solar irradiation than Scotland, although it is still beneficial to install solar PV(SISER,2018).This is because if we look at the map below for UK and Scotland, the variation in energy difference with Germany(had 24.7 GW of PV installed at the end of 2011) is not significant. Moreover, according to statistics a 3kWp system in Scotland can generate around 2,300 kilowatt hours of electricity a year(Birol,2018).
What is Power Coefficient (Cp) for Wind Turbine?
The term power coefficient (Cp) is used by much of the wind power industry to represent the overall efficiency of the turbine. It combines the efficiencies of the blades, mechanical, and electrical components which manufacturers tend to specify based on certain ideal standardized conditions. Knowing the Cp at a given wind speed, provides a simple approximation of what the actual electrical power produced by the wind turbine will be and it is helpful for comparison with other wind turbines(Watson, 2015).
The grey line from the graph below (fig 6) indicates the wind power into the turbine blades. As it increases with the cube of wind velocity, it dramatically increases with increasing wind speed.
Biogas
Why are we using biogas?
The use of biogas in a hybrid system with wind, solar and energy storage can mitigate wind/solar prediction errors and provide a more predictable source of energy. The introduction of the biogas will contribute to an increase in efficiency on the energy system and is capable with very short advanced stable power requirements. The system as a whole can reduce reliance on the grid and is more competitive as energy reliability requirements.
From the global wind velocity map below, it is evident that wind is the main source of energy for Scotland whereas the wind speed changes for countries lying closer to the equator. On the other hand, the solar irradiance map indicates a shift in the rise of solar energy for the equatorial regions.
The biogas can also be used as a CHP (Combined Heat and Power Plants) as energy conversion efficiencies of biogas are 30% in internal combustion engines. Efficiencies of biogas can reach 80% through the use of CHPP. The H2S content must be removed as it has high toxicity and is corrosive with steal. The content of the CO2 is not a limiting factor.
Anaerobic digestion produces as well as biogas another by-product, digestate. This consists of a solid and liquid fraction. This can be used as a fertilizer as it has similar characteristics as compost.
Biogas plant used in the Mackie’s case study is needed to fill the gaps in the plants demand when solar and wind fail to produce enough to meet the energy requirements. Biogas is chosen for this as it provides a constant energy supply and is used a buffering technology as the biogas can be stored in the storage tank and the output controlled. The constant supply of the biogas plant is ensured as the storage tank is capable of storing 4 days’ worth of biogas, so any shut down of the digester for maintenance can be mitigated against.
Battery Energy Storage Options
Wind speeds are not just seasonally variable, daily and hourly variations affect the power wind farms can generate. Capture and storage of power predicates security and safety of supply essential to the aim of self-sufficiency. There are numerous technologies capable of providing storage capacity, amongst those most promising are batteries, super-capacitors, flywheels and superconducting coils. This section examines battery energy storage solutions capable of providing adequate energy storage for the needs of our case study Mackie’s, provides a brief overview of potential battery technologies and rationale for the selection of the proposed technology, along with technical specifications. The proposed technology should offer the best possible fit as an energy storage medium for Mackie’s of Scotland business operations and energy self-sufficiency aspirations.
Costing Energy Storage Options
Many factors contribute to accurate pricing of an energy storage system. It is essential that all contributing costs are established at the onset. Upfront capital costs encompass DC block; AC equipment; housing, grid, controllers, and interconnections; installation and commissioning costs; and importantly, site-specific delivery factors (cost and time). Ongoing annual costs of operating and maintaining these assets are significant and comparison of key factors of the systems investigated was made prior to selection of an appropriate storage system. Characteristically, all batteries have parasitical AC systems and some degree of energy loss during operation therefore comparison of efficiency measures is essential. Battery lifetime and degradation rates measured in years and cycles is important as is the warranty period. Furthermore, the system must be scalable to futureproof the proposed investment.
A comparison of two key battery energy storage technologies utilised in renewable energy projects is provided below in table 1.
It must be noted that he efficiency figure quoted for Li-ion Powerpack refers to AC/DC efficiency whereas Hi-Cube efficiency figure relates to DC/DC convertor. It is widely acknowledged that AC/DC convertor losses are greater than DC/DC convertors.
Analysis
Li-ion batteries are dominant in the renewable energy storage industry at present. They are less expensive to purchase in terms of cost per kWh however they deteriorate substantially over time requiring replacement. Cycling rate is a key factor in the rate of deterioration as is depth of discharge. Whilst flow batteries allow for stacking, Li-ion batteries do not due to risk of fire and explosion. They do however have an In-built Thermal Controller. Powerpack has a significantly smaller footprint than the flow battery unit. Flow batteries provide for optimisation as power and energy components can be sized independently. An important advantage of Li-ion Powerpack is the In-built bi-directional Inverter which is an additional cost factor should RedT Hi-Cube be selected. Table 2 below provides a comparison of key characteristics of Li-ion Powerpack and RedT Hi-Cube Flow Battery system.
Based on cost, integrated bi-directional Inverter, space considerations, and proposed usage pattern, Li-ion Powerpack is deemed the most appropriate system at this time. Five Tesla Powerpacks with the total capacity of 1.05MWh are required.
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