Software Analysis

To allow analysis to be performed on Mackie’s Hybrid Energy Generation System with our proposed system of adding a biogas generator as dispatchable energy generation and energy storage as energy buffering, we modelled and optimised the system based on the information provided by Mackie’s Scotland.

The energy generated from the wind turbines and biogas generator would be converted from AC to DC through a rectifier unit to charge the batteries and to meet the demand of the site. Energy discharged from the batteries will be converted to AC through the inverter to meet the demand before the biogas generator is fully functioning.

Solar energy from the PV panels are on the hand would be converted from DC to AC through an inverter unit to be fed into the grid or to the site. The excess solar energy can be used directly to charge the batteries.

As illustrated in the schematic, the system is separated into AC and DC bus with converter as an intermediary component in dealing with the transition between the two.

The AC bus houses the electric load of the site, biogas generator and the wind turbines while the DC bus houses the solar panels and Li-ion batteries.

Homer Pro was used to carry out analysis for our case study on Mackie’s Scotland. Constraints were set to achieve 100% renewable fraction and full energy autonomy from the grid.

Search Space feature was used to integrate existing energy generation system into the simulation and analysis. The number of components and their parameters were altered to simulate and determine optimum solutions to achieve project aims and project feasibility financially and technically.

Homer Pro calculated the optimum number of batteries required to meet the transient demand as energy buffering, and the capacity of biogas generator as dispatchable energy generation to meet the peak demand when the renewable energy fails to. By simulating different combinations of energy generation systems, the optimum hybrid energy generation system was determined by its cost effectiveness and technical feasibility.

In order to achieve full renewable energy autonomy, the power supply and demand of the site must be balanced throughout the year. Any unmet electrical load would cause disruptions to the operations of the industrial site, which can lead to severe capital losses. Hence, the demand profiles of the industries play an important role in establishing full renewable energy autonomy. In the case study of Mackie’s Scotland, the provided demand profile is in monthly basis. Hence, assumptions were made to create a domestic demand profile for the simulation in Homer. The hourly demand profile was scaled up to approximate 4 GWhr a year as provided.

The graph below represents the electrical supply and demand of Mackie’s of Scotland with our proposed system. The grey columns represent the excess energy that will be exported to the grid. Less excess electricity is produced during the summer due to the lower wind speed and higher demand from the site. In addition, our proposed system is to establish full renewable energy autonomy by ensuring all electricity load on site are served and zero energy shortage throughout the year.

Transition seasons were used to analyse the system feasibility in equatorial areas where the climate is more consistent and less extreme.

However, the results of the summer and the winter were used to analyse the performances of the hybrid energy generation system in extreme weather conditions.

As the graph above shows, biogas generator can be used as dispatchable energy generation to meet the transient demand. While Li-ion Batteries as energy buffering to offer buffering time for biogas generator to operate.

It is important to note that the case study is based in Aberdeenshire, Scotland. In our case study, wind energy is the main source of renewable energy, which provide more than 75% to the site. However, PV panels are installed to compensate the wind loss during the summer. Biogas generator and Li-ion Batteries are used as dispatchable energy generation to meet the peak demand. However, as we move closer to the equatorial areas, where wind speed is lower but with greater amount of solar radiation. Solar energy would become more favourable main source of renewable energy. While wind is becoming less favourable in equatorial areas, it can still be utilised to compensate the energy loss during the night and monsoon season.

Furthermore, source of feedstock for anaerobic digestion would change accordingly with the location and the availability of the resources. This has significant effect on the design and the viability of the energy generation system as the nature of the feedstock dictates the yield of biogas, rate of reaction and ultimately the amount of feedstock required to suffice the needs.

The idea of introducing biogas generator as dispatchable energy generation is to maximise generation with minimum capacity of energy storage. This will effectively minimise the capital cost of proposed system due to high capital cost of energy storage and additional profits from exporting excess electricity to the grid. However, it is important to take into account that the capacity of the biogas generator is dictated by the amount of feedstock available. Additionally, the demand profile of the site also dictates the capacity of the biogas generator. As biogas generator is required to supply enough electricity for the site in the situation when wind and solar are not able to provide.

Secondly, the capacity of energy storage is sized that it can provide enough energy to the site before the biogas generator can respond to the sudden rise of the demand. Such design allows energy storage to act as an energy buffering due to its fast response time. Hence, the demand profile of the site will dictate not just the capacity of energy storage but also the capacity of biogas generator. It can be observed that a steady energy pattern will require a smaller capacity of biogas generator and energy storage due to less extreme energy transient demand.

EIA will be required for implementing such system due to the installation of biogas generator and wind turbines. This may pose challenges to the feasibility of proposed system due to the odour concerns on biogas generator and the lack of ideal location for wind turbines and PV farms. Further details about EIA can be found at here.