Before assessing the prospective installation of any other generative technologies, it is necessary to first check whether the current configuration can satisfy Eigg’s space heating demand after installing heat pumps.
The section on demand reduction details the fact that upgrading Eigg’s buildings greatly reduces the heating demand. For this heat demand to be satisfied by electricity, heat pumps must be installed. Ground source heat pumps (GSHP) require less electrical work input than air source heat pumps (ASHP) to produce the same thermal energy output, the full details of which can be read here. Given all this information, it is clear that the scenario which leads to the lowest electrical demand is that of upgraded buildings with GSHP installed. Can the renewable energy in the current configuration satisfy this load along with the present?
The graph below would indicate not and, indeed, HOMER gives an output of only 66.3% renewable penetration. In the pursuit of 100% renewable penetration and the potential unsuitability of GSHPs financially (as discussed in the cost analysis), it is therefore prudent to investigate increasing the existing microgrid’s capacity to provide heating.
Various technologies were investigated but most were deemed inappropriate, as discussed here. The simplest and most effective technologies to implement are wind turbines, batteries and converters, along with retaining the present PV and hydropower schemes. A number of wind turbines were highlighted as potentially fulfilling the requirements of the new electrical demands. The price of the different turbines were found from numerous sources as noted: Northern Power NPS100C (1), BENZ PMG DD100 (2), CIESSE 100Wheel PM DD (3) and Vestas V17 75kW (4). References for technical data including power curves can be found here. Technical information for the NPS100C was provided by HOMER software.
Eigg's Current Wind Generation
Eigg's Current Battery Bank
Wind and Sun Inverters (5)
100% electrification of Heat
The tables below show ways in which 100% renewable penetration can be achieved using the different wind turbines for four potential scenarios. Those highlighted in yellow indicate the most cost-effective option for the given scenario, discounting the capital required for heat pumps and upgrading the buildings (the analysis of which appears here). The four configurations highlighted in yellow are compared with one another in the cost analysis. All consist of three Vestas V17 75kW turbines, indicating that they are clearly the most cost-effective turbine option.
NB: Costs for batteries and converters does not take account of those already there. In reality then the NPC (net present cost) will be lower. This means of analysis still provides a fair basis for comparison as this inaccuracy is consistent for all configurations. The true NPC is later calculated for the best configuration in the costs section.
The biomass section explains how 30% of Eigg’s heating needs can be met by a sustainable local biomass industry after upgrading buildings. In order to account for uncertainty and increase security of supply, a more conservative estimate of 20% biomass is considered for the following part of the project – to include both electrification of heat with locally sourced biomass.
Demand profiles for the four scenarios had to be adjusted to account for the inclusion of biomass. For the upgraded buildings with ASHPs and GSHPs this consisted of multiplying the scaled yearly average by 0.8. As detailed in the biomass section, the biomass fuel could also provide approximately 14.4% of the present buildings’ heating requirements. Hence, the former profiles for non-upgraded buildings were scaled by 0.856.
The same process as for the previous scenario was then performed for determining the optimal configuration, now with a reduced electrical load.
As expected, the Vestas model again provide the most cost-effective solution. Financial viability between upgraded/non-upgraded and ASHP/GSHP configurations are explored within the cost analysis section, along with an overall comparison between the biomass and 100% electrified space heating options.
© University of Strathclyde | TEC Eigg | Sustainable Engineering 2016