The specifications of the components that we have used to develop our models are described below. We have considered the project life time as 25 years with discount rate at 2.5%.

by considering the expected higher demand during the summer season because of the number of second homes in Cumbrae, we have also considered Photovoltaic renewable generation.

The different parameters that we have used to model PV are:

**Capacity**- The Isle of Cumbrae already has a permission in place to install 5MW PV system. Therefore, for all our models we have used PV of size 5MW.**Lifetime and derating factor**- As most of the manufacturers usually offer solar panels with 20-25 years of warranty, we have considered lifetime as 25 years. Derating factor is the scaling factor at which the PV array power output to account for reduced output in real-world operating conditions compared to the conditions under which the PV panel was rated. It accounts for factors as such as soiling of the panels, wiring losses, shading, snow cover, aging etc. We have considered derating factor of 80%.**Cost**- In order to model PV effectively in HOMER, we need to consider Capital cost, Replacement cost, and Operation & Maintenance(O&M) cost. We have considered the capital cost as £4.3 million for the entire project including the inverter (5MW in our case) and other hardware expenses, BoS (Balance of System) costs, connection costs (vary depending on the real conditions at the site) etc.[1]. The replacement cost is considered as 60% of capital cost, and O&M/year as 1% of the capital investment.**Weather data**- It is necessary to give accurate solar global horizontal irradiance (GHI) to calculate the generation of electricity from PV. For this, we have used 2017 horizontal irradiance(W/m2) data that we got from the Hunterston power station on the mainland close to the Island. The obtained monthly average Solar global horizontal irradiance values are shown in the figure.1.

The different parameters that we have used to model Wind turbine are:

**Capacity**- We have considered different wind turbine manufactures in the HOMER library to select the best one by considering the maximum hub height possibility at the Cumbrae. We have identified two different manufacturers, 'Vestas' and 'EWT', which provide the good rating at a hub height around 50 meters. Then, we conducted trail runs to select the best one along with the 5 MW solar farm. Table 1 shows the different cases that we have considered. Finally, we selected the wind turbines from the manufacturer 'EWT' as it given higher Renewable fraction. The leaflet of EWT wind turbine can be downloaded from*here*.**Hub height**- We have selected the wind turbine with a hub height of 40m. Since the island has a lot of hill areas with height roughly around 25m from the sea level, we considered the effective hub height as 65m.**Weather data-**In order to model wind turbine effectively in HOMER, it is necessary to give accurate wind speed input. For this, we have used 2017 wind speed(m/s) data that we got from the Hunterston power station on the mainland close to the Island. The resulted monthly average wind speed values are shown in figure 2.**Roughness length**- The surface roughness length is a parameter that characterizes the roughness of the surrounding terrain or area. Table 2 shows representative surface roughness lengths [3].**Velocity at the hub height-**We got the wind speed(m/s) input from the Hunterston power station based on an anemometer placed at the height of 10m from the sea level. Therefore, it is necessary to calculate the wind speed at the hub height to know about the electricity generation from the wind turbines. We have used the Logarithmic profile equation to calculate it. The following equation gives the ratio of the wind speed at hub height to the wind speed at anemometer height.**Lifetime & Costs-**In the case of wind turbines, we have considered lifetime of 20 years with capital cost, £1.2 million[4]. The replacement cost is considered as 60% of the capital cost, and O&M cost as £35,000/year[5].

**Capacity**- At the Island, the maximum grid export capacity is limited to 5MW. Hence, in our design also we fixed the same constraint.**Power price**- The buying electricity price, the price at which the consumer buys electricity is fixed as 10 pence/kWh. The selling electricity price is according to the wholesale rate in the UK. To know more about the electricity prices, please click*here*.

**Generator**- The Isle of Cumbrae already has a backup diesel generator of 4MW. Hence, in our scenario's we modeled the same. As it is already available the capital price is fixed as £0 with fuel price as £1.23/liter. However, in our models, we forced off the generator for most of the time as we do not aware of any fixed power cuts or blackout per year in the Island. Also, as renewable generation from the island is increases the generator is no more needed in the island, and they can sell it.

**Capacity**- The Isle of Cumbrae already has a permission in place to install 1MWh lithium-ion storage along with the proposed 5MW solar farm. Hence, we selected the same storage option for the model that uses the battery.**Lifetime & Cost**- We have considered the capital cost of the lithium-ion battery as £0.5 million[6] with 20 years lifetime. The replacement cost is considered as 60% of capital cost, and O&M/year is considered approximately 1.5% of the capital cost**[7].**

The main parameters that we considered to model pumped hydro in HOMER are:

**Power**- For hydropower generation, a cubic meter of water, weighing 1000 kg, falling a distance of 1 m, acquires 9810 J (N.m) of kinetic energy[9]. The energy generated in 1 sec equals the watts of power P produced. Hence, an average flow Q (m3/s) falling a height H (m) and affected by an efficiency in conversion , combine in the following equation to calculate the yield in kilowatts of power:*Power(kW) = pgQH*efficiency/1000*Where: p= Mass density of sea water(1,028 kg/m3) g= Acceleration due to gravity (9.81 m/s2) Q= Discharge through the turbine (m3/s) H= Effective head(m) Pump efficiency is assumed as 90%**Energy**- In order to find the energy stored in the reservoir we used the following formula.*Energy (kWh) = pgH*Volume*efficiency/3600*Where: p= Mass density of sea water(1,028 kg/m3) g= Acceleration due to gravity (9.81 m/s2) Volume= Volume of the water stored in m3 H= Effective head Pump efficiency is assumed as 90%**Nominal capacity**- The capacity of a battery is usually specified in Ampere-hours (A.h) units. The fact that the capacity for hydropower generation is not as heavily affected by its output is the main reason that justifies creating an equivalent battery for modeling a pumped hydro. The formula used to calculate nominal capacity is*Nominal capacity(Ah) =Energy *1000/ nominal voltage*Where: nominal voltage is taken as 240V for simplicity. Energy is the stored energy in the reservoir as given in the above formula.**Maximum charge /discharge current**- To describe the filling of the reservoir on the equivalent battery representation, there are two defining variables. For a battery, the maximum charge current variable imposes an upper limit on the allowable charge current, regardless of the state of charge. The maximum charge rate variable imposes a limit on the rate at which the system can charge the battery bank. That limit is directly proportional to the amount of “unfilled capacity” (headroom) in the battery. As the battery fills up, the headroom decreases, so the maximum charge rate starts to become the limiting factor. The formula used to find this variable is,*Maximum charge/discharge current= Nominal capacity / Number of hours taken to discharge 90% from the reservoir*Note: By considering the possible visual effects, we haven't considered 100% of discharge or empty the reservoir.**Round trip efficiency**- The fraction of energy charging input that is recovered when discharging. This can include electrical losses, hydrodynamic losses, frictional losses, and other sources of loss. In our case, we assumed this efficiency as 80%.**Lifetime & Cost**-

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