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HOMER Components

Mobirise

HOMER is one of the widely used software which helps developers to determine which renewable, storage technologies are cost-effective, what load management is effective, and what is an optimal capacity for each of these.
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%.

Photovoltaic

Even though the wind potential is higher compared with Photovoltaic(PV) in Scotland,
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: 

  1. 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. 
  2. 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%.
  3. 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.
  4. 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.                                                                                                                                                         

Wind Power

As onshore wind is considered as a significant source of renewable electricity in Scotland [2], we explored wind potential along with the proposed 5MW Solar farm. However, based on a feasibility study, a maximum of 3-4 wind turbines with hub height around 40-50 meters is only possible at the Island by considering the visual impacts. Hence, we considered and analysed a maximum of 4 wind turbines at the Island.

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

  1. 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.                                                                                                                                                
  2. 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.
  3. 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.                                                                                                      
  4. 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].                                                                                                                                                                                                                                                                                                                                                                             We have selected roughness length as 0.1m( few trees) for our models.
  5. 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.                                                                                                                                                                                                                                                                                                   
  6. 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]. 

Grid & generator

To make our model more realistic, it was necessary to design grid similar to the real conditions at the Cumbrae. The different parameters that we used to design grid in HOMER are given below.

  1. Capacity- At the Island, the maximum grid export capacity is limited to 5MW. Hence, in our design also we fixed the same constraint.  
  2. 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.             

In addition to the above options, in order to store electricity in our storage solutions when the price is low (during off-peak hours), and export it when the electricity price is high(during peak hours), we set a parameter such that prohibit export electricity to grid when the electricity selling rate is below 10 pence/kWh.

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.

Battery

We fixed following paramters for the models that use the battery for storing excess electricity generation from the renewables.

  1. 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.
  2. 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].

Pumped hydro 

We have identified that Cumbrae has the potential to store and use excess energy from renewable as pumped hydro scheme. Hence, we propose seawater pumped hydro storage for the island. In order to build this scheme,it was necessary to model the same in the HOMER. However, HOMER doesn't have a dedicate flexible pumped hydro system in its library. Therefore, we modeled the pumped hydro as a battery storage based on a research paper[8].
The main parameters that we considered to model pumped hydro in HOMER are: 

  1. 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% 
  2. 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%                                               
  3. 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.
  4. 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.
  5. 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%.
  6. Lifetime & Cost- We have assumed the capital cost of the pumped hydro project as £1.5 million[10]. As the hydro system is expected to have a longer period of life around 40-50 years, the replacement cost is considered as 40% of the capital cost (by considering the replacement of only turbine, generator etc.), and O&M/year is considered approximately as 1.5% of the capital cost.                                                                                                    

References

  1. Ryan, L., Dillon, J., Monaca, S., Byrne, J., and O'Malley, M. (2016), Assessing the system and investor value of utility-scale solar PV, Renewable and Sustainable Energy Reviews[online], 64(5), 506-517. Availabe from: https://www.sciencedirect.com/science/article/pii/S1364032116302210 [Accessed 13 Feb 2018]
  2. Scottish Government. ([ca.2010]). Onshore Wind [online]. Availabe from: http://www.gov.scot/Topics/Business-Industry/Energy/Energy-sources/19185/17852-1 [Accessed 03 Feb 2018]
  3. Manwell J.F, McGowan J.G, Rogers A.L (2002), Wind Energy Explained, Wiley, New York, NY
  4. openEnergyMonitor. ([ca.2017]), Looking at costs[online], Availabe from: https://learn.openenergymonitor.org/sustainable-energy/energy/costs [Accessed 18 Mar 2018]
  5. Renewables first. ([ca.2017]), What is the cost to operate wind turbines ?[online], Availabe from: http://www.renewablesfirst.co.uk/windpower/windpower-learning-centre/how-much-does-a-wind-turbine-cost-to-operate/ [Accessed 18 Mar 2018]
  6. IRENA (2017), Electricity Storage and Renewables: costs and markets to 2030 [online], Availabe from: http://www.irena.org/publications/2017/Oct/Electricity-storage-and-renewables-costs-and-markets [Accessed 20 Mar 2018]
  7. Naumann, M., Karl, R., Truong, C., Jossen, A., and Hesse, H. (2015), Lithium-ion Battery Cost Analysis in PV-household Application, Energy Procedia[online], 73, 37-47. Availabe from: https://www.sciencedirect.com/science/article/pii/S1876610215013235 [Accessed 13 Feb 2018]
  8. Canales, F., and Beluco, A. (2014), Modeling pumped hydro storage with the micropower optimization model (HOMER), Journal of Renewable and Sustainable Energy [online], 6.  Availabe from: https://aip.scitation.org/doi/abs/10.1063/1.4893077?journalCode=rse [Accessed 13 Mar 2018]
  9. Loucks, D. P. and Van Beek, E.(2005),  Water Resources Systems Planning and Management: An Introduction to Methods, Models and Applications, UNESCO, Paris.
  10. McLean, E., and Kearney, D. (2014), An Evaluation of Seawater Pumped Hydro Storage for Regulating the Export of Renewable Energy to the grid, Energy Procedia[online], Availabe from: https://www.sciencedirect.com/science/article/pii/S1876610214001842 [Accessed 08 Mar 2018]

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Department of Mechanical & Aerospace Engineering,  James Weir Building, Level 8,  
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Glasgow G1 1XJ
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