HOMER is a computer simulation program designed by the National Renewable Energy Laboratory (NREL) in the United States. Coined as the Optimisation Model for Distributed Power, HOMER allows the modelling of both grid and non-grid connected power systems consisting of conventional and renewable technologies. The program considers the economic and technical feasibility of desired power systems and delivers comprehensive reports covering a range of subjects from the net present capital cost of the system to the renewable penetration.

HOMER is a very sophisticated program having been constantly developed and updated since its beginnings in February 2000. Now in its sixth year, HOMER (Version 2.2 beta) is used by thousands around the world as the basis from which to begin designing simple power systems. It allows the input of renewable resources such as wind speeds, battery data, demand load data, capital and operation and maintenance (O&M) costs among others, as well as sensitivity analyses modelling the impact on the system to variations in any input.

HOMER performs simulation calculations on an hourly basis, for each of the 8,760 h in a year. The calculations are based on energy balances and compare the thermal and electrical energy both provided and demanded by the system. Based on these calculations, HOMER determines the optimum system configuration as well as an analysis of the system cost. For systems that include batteries and/or fuel powered generators, HOMER determines for each hour whether to charge or discharge batteries and whether to run the generator.

The most significant assumption made by HOMER relates to the round trip efficiency. The round trip efficiency of the battery is assumed to be constant over the load and power range and over the lifetime of the battery. In reality, the round trip efficiency (energy efficiency) of the cell will vary depending on the power drawn from the battery and the surrounding temperature, among other factors. Other assumed constants include the nominal voltage, the capacity curve, the lifetime curve and the minimum state of charge of the battery [9].

In calculating the total net present costs and annualised costs of the system, HOMER assumes a project lifetime of 25 years with an interest rate of 6% per annum.

For more detailed information on assumptions and algorithms within HOMER see the article entitled Micropower System Modelling with HOMER by Lambert et al. [9].

Actual wind data was used for the wind resource input for HOMER. Hourly wind speeds provided by V-Fuel Pty Ltd from an undisclosed source over the 2004 calendar year were used and are summarised by month in Figure 1. The wind speeds were recorded at an elevation of 46 m and have an annual average speed of 9.63 m/s.

The load data used was taken from the Australian National Electricity Market Management Company (NEMMCO) website [10]. Historical demand data from the last eight years is accessible to the public from this website. In this case, hourly data from 2004 for NSW was used and scaled to achieve a 1 MW peak load for the purposes of representing a typical load size for a wind/diesel/battery system. Scaling the data down is still representative of a typical load as the hourly, daily and seasonal trends are maintained. In this case, the scaled annual average daily load is 15,806 kWh/day. Figure 2 graphically summarizes the average daily demand data.

HOMER has the data for a number of manufacturers of wind turbines and their sizes already built into the program. For this simulation, the Fuhrländer 250 wind turbine was chosen for the size of its rated output (250 kW). A wind turbine of this size is typical for a wind/diesel/battery system with a 1 MW peak load. HOMER was directed to consider up to eight wind turbines. The Fuhrländer 250 turbine has an expected lifetime of 15 years, a hub height of 25 m and a power curve as shown in Figure 3.

HOMER has the data for generic diesel generators built in. For this system, two diesel generators were considered of either 300, 600 or 1,000 kW rated power output. The generators are assumed to have an operating lifetime of 15,000 h with a minimum load ratio of 30%. The efficiency curve relating the fuel consumption to the power output for the generators is included as Figure 4.

The efficiency curve above demonstrates two aspects of diesel power generation. Firstly, in general, efficiency is very low. Operating at a peak of around 35% efficiency, most of the energy supplied to a diesel generator is wasted. Secondly, there is a marked reduction in efficiency with a reduction in power output. For this reason, a constraint is placed on the system within HOMER to limit the minimal power output of the diesel generators to 30% of their overall capacity.

The base price of diesel used in the simulations was $1.50/L and is based on Figure 5 depicting the 5-year historical trend in the price of diesel in a remote part of Western Australia.

It is important to note that HOMER assumes the price of diesel to remain constant over the 25-year life of the project. As Figure 5 indicates, this is a very conservative assumption for the purposes of calculating fuel costs.

The VRB round trip efficiency was assumed to be 80% for this system. This is a realistic assumption based on significant research during the battery’s development [2,3,4,5]. In each case the cell stack is assumed to have a lifetime of 20 years and the electrolyte a lifetime of 50 years, based on figures provided by V-Fuel Pty Ltd. The simulation considered between 400 kW and 1,200 kW rated power batteries with between 8,000 kWh and 20,000 kWh of storage. The price of vanadium pentoxide on world commodity markets is shown in Figure 6 and forms the basis for the $300/kWh cost of the electrolyte system (although substantial electrolyte cost reductions would be possible if vanadium production capacities were to increase in the future).

The following typical costs data (Table 1) was adapted from a table within a report prepared for the Australian Greenhouse Office entitled Assessment of Diesel Use in Remote Area Power Supply.

As the above data is based on typical costs in 1999, it was adjusted for inflation (Data was adjusted using the inflation calculator on the Reserve Bank of Australia website) [15] and presented in Table 2.

Using the above data, the appropriate capital and operation and maintenance costs were entered into HOMER. In each case the high cost figure was used to take into account the additional costs incurred in implementing such a system in a remote area and so that a worst-case cost scenario simulation is run. Included below is a summary of the cost data entered into HOMER (Table 3).

Results from the simulation suggest significant environmental and financial benefits are to be gained in installing a VRB as an integration technology for remote wind/diesel power systems. HOMER calculated the following optimal system architecture from the input data (Table 4):

As can be seen, the rated power output of the optimal VRB (1,000 kW) matches the peak load of the system suggesting favourable use of the VRB over the alternative of supplementing wind power with diesel generation. Furthermore, the very large optimal storage capacity of 18,000 kWh also suggests HOMER favours the use of the VRB over the diesel generators in times of minimal wind turbine output.

Good use is made of the VRB throughout the year, especially in times of little or no wind. Figure 7–Figure 9 indicate the output of power from the wind and VRB for each month of the calendar year. As is evident from the diagrams, in times of little or no wind power output, the VRB is used, demonstrated by the low state-of-charge (SOC) at these times. The frequency histogram (Figure 9) demonstrates that the VRB is in a minimal state of charge for approximately 12% of the year and a high SOC for about 45% of the year, suggesting there is still potential for more VRB use; an important consideration given the intermittency of wind power and the potential variations in wind output from year to year.

A significant reduction in the use of the diesel generators and the reliance on diesel fuel is observed with the inclusion of a VRB in the wind/diesel system. This is made evident by a comparison of identical systems with and without a VRB installed. Figure 10 and Figure 11 graphically displayed the use of the 600 kW diesel generator in these systems.

As these figures demonstrate, there is far greater reliance on the diesel generators in the absence of a VRB. With the inclusion of a VRB the role of the diesel generators shift from the primary base source of power to a necessary supplement in the event of sustained periods of little or no wind.

An examination of the breakdown of system costs gives further indication as to why HOMER favours the use of the VRB over diesel generators. Table 5 summarises the system costs as calculated by HOMER.

The figures presented in Table 5 are calculated by HOMER assuming a 25-year project lifetime. The total net present cost (NPC) at the end of the 25-year project is $21,832,144 giving a levelized cost of energy of 29.6 ¢/kWh. Although the 1 MW/18 MWh VRB has a high initial capital cost of $6.05 million, the total annualized cost is comparable to the two diesel generators, despite this high initial capital cost.

It is in the cost of diesel and the operating and maintenance costs that the VRB distinguishes it from the diesel generators. The VRB has relatively low annual operating and maintenance (O&M) costs with the added benefits of a negative annualized replacement (owing to the long life of the electrolyte) and the lifetime fuel source effectively covered in the initial capital cost. In contrast, the diesel generators have higher O&M costs with the disadvantage of being dependent on an increasingly expensive fuel source. It is for these reasons that HOMER favours the use of a VRB to diesel generators. To further illustrate this point, cost data from the comparison between identical wind/diesel systems with and without a VRB installed is included in Table 6.

As Table 6 indicates, considerable financial and environmental benefits are to be gained in installing a VRB in this system. An improvement of over 10% renewable penetration is achieved with the use of a VRB. This equates to $11.9 million in savings over the 25-year life of the project and over 1,000 tonnes per year reduction in carbon emissions. The consumer is afforded a 16.1 ¢/kWh break in the cost of electricity. Furthermore, the wind/diesel system has far more financial exposure to the rising price of diesel. With the inevitable increase in the cost of diesel fuel, the cost differences between the two systems are realistically expected to be even greater.