To model a geothermal plant or turbine that produces a steady amount of power, create a new generator with a new fuel called "Steam" or "Hot water". To optimize the capacity of the geothermal plant/turbine, you must use one of HOMER's "size-your-own" generators, which can be found in the Complete Catalog with other Generic generators. Alternatively, you may convert it your geothermal generator to "size-your-own" by unchecking the "Fixed generator capacity" box in the General tab of your generator under the Library view. If you would like to fix the capacity of your geothermal generator, you may do so here. If the power output of the geothermal plant is fixed, then set the minimum load ratio to 100% so it can only run at full blast. If you scheduled your geothermal generator on all the time (using the Schedule tab), it will produce the same kW all the time.
Be sure to enter cost data in the cost table or cost fields to properly describe the cost curve of a geothermal system in the location of interest. You may also wish to adjust the lifetime of your generator to specify how many hours of operation occur before the generator replacement cost happens.
Now, adjust the properties of the new fuel and enable it to mimic the geothermal resource. Be sure to set the fuel's carbon content to zero to account for the carbon-free nature of geothermal power. You can model it as a free fuel (set the fuel cost to zero) or your can set the fuel properties and generator fuel curve so that you can enter a $/kWh price in the fuel. We have created a sample HOMER model (attached) that contains a geothermal generator with no capital, replacement, or O&M costs, and created a fuel "Steam" with fuel properties such that you can enter a desired $/kWh price into the fuel's $/L price. Please see the attached HOMER Pro file.