While solar and wind projects are now commonplace, geothermal power is less developed. Geothermal systems typically use heat found deep underground to make steam and generate electricity. WaterFurnace Renewable Energy in Fort Wayne, Ind., builds heat pump systems that don’t require deep drilling for homes and businesses. The technology takes advantage of modest but consistent temperatures of about 55 degrees found a few feet underground. Air pumped underground is heated or cooled, which reduces the load on traditional heating and cooling systems and cuts energy bills by about two-thirds. Over time, that offsets installation costs. Revenue is growing 50% a year, and installations haven’t been hurt by the credit crunch, says Jack Robinson, lead manager of the Winslow Green Growth Fund. Guinness’ fund owns Energy Development Corp., a Philippine utility that oversees a dozen geothermal plants and consults on projects for others.