4R.Gordn Bloomquist, PH.D: Geothermal Heat Pumps Plus Decades of Experience in the United States, Proceedings World Geothermal Congress 2000
5Rybach,L and Sanner,B.Ground-source heat pump systems-the European experience.Geo-Heat Center Quarterly Bulleyin Klamath Falls.Oreg.Oregon Institute of Technology 2000,21 (1):16-26.
