This project was initiated by the Department of Energy in response to a request from the HVAC industry for consolidated information about alternative heating and cooling cycles and for objective comparisons of those cycles in space conditioning applications. Twenty-seven different heat pumping technologies are compared on energy use and operating costs using consistent operating conditions and assumptions about component efficiencies for all of them. This report provides a concise summary of the underlying principals of each technology, its advantages and disadvantages, obstacles to commercial development, and economic feasibility. Both positive and negative results in this study are valuable; the fact that many of the cycles investigated are not attractive for space conditioning avoids any additional investment of time or resources in evaluating them for this application. In other cases, negative results in terms of the cost of materials or in cycle efficiencies identify where significant progress needs to be made in order for a cycle to become commercially attractive. Specific conclusions are listed for many of the technologies being promoted as alternatives to electrically-driven vapor compression heat pumps using fluorocarbon refrigerants. Although reverse Rankine cycle heat pumps using hydrocarbons have similar energy use to conventional electric-driven heat pumps, there are …