Mark joined NREL in 1986 and manages the Thermal Systems R&D group at NREL, which includes the Concentrating Solar Power (CSP) Program. Since 2001, he served as the CSP program manager.

Mark is the leader for the International Energy Agency's SolarPACES "Solar Thermal Electric Power Systems" task, which focuses on the development of international guidelines for modeling and acceptance testing of CSP systems. He has participated on and conducted analysis for several task forces, including New Mexico Governor Richardson's Concentrating Solar Power Task Force, the Solar Task Force for the WGA Clean and Diversified Energy Initiative, and more recently the U.S. Department of Energy's Solar Vision and Renewable Energy Futures studies. He is actively involved in the development of several new grid-integration studies investigating the interaction of CSP systems with thermal energy storage within the western U.S. interconnect.