Optimization of a Hybrid Ground Source Heat Pump using the Response Surface Method
http://www.ep.liu.se/ecp/057/vol5/015/ecp57vol5_015.pdf

Simulation and optimal control of hybrid ground source heat pump systems
http://gradworks.umi.com/32/91/3291353.html

Optimal Design for a Hybrid Ground-Source Heat Pump
http://repository.tamu.edu/bitstream/handle/1969.1/5468/ESL-IC-06-11-307.pdf

Optimization of cooling-dominated hybrid ground-coupled heat pump systems
http://www.thefreelibrary.com/Optimization+of+cooling-dominated+hybrid+ground-coupled+heat+pump...-a0201591040