35 u00b0 c which helps enable it to maintain two phases under normal operating conditions exemplary heat transfer fluids for use in the first fluid circulation loop include but are not limited to saturated hydrocarbons e g propane isobutane unsaturated hydrocarbons e g propene r32 r152a ammonia an r1234 isomer e g r1234yf r1234ze r1234zf r410a and mixtures comprising one or more of the foregoing the heat transfer fluid used in the second fluid circulation loop has an ashrae class a toxicity rating and an ashrae class 1 or 2l flammability rating exemplary heat transfer fluids for use in the second fluid circulation loop include but are not limited to sub critical fluid co 2 a mixture comprising an r1234 isomer e g r1234yf r1234ze and an r134 isomer e g r134a r134 or r32 2 phase water or mixtures comprising one or more of the foregoing in another exemplary embodiment the second heat transfer fluid comprises at least 25 wt and more specifically at least 50 wt sub critical fluid co 2 in yet another exemplary embodiment the second heat transfer fluid comprises nanoparticles to provide enhanced thermal conductivity exemplary nanoparticles include but are not limited to particles having a particle size less than 500 nm more specifically less than 200 nm in an exemplary embodiment the nanoparticles have a specific heat greater than that of the second fluid in yet another exemplary embodiment the nanoparticles have a.