59 FIRST EXPERIENCES WITH COAXIAL BOREHOLE HEAT EXCHANGERS J. Acuña, B. Palm jose.acuna@energy.kth.se . Department of Energy Technology/Applied Thermodynamics and Refrigeration/KTH Brinellvägen 68, 100 44 Stockholm, Sweden ABSTRACT Some experiences with coaxial borehole heat exchanger prototypes are discussed here. Four dif- ferent designs are described as they have been part of a research project at KTH: two pipe-in- pipe annular designs, one multi-pipe and one multi-chamber design. A special focus is given to two of the prototypes, a pipe-in-pipe design with the external flow channel consisting of an an- nular cross section and partly insulated central pipe, and a multi-pipe design with twelve paral- lel peripheral pipes and an insulated central channel. The secondary fluid temperature profiles at low volumetric flow rates are presented for these two prototypes, measured with fiber optic ca- bles during thermal response tests and allowing a detailed visualization of what happens along the heat exchanger depth. It is the first time this is carried out in these types of borehole heat ex- changers. The measurements indicate good thermal performance and point at potential uses for these heat exchangers in different ground coupled applications. 1. INTRODUCTION The performance of Ground Source Heat Pumps (GSHP) coupled to vertical energy wells partially depends on the thermal resistances in the ground and in the borehole. Different locations may imply different design needs and the thermal response of the ground at a certain location is difficult to control by other means than changing the position of the boreholes and controlling the heat rates from and to the ground. However, it is possible to affect the resistance in the borehole by changing the Borehole Heat Exchanger (BHE). This resistance depends on the heat transfer between the secondary working fluid and the borehole wall and depends on the arrangement of and the heat transfer in the flow channels, possible convection in the borehole, the thermal properties of the BHEs as well as of the borehole filling material. All these different parts are normally added together and called borehole thermal resistance, defined as R b by (Hellström, 1991). It is of great relevance to design cost effective BHEs characterized by moderate temperature differences (low R b ) between the secondary fluid and the surrounding ground. A one degree temperature dif- ference flowing from the BHE into the heat pump evaporator may represent 2-3% change in the COP of the system. This paper shows measurements from an annular coaxial design with partly insulated central pipe and a so called TIL (Thermal Insulated Leg) design that present possibilities for solving this issue. These are being tested as part of two research programs, EFFSYS2 (www.effsys2.se ) and EFFSYS+ (http://effsysplus.se ). Padua, Italy, April 5-6-7, 2011 Sources/Sinks alternative to the outside Air for Heat Pump and Air-Conditioning Techniques (Alternative Sources - AS)