GUIDELINES - Choosing energy collector


On the market today there are two major parts of standard energy collectors, smooth and fined PE collectors. Discussions about usage of the energy collectors in projects are often an issue and many installations are made without proper calculations. Scope of work for energy collector design is to fulfil the highest allowed pressure drop in the PE pipe system together with a low resistance for heat transfer inside the pipe.


Energy collector configuration

First step in collector design work is to determine if single U or double U collector should be used. Based on the nominal flow rate in the system, the number of boreholes and type of heat carrier fluid the collector design is chosen to achieve a good flow situation in each energy collector. As a rough design value 0.5 lit/s for PE 40x2.4 U bend and 0.3 lit/s for PE 32x2.0 U bend could be used. For double U configuration the flow rate to each borehole is twice these values.


Smooth or energy collector with fines

Based on the boundary temperature levels from heat pump/ cooling system design calculations of Reynolds number (RE) should be done. Our recommendation is to have a RE number that is over 2300 and below 4000 for a smooth pipe. If RE is low collectors with fines is recommended, if the RE number is high smooth pipes is recommended. It is important that RE number is calculated on actual heat carrier fluid and working temperatures. For systems that normally is working between RE 2400 and RE 4000 but in peak load situation has a RE number under RE 2400, recommendation is to always use collectors with fines. As the RE number is decreasing with decreasing temperature of the fluid the critical point is the lowest temperature regarding heat transfer. For pressure drop the critical point is the highest system temperature. For system solutions with both heating and cooling the situation often is different as the temperature difference is not equal in heating and cooling mode. In that case it is important to find the critical points for design.

This recommendation is based on standard energy collector design where low pressure drop together with good heat transfer performance is the design point. All systems must be calculated in the design phase to fulfil actual demands in each project.


Example of fined energy collector