new magic fluid, page-9

Heat transfer in the primary circuit.

You are raising some most interesting brainstorming questions: how can the efficiency of heat transfer from underground be improved? One aspect is to consider the specific heat capacity of the fluid. Others might be, how can greater flow rates be effected through existing fractures and, how can heat transfer from the fracture face to the fluid be enhanced? Do theoretical solutions even exist?

Try a few bites at the problem. The specific heat of a material is the energy required to raise unit mass by a temperature of 1 C. It is a key thermal property of a material; the higher the value the more heat can be contained or transported. It is interesting to note that a scan along any list of properties of materials always clearly shows that water has the highest value by far at 4180 J/(kg.C) at 20 C. Only liquid ammonia comes close. If it were to be mixed with some other fluid the average specific heat could therefore only decrease as would the total energy transported. [This characteristic is determined by the number of possible modes of vibration of the atoms within a molecule and is therefore unalterable unless the molecule rearranges with increased temperature. I can't see how trapping molecules within fullerenes would change this.] Regardless of this it is tempting to theorize on the use of other liquids such as mercury (ignoring cost and other absurdities). For Hg the volumetric heat capacity has value 1890 and is largely invarient to temperature change while for water the value rises to 10 850 at 250 C making it even more desirable as a medium. I've also considered low temperature fuseable alloys such as Wood's metal which melts at 70 C but the relevant data is not readily available.

For heat transport the speed of fluid flow could become important which in turn is related to the viscosity or resistance to flow. Coefficients of viscosity decrease as temperature increases. For water at 300 C it falls to a value of 10% of that at 20 C, and amazingly, it collapses to just 3% at 400 C. It seems that plain water is the magic fluid at high temperature. I wrote a post here on viscosity effects and its implications on 28 Oct 2008. (I don't think anyone read it.)

Just supposing we could invent and push through a new magic working fluid that had properties that could transport 10 x the present heat energy to the generating plant would we gain anything useful? The fluid washes past the hot rock surface to absorb any heat hotter than the fluid itself. Will the rate of heat conduction through to the rock surface be sufficient? There will be an optimum rate of fluid flow to maximize heat transfer. It is this basic rate of conduction within the rock that is the bottleneck. Improving the ideal properties of the fluid will then not help. After that further heat can only be extracted from the underground heat exchanger by increasing the surface contact area and this means increasing the number of permeable fractures between the wells. Just where the optimum values occur would require setting up a detailed numerical model.

(I would have posted sooner but I'm having a problem copying from Microsoft Word at the moment. Apologies.)

Juke