                          312  1 Model of vacuum glass heat pipe solar collectors     Università degli Studi di Firenze, Dipartimento di Energetica “Sergio Stecco” Via C. Lombroso 6/17 – 50135 Firenze – Italy Daniele.fiaschi@unifi.it Giampaolo.manfrida@unifi.it Abstract:                                                         !""#$   %                      & ’($          %)%       &      *    * +                          &&  %   ,                     -    -              .  --                                   %                            Keywords: + /  ’ (0 $ ’($ % )  % %)% 1. Introduction The glass heat pipe design has become a bestseller in advanced applications of solar energy thermal utilization, with special reference to systems designed for providing heat in regions with low radiation, or during winter; the most performing collectors use a doublepipe design for the absorbing section, with deep vacuum in between, forming a kind of “Dewar” bottle which is very effective in reducing convective heat transfer, and allows thus to reach high absorber temperatures: a typical example of this solution (DGVHP for Double Glass Vacuum Heat Pipe) is shown in Figure 1, while Figure 2 represents two detached heat pipes. Currently, most of these devices are produced in China [1] and are rebranded for worldwide commercialization; their application is limited to hot water production. The idea pursued in this paper is to develop a mathematical model of the DGVHP, in order to be able to modify its design so that it can be adapted to producing a fluid flow at temperature interesting for applying thermodynamic energy conversion to small CHP applications, providing heat and electricity to small, distributed users (residential, commercial, SME industrial production processes). The DGVHP, within such systems [2, 3], represents totally or partially the component dedicated to the conversion of radiation into thermal power, which is then transferred to a suitable working fluid (water/steam, organic or engineered fluid). It has been