D. Manolakos, G. Makris G. Papadakis and S. Kyritsis

Agricultural University of Athens
Dept. of Agricultural Engineering, Farm Structures Laboratory
75, Iera odos street 11855 Athens, Greece
Tel: +30-210-5294033 Fax:+30-210-5294023 e-mail: dman@aua. gr

K. Bouzianas

Hellas Energy K. Bouzianas P. Moschovitis & Co
10, Saint George Square 11257 Athens Greece
Tel: +30-210-8222519 Fax: +30-210-8238314 e-mail: kostasb@compulink. gr

The research regards the development, application testing and performance evaluation of a low temperature solar organic Rankine cycle system for Reverse Osmosis (RO) desalination. Below is given a technical description of the system under development:

Thermal energy produced by the solar array evaporates the working fluid (HFC — 134a) in the evaporator surface. The super-heated vapour is driven to the expanders where the generated mechanical work produced by the Rankine cycle drives the RO unit pumps (high pressure pump, cooling water pump, feed water pump) and circulating pump. The saturated vapour at the expanders’ outlet is directed to the condenser and condensates. On the condenser surface, seawater is pre-heated and directed to the seawater reservoir. Seawater pre-heating is applied to increase the fresh water recovery ratio. The seawater tank is insulated. The use of seawater on the condenser surface decreases the temperature of “Low Temperature Reservoir” of Rankine cycle thus a better cycle efficiency is achieved. The saturated liquid at the condenser outlet is pressurised in a special pressurisation arrangement consists of two vessels and three valves, substituting a pump. The sub-cooled liquid at the pressurisation arrangement outlet is driven to the economiser. The economiser acts as working fluid pre-heater. In the economiser outlet saturated liquid is formed, which is directed to evaporator inlet and the cycle is repeated.

For the prototype system 240 m2 of vacuum tube solar collectors will be deployed. The evaporator and condenser capacity is estimated to be about 100 kW. For these systems’ characteristics and considering a water recovery ratio of seawater RO desalination unit of 30%, the average yearly fresh water production is estimated at 1450 m3 (or 4 m3 daily).

Specific innovations of the system are:

Low temperature thermal sources can be exploited efficiently for fresh water production; solar energy is used indirectly and does not heat the seawater; the RO unit is driven by mechanical work produced from the process; the system condenser acts as sea water pre-heater and this serves a double purpose; (1) increase of feed water temperature implies higher fresh water production (2) decrease of temperature of “low temperature reservoir” of Rankine cycle implies higher cycle efficiencies.