The research regards the development, design and application (installation), testing and monitoring and performance evaluation of a Low Temperature SORC system for sea (or brackish) water desalination. The system consists of the following sub-systems and components (Fig.1):

1) High efficiency vacuum tube solar collectors’ array

2) Circulator

3) Alternative thermal source (thermal wastes, geothermal or other)

4) Evaporator

5) Condenser

6) Economiser

7) Expanders

8) Pressurisation unit consists of 2 vessels V1, V2 three automatically controlled valves (VL1, VL2, VL3)

9) RO unit

10) Insulated seawater reservoir

11) Fresh water reservoir

12) RO energy recovery system

The system operation is described briefly below:

Thermal energy produced by the solar array (1) preheats and evaporates the working fluid (HFC-134a) in the evaporator surface (4). The water temperature at collector inlet is about 70 oC and the outlet temperature about 77 oC. The super-heated vapour is driven to the expanders (7) where the generated mechanical work drives the RO unit pumps (high pressure pump, cooling (heating) water pump, feed water pump) and circulating pump (2). The saturated vapour at the expanders’ outlet is directed to the condenser, after passing through an economiser (de-super heater) (5). On the condenser surface, seawater is pre­heated and directed to the seawater reservoir (10). Seawater pre-heating is applied to

increase the fresh water recovery ratio (in RO technology, higher feed water temperatures imply higher fresh water recovery ratio). The seawater tank is insulated. The use of seawater for condensation purpose on the condenser surface decreases the temperature of "Low Temperature Reservoir” of Rankine cycle thus a better cycle efficiency can be achieved. The saturated liquid at the condenser outlet is pressurised in the special pressurisation arrangement consists of two vessels and three valves (8) 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.

An energy recovery system is coupled to the RO unit thus declining the energy consumption to 3 kWh/m3 product.

Below the thermodynamic analysis of the states described above is presented (Fig.1,2):

Table 1: States of Rankine cycle State T P H S (oC) (kPa) (kJ/kg) (kJ/kgoK) S1 Super-heated vapour, evaporator outlet, expander inlet 75.8 2200 435.7 1.7138 S2 Saturated vapour, expander outlet condenser inlet, isentropic expansion 35 887.91 417.5 1.7138 S3 Saturated liquid, condenser outlet 35 887.91 249.2 1.1676 S4 Sub-cooled liquid, pre-heater inlet «35 2200 «248.0 1.1676 S4’ Saturated liquid, evaporator inlet 71.7 2200 307.8 1.3433 Calculation of theoretical and actual efficiency of the system

rankine

AHq

AHW = Hsl — Hs2 AHq = Hsl — Hs3

hrankrne = °.°976

Where: n, Efficiency; H, Enthalpy; T, Temperature

The theoretical Organic Rankine system efficiency is 9.76 %

The Carnot cycle efficiency is 10.6 %

The actual system efficiency is estimated to 65% of theoretical that is 6.34 %.

For the prototype system 240 m2 of vacuum tube solar collectors will be deployed. Both the evaporator and condenser are plate heat exchangers of brazed type. The heat exchanger area of the evaporator is 6.8 m2, while that of condenser is 11.5 m2. For this number of collectors and considering a water recovery ratio of seawater RO desalination

SHAPE * MERGEFORMAT

Enthalpy (kj/kg)

Figure 2: Molier chart of Rankine cycle (working fluid states)

unit of 30%, the average yearly fresh water production is estimated at 1450 m3 (or 4 m3 daily)2. Table 2 presents the distribution of fresh-water production throughout a year.

Table ^ 2: Fresh water production (Average daily ^ water production (m3/day) Jan Febr Mar Apr May Jun Jul Aug Sept Oct Nov Dec 0.81 1.98 2.96 4.76 5.76 5.85 6.72 6.98 5.86 3.54 2.18 0.89