. Two loop system
The concept of the “two loop “system is different from the compact system and will be favourably from the economical point of view for daily capacities higher than 1000 litre. A sketch of the principle set up is given in figure 5. The design keeps four main differences compared to the compact systems:
A thermal storage tank is used to enable an extended operation time of the MD-modules even after sun-set ^Decreasing specific module costs
The system consists of two loops. The desalination loop is operated with sea water and is separated from the collector loop which do not have to be sea water resistant ^ Cheaper standard components (storage, collectors, pumps) can be used
Several MD-modules are operated in parallel.
A controller is used for charge and discharge of the heat storage and controlling a set point temperature for the evaporator inlet ^The operation conditions can be optimized regarding performance conditions of the MD process.
Simulation computations were carried out for the system design and the development of an adapted control strategy for two different pilot plants. The design capacity for the Aqaba system was 700 - 900 l/day and for the Gran Canaria system 1000 - 1600 l/day. Table 1 provides the key data for both systems.
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Table 1: Key data of the two “two loop systems” as designed by simulation computations
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Figure 6 shows a picture of the collector field (left hand side) and the heat storage, hydraulic board and the desalination unit (right hand side) of the Aqaba system installed in December 2005. The two loop system in Gran Canaria was installed in March 2006.
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Experimental investigations Compact System
The graphs in figure 7 (left side) represent one day of operation of the Compact System in Freiburg on 16th July 2006. The solar irradiation (Irrad) in the collector plain increases up to 1000 W/m2 at noon. The feed pump switches on when a set point temperature of 55°C at the collector outlet is reached. The feed mass flow is in the beginning about 300 l/h and increase with rising solar irradiation to a maximum of 500kg/h at noon.
The distillate production starts immediately after the system start up and increases continuously with the rising evaporator inlet temperature and feed volume flow. The maximum distillate mass flow during noon is about 25 l/h. The maximum evaporator inlet temperature is almost 90°C. The temperature at the condenser inlet (Tcond_in) increases during the day, due to the brine recirculation to the feed tank. At 12:00PM the refilling of the feed storage starts initialized by reaching the critical feed temperature of 50°C at the condenser inlet. As can be seen the temperatures (Tcond_in) decrease immediately by 10 K. The total daily distillate production at the 16th July 2006 was about 140 liter.
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The first Compact System was installed in Gran Canaria in December 2004 and is still in daily operation with the first MD-module. Figure 6 (right hand side) presents a part of these long term measurements in Gran Canaria. The daily distillate gain is plotted versus the cumulated daily gain of solar energy. The considered period starts in the middle of June 2005 and ended in the middle of June 2006. As can be seen from the one year measurements there is no decrease of specific energy demand during the observed period. For example a daily solar gain of 7kWh/m2 enables an average distillate production of 60 l/day in June 05 as well as in June 06. Differences, in both directions, occur due to specific weather conditions an operation conditions.
Fig 7: Daily measurement - Compact System Freiburg (left), Long term measurement Gran Canaria - daily
permeate production vs. sum of daily solar irradiation (right)
A plot the operation performance of the two loop system in Gran Canaria is presented in figure 8 for a fine day in March. The “Irrad” graph represents the global radiation on the tilted collector surface. The “TCol_out” graph is the collector outlet temperature and the “T_evap_in” line represents the evaporator inlet temperature as adapted by the control unit. As can be seen the collector outlet temperature and the evaporator inlet are rising comparably until 11:00 AM (IG=500W/m2) when the set value at the evaporator inlet is reached. Then the collector outlet temperature continues rising while the evaporator inlet is set to 80°C.
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Fig. 8: 24 hour measurement and collector field efficiency of the two loop system in Gran Canaria in March 2006. |
From 6:30 PM to 7:30PM the controller switches to storage discharge. As can be seen the temperatures are fluctuating and can not be controlled to the set value. The reason can be found in the slow reaction time of the control unit respectively the slow movement of the Valve. From 7:30PM to 22:30AM the temperature control operates successfully again and the evaporator inlet temperature is set again to 80°C until the storage top temperature decreases below that value. The system is operated with a decreasing evaporator inlet temperature and decreasing distillate flow until 4:15AM, March 24. Then the switch of temperature of 58°C at the evaporator inlet is reached. The distillate volume flow during operation on the set point temperature is about 75 and 80 kg/h. The cumulated distillate gain from the operation period between 10:00 AM March 23 and 6:15 AM March 24 is 1240kg.
The specific energy consumption of the MD unit is in the range of 260 kWh/m3 distillate for low evaporator inlet temperatures of 55°C and decreases down to 180kWh/m3 distillate at a set point operation temperature of 80°C. The specific energy supplied by the collector - respectively the storage loop is 14% to 23 % higher than consumed by the MD unit. That difference can be considered as system losses.
The diagram on the left hand site of figure 8 presents the collector field efficiency (nth) of the Gran
Canaria two-loop system. It was calculated from the temperature difference between collector field in - and outlet, the collector loop mass flow rate, the specific heat capacity of water (Cp) and the measured global radiation in the collector plain.
nth = (T_coll_out - T_coll_in)*mp_coll*Cp / IGt
It can be seen from the graph that in the range of standard operation between 0.06<(dT/G)<0.08 (for G =1000W/m2 and Tamb=20°C this is equal to an average collector temperature range of 80 - 100°C) the collector field efficiency is between 0.61 and 0.5. The stagnation temperature for G=1000W/m2 and Tamb=20°C can be calculated from the efficiency curve with 200°C.
