Control strategies for rankine solar cooling systems

Solar & WindTechnologyVol. 1, No. 4, pp. 259-264,1984

0741~983X/85 $3.00+ .00 PergamonPressLtd.

Printedin GreatBritain.

TECHNICAL

NOTE

Control strategies for Rankine solar cooling systems ABDUL-RAOUFMANNAA, M. GI-IAZIand ADELKHALIFA Mechanical Engineering Department, King Abdulaziz University, P.O. Box 9027, Jeddah, Saudi Arabia (Receioed I Au#ust 1984; 30 Auoust 1984)

Aktraet--Control strategies are designed to meet the normal, abnormal, startup and shutdown operations for three solar powered/fuel assisted steam Rankine engines, each system operates a 22-ton air conditioning unit. The stability of the first system is maintained by the use of an auxiliary oil heater in the concentrator collector loop, thus maintaining a steady energy input to the Rankine cycle. The steady output of the turbogenerator is interfaced with the public utility grid to match the required cooling load. The second system is similar to the first one but includes a thermal energy storage integrated to the collector loop. In the third system the public utility grid interface is eliminated and both the compressor of the vapor compression cooling cycle and the generator are directly coupled to the gear box. A cold storage is used as a capacitor to overcome the fluctuation of the cooling load. The control strategy of the first system is modified to be adopted to the other two systems.

Schlesinger and Peltzman [4] discussed general design considerations for data collection systems for three classes of solar energy studies. Typical design tradeofffor each type, and some technical and cost factors for each were also given. In this work, the controls and data acquisition system for Rankine solar cooling system are discussed. Two modifications of this system and their controls are also outlined.

INTRODUCTION Instrumentation and controls are an essential part of all Rankine solar cooling systems. They serve to assure safe operation, optimum performance, economic and reliable operations of equipment. Many researchers provided significant contributions in this important subject. Luffey and Black [1] presented a "soft driven" digital computer based control system interfaced with analog control elements for the Fort Hood Solar Energy System. In this design, a master computer located in the central control room communicated disynchronously with field distributed process units via parallel linked transmission lines. These distributed process units contained control algorithms, control commands, and set points which were relayed to either conventional analog controllers or analog actuators via signal conditioned I/O terminal boards located in the distributed process units. Barber and Dillard 12] outlined the controls of a 77-ton Rankine cycle water chiller. The controls of this system were divided into two main groups according to their functions. The primary controls were to regulate the temperature of the chilled water delivered from the water chiller evaporator. The secondary controls included logic for sequencing the startups and shutdowns for both normal and emergency conditions. Farrell and Reska [3] discussed a"unique" computer-based digital control system-designed by General Electric--to control, monitor and acquire data from the world's largest industrial solar thermal energy system at Shenandoah, Georgia, U.S.A. The control system employed distributed microcomputer architecture, a human factors engineered control console, and large scale integrated electronics hybrid signal conditioning to control solar energy collection, storage, conversion, and utilization. This total energy system can thus operate under either automatic or manual control to produce electricity, process steam, and provide space heating and cooling for a large knitwear manufacturing plant.

SYSTEM DESCRIPTION The solar cooling system in general (Fig. 1) receives the input energy from an intermittent source which is the solar radiation absorbed by the collectors field. Also, its output cooling load is not steady since it depends on the demand of the air conditioned space. In order to utilize the input energy and provide the required output cooling load while maintaining the system stability several solutions are possible. Three of them are selected and discussed in this paper. The main system is a solar powered fuel assisted steam Rankine cycle (Fig. 2). The system has two heat sources : the solar energy and an auxiliary gas fired oil heater to supply the deficit in energy supply demanded by the Rankine cycle loop which is maintained at fixed conditions and a constant output of 36 kW. In case of surplus solar energy some of the solar collector rows have to be shut down in order not to exceed the required input heat. The primary solar source is a 400 m 2 parabolic trough collectors field, both primary energy sources supply the heat to a preheater and a boiler in the Rankine cycle loop. The steam is then superheated with a natural gas-fired superheater to improve the cycle efficiency. The steam powers a two-stage, 60,000 rev rain-t turbine. The Rankine heat engine uses an air-cooled condenser and electrically driven boiler feed pump. The turbine shaft speed is reduced to 3600 rev min- t by a gearbox and drives an induction generator that produces 36 kW. The power produced by the generator is 259

260

Technical Note

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Fig. 1. Input-output diagram for a solar cooling system. The control for the system consists of four subsystem controllers connected to a main controller as shown in Fig. 4. The collector tracking controller (CTC) communicates with the individual controllers of each solar collectors' row. Its main function is to initiate the tracking of the collectors, stow the collectors in cases of dust storms, power failure, or emergency and normal shut-offs, it also eliminates some of the collector rows in case of excess solar energy input at the command of the energy management controller (EMC). The CTC is connected to the main controller to receive signals for the startup of the collector loop by a gradualincrease of the oil pump speed until the oil preheater exit reaches the normal working condition. The second controller is the energy management controller (EMC). Its function is to maintain the prespecified value of the oil temperature at the solar boiler inlet. If the actual oil temperature drops auxiliary energy is demanded. The EMC will then activate the natural gas oil heater, shut the oil heater bypass off, and open valve at the heater inlet. If the solar insolation is below a prespecified value the EMC will shut the valve at the inlet to the collector field and open the bypass valve to short circuit the collector field and the collectors will be kept in stow position. Under this condition the oil heater will be the only source of energy. In case of oil temperature

interfaced with the public utility grid which acts as a back-up power when the Rankine cycle is down. Also, when the load requirements are low, the excess generated power is dumped into the utility grid. Thus the problem of the fluctuations in the output cooling load is overcome by interfacing the fixed output of the Rankine cycle with the public utility grid.

INSTRUMENTATION AND CONTROL The piping and instrumentation diagram for the main system is shown in Fig. 3. The instruments types and positions were selected to measure all parameters, solar, weather, and air conditioned data, and to transmit information to the controllers. The instruments include thermocouples, pressure guages, flow meters, speed and horsepower meters. Electric signals from thermocouples, flow and pressure transmitters, torque, speed and horsepower meters are directly sent to a terminal board and then fed to a data acquisition subsystem and a monitoring board. The data acquisition subsystem will multiplex signals, digitize and send them to a recorder and/or to a computer terminal. The analized data can be presented on a video screen, plotted by a graphic plotter or printed using a fast line printer.

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Technical Note

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Fig. 3. Piping and instrumentation diagram of the main system.

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262

Technical Note

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Fig. 4. Layout of the main system control.

increase above the prespecified value, the EMC will shut the oil heater off(if it is on), convert the oil flow to bypass the heater and eliminate some rows of the solar collectors, if necessary. The third collector is the Rankine cycle controller (RCC). Several functions are done by this controller. During startup the RCC will receive a signal from the main controller upon which it will operate the condenser fan and the boiler feed pump, and will activate the gas-fired superheater when the boiler generates enough steam to avoid superheater tubes burn out. In cases of normal or emergency shut offa signal will be received from the main controller upon which the RCC will shut the natural gas supply to the superheater, and then it will turn the boiler feed pump and the condenser fan off.

The last controller is the interface controller (IC) which manages the energy transfer between the public utility, the system generator, and the load (airconditioning unit). The main controller communicates with the other four controllers. It transmits signals for start up, normal and emergency shut off to the subsystem controllers. Emergency shut down is necessary in cases of boiler overpressure, condenser overpressure, superheater over temperature, turbine overspeed, loss of gearbox lubricating oil, leakage, or public utility break down. It also conveys the output alarms to the diagnostic panel. The manual interaction can be done through communication between the operator console and the main controller.

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Technical Note

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MODIFIED SYSTEM I

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The modified system I is similar to the main system, but the gas fired oil heater is replaced by a hot storage which is integrated to the collector loop as shown in Fig. 5. The main function of the hot storage is to store the excess thermal energy and augment the solar boiler during extended periods of low solar insolation. The control strategy for this system is the same as the main system except that :

In the modified system II, an auxiliary boiler and a cold storage are integrated to the power loop and the air conditioner, respectively, as shown in Fig. 6. By these additional components, the fluctuations problem of solar insolation and cooling load have been solved. The cooling loop, of that system, consists of a packaged air conditioner, a cold storage and fan coils. The refrigeration effect is stored in a cold storage which provides the required cooling load through fan coils. The cold storage can be used during emergency and normal shut-offs. Thus it can be easily concluded that the modified system II has the advantages of being used in isolated areas and to avoid the interface problems with the public grid. The difference between the control strategy of modified system II, Fig. 6, and the main system strategy, Fig. 2, can be presented in brief as follows:

1. The energy management control (EMC)is used to ensure that the power loop operates at steady condition by regulating the oil flow rate through the solar boiler, by controlling the variable speed pump. 2. Any excess of energy will be stored in the hot storage, therefore, no elimination of any collector rows by the controllers is necessary. The net output of the turbogenerator will still be fixed at 36 kW and the fluctuations in the cooling load will be handled by the interface with the public utility grid.

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Fig. 7. Schematic diagram of the modified system II controllers.

264

Technical Note

for actuating the auxiliary boiler when solar energy is not sufficient. 2. Any excess of energy will be stored in the cold storage therefore no elimination for any collector rows by the controllers will be done. 3. The interface control (IC) is replaced by thermal comfort control (TCC). The TCC will receive information from conditioned space sensors (temperature and humidity) upon which it will manage cooling loop to give the required refrigeration effect, see Fig. 7. CONCLUSIONS The solar cooling system can be modeled as a system with a transfer function which has a fluctuating input and output. Additional components and proper control strategies must be integrated into the basic system to ensure stability, optimum performance, and safe operation. A control strategy has been outlined for a solar powered/fuel assisted steam Rankine engine which operates a commercial air conditioning unit (the main system). For the purpose of using the system as a demonstration plant the instruments and data collection devices have been selected. The resulting modular features of the control strategy have been applied to two other systems, modified system I and modified system II. The modified system I is similar to the main system but it has a hot storage instead of the auxiliary heater. Modified system II includes an auxiliary boiler and a cold storage integrated to the power loop and the air conditioner, respectively. The latter system has the advantage of possible utilizations in isolated areas.

Acknowledgement--The ideas presented here have resulted from the work on a project supported by Saudi Arabian National Center for Science and Technology, under the joint United States/Saudi Arabian Program for Cooperation in the field of Solar Energy (SOLERAS). This support is greatly appreciated.

REFERENCES

1. F.C. Luffey and D. L. Black, The control system for Fort Hood solar total energy system. Proceedings of the

Fourteenth lntersociety Energy Conference, August, 1979.

Conversion Engr.

2. R. E. Barber and J. E. Dillard, Jr., Design, fabrication and testing of a 77-ton solar powered water chiller for the National Security and Resource Study Center at Los Alamos, New Mexico. Proceedings of the Third Workshop on the Use of Solar Ener9yfor the Cooling of Buildings, San Francisco, California 1978. 3. J. O. Farrell and R. S. Reska, A distributed microcomputer-based control system for a large scale solar total energy system. Proceedings of the Fourteenth Intersociety Energy Conversion Enyr. Conference, August, 1979. 4. R. J. Schlesinger and E. S. Peltzman, Solar Energy Data Acquisition System. Rho Sigma Inc., 15150 Raymer St., Van Nuys, CA 91405, U.S.A. 5. H. Gari, A. Radwan, A. Mannaa, A. Khalifa and M. Ghazi, Solar cooling project. Report submitted to Saudi Arabian National Center for Science and Technology, January, 1984.