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Optimization of aquaculture systems in Spain
Energy
Pergamon PII: s01%-8904(%)ooo94-5
OPTIMIZATION
OF AQUACULTURE M. IZQUIERDO
Comers. Mgmt Vol. 38. No. 9. pp. 879-888, 1997 0 1997 Elsevier Science Ltd. All rights reserved Printed in &eat Britain 0196-8904/97 $17.00 + 0.00
SYSTEMS IN SPAIN
and M. CARRILLO’
Instituto Ciencias de la Construction Eduardo Torroja, (CSIC), c/Serrano Galvache s/no, 28033 Madrid, Spain and ‘Instituto de Acuicultura de Torre la Sal &SIC), Ribera de Cabanes, 12595 Torre de la Sale, CastelIon, Spain (Received 21 September
1995)
Abstract-An analysis of present heat production systems, using fossil fuel combustion, employed for sea water heating in Spanish hatcheries is given in this paper and compared to a technical solution based on the employment of a heat pump. Price per unit of produced energy is calculated for each system using liquid and gaseous fuels, and then these prices are compared to the price obtained for a heat pump. The heat pump system is also compared, from the point of view of its precision in maintaining temperatures, to the systems used at the present. A project prototype for thermal conditioning and temperature control in aquaculture rearing tanks is described. The waste heat of the return water is recovered: first, by a static recoverer and second, by a recoverer connected to the evaporator of a refrigeration unit. The system should be used in such a way that economic benefits are obtained from its two heat sources of the frigorific system in simultaneous production of warm and chilled sea water. 0 1997 Elsevier Science Ltd Aquaculture
Fuels
Heat recoverer
Heat pump
Heat cost
NOMENCLATURE C = Cost of produced energy (PTA/kWh) COP = Heat pump coefficient of performance CV = Calorific value (kWh/kg) E = Input electrical energy (kWhe) F = Fuel (kWh/m’) h = Enthalpy (kJ/kg) rn = Sea water flow (kg/h) P = Cost of fuel (PTA/m’ year) p = Pressure (bar) Q = Calorific power (kW) R = Recoverer Sh = Specific heat (kJ/kg”C) t = Temperature (“C) x = Waste heat recovered (%) Greek leiters q = Efficiency
T = Compression ratio Subscripts
bu = c= d= e= el = er = hp = i= If = me = ng = ove = p= sea = I. . n =
Burned Condensation Design Evaporation Electric Exit recoverer Heat pump Indicated Liquid fuel Mechanical Natural gas Overall Process Sea water Recoverers; points of cycle
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INTRODUCTION
The increasing demands for proteins by our overpopulated world have forced the search for alternative methods of food production distinct to agriculture of fisheries. Aquaculture, as the husbandry, management, nutrition and breeding of useful aquatic organisms, such as fish, crustaceans, molluscs and algae, has been considered as the above mentioned alternative. The FAO estimates that aquaculture production is capable of considerable improvement by the year 2000 through optimal use of existing resources and the application of science. Intensive culturing of most commercially valuable sea fish and crustacean species requires a continuous water supply in order to maintain their quality and to favour the growth and reproduction physiological processes of these organisms [l l] in hatcheries. A detailed knowledge of these parameters, such as oxygen demand or fish weight as a function of temperature, enables the calculation of water flow requirements. These are critical [9] when it is intended to define production capacity in an aquatic medium. In large masses of water, optimum temperature must be maintained for every species; larval stage, year period or geographic latitude. Every cultured sea species, given adequate nourishment conditions, have temperature as a basic parameter controlling its growth. In addition, each species has an optimal development temperature [lo]. An important implication of this is that growth cycles of each species vary as a function of average water temperature, being shortened or prolonged as temperature is raised or lowered. On the other hand, the seasonal variation of temperature is predisposed to stop production in winter periods when the temperature is lower, since growth and nourishment parameters are sharply reduced [ 111. The employment of sexual, thyroid and pancreatic hormones in aquaculture [4] could maximize production per cost unit in sea hatcheries. In spawning, induced production of egg and semen by the administration of gonadotrophic hormones is widely used [5], [13]. Nevertheless, when reproduction cycles are modified by environmental manipulation leading to induced spawning out with the natural season, the larvae often suffer serious defects. These manipulations of photoperiod and temperature could be a major cause of these defects [ 1l] because the role played by temperature in the reproduction process has not been clearly identified, due to its difficulty to control and its need to be differentiated from photoperiod effects. Its influences on gonadal, spawning and post-spawning development [9], has been applied to induced spawning of sea bass (Dicentrarchus lubrux L.) out of season in a combined action of photoperiod and temperature (and eliminating hormone administration). This verified a spawning thermal threshold at a temperature around 16°C. For these reasons, it is necessary to develop a technology applicable to hatcheries such that it is possible to reproduce optimum temperatures at every stage of marine life (reproducer maintenance, egg spawning, larval and alevinal development) in these installations. Prefattening and fattening stages, however, will not be included in these studies [8]. To obtain these temperatures according to the tolerances demanded by the biologist and, in turn, to achieve them with the minimum expense of energy and production costs are the main aims of this work. The studies to be conducted and the ideas to be developed in the near future will be summarized below. This paper will analyse the present situation with respect to the technology employed in thermal energy production and temperature control of hatcheries installed in Spanish aquaculture facilities. This technology consists basically of heat production by means of the burning of liquid fuels. As an alternative to this technology, it is proposed to employ heat pumps which use mechanical compression to produce thermal energy. The advantages of this system, are analysed from an economic stand point as well as from its precision in temperature regulation and control. A prototype was designed by the CSIC at the Instituto de Acuicultura de Torre la Sal, Castellon (Spain). This prototype was designed according to energy saving criteria and employs a refrigeration unit in which mechanical compression of vapour is used to obtain economic benefits from both cold and hot (around 7°C and 40°C) thermal sources simultaneously. With the heat obtained from the cold source, chilled sea water (13°C) is produced that is employed in treatment of spawning induction tanks for fish, e.g. sea bass (D. lubrux L.) and sea bream (Spurus uurutu). The heat obtained from the hot source is utilised to produce warm sea water (18”C-28°C) that is
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employed in spawning induction for crustaceans (e.g. Pennaeus keraturus) as well as in obtaining an optimal temperature for larval and alevinal development of crustaceans, fish and molluscs. To keep this parameter throughout the year in the optimal ranges determined and with minimal energy costs is a task that overcomes biologists functions and results in the need for collaboration among biologists and engineers in order to find technical solutions which permit optimal conditions of life for these animals to reproduce in hatcheries. The project depicted in this paper is the result of this collaboration. OF REARING
HEATING
TANKS
USING FOSSIL
FUEL COMBUSTION
Figure 1 shows a simplified flow diagram of heating sea water in Spain. Preheating, using the residual heat of the waste water, takes place in heat recoverer, Rl. The required process temperature (28°C) aided by the boiler producing hot water at 8O”C-90°C is obtained in heat exchanger, R2. Sea water temperatures on Spanish coasts are approximately sinusoidal and range from 8°C to 20°C in the North and 11°C to 28°C in the South and East. In the North, it is usual to culture oysters or mussels with the optimal temperatures being around 20°C. In the South, shrimps are cultured at temperatures of about 28°C. In either case, it is necessary to heat the sea water in order to obtain these temperatures. This task is accomplished by the installation shown in Fig. 1. The problem, however, in these installations, employing a hot source of 8O”C-90°C is the difficulty in controlling process temperatures to 18”C-28°C with a tolerance of + 1°C. Energetic analysis
The difficulty in controlling temperatures mentioned above can be understood after taking an overall energetic analysis of the system and studying each one of its components. The thermal behaviour of these systems can be summarised as follows: As can be seen in Fig. 1, these systems work on an open circuit with a constant renewal of the water to the rearing tanks, such that the same flow of water extracted is returned to the sea. In heat recoverer Rl, the heat is partially recovered from the waste stream before it is discharged to the sea. In the heat exchanger R2, the water is conditioned in order to reach the required temperature for the rearing tanks. Accordingly, and assuming that there are no heat losses to the surroundings, the heat Qd required to obtain the process temperature, t,, can be calculated, Q,, = m . Sh . (tp - txa).
(1)
Since the sea temperature is variable, Q,, will be a maximum when t,, is a minimum, and this will be the design value for the installation.
to sea
IL.. FJ
F
month I
WLa Fig. 1. Fossil fuel system for sea water heating.
4
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Heat recoverer RI
There are different criteria for energy optimization in the design of this exchanger, and these must be applied to each given installation. For an operating temperature about 3O”C, a result, compatible with a reasonable exchange area, is that which permits us a maximal residual heat recovery from the return water of about 60%. According to this criterion, the heat recovered QR, will be
QRI= 0.6. Q,,.
(2)
QR, = m . Sh * (t,, - tsea).
(3)
This heat can also be expressed as
Water conditioner of process R2
The heat needed to be delivered to sea water in this exchanger will be QR2which, in turn, will be the energy that must be produced by the boiler
QRl= Qci- QRI
(4)
which, expressed as a function of mass flow rate, will be QR2= m * Sh * (tp - t,,).
(5)
With a variable sea water temperature, the resulting Qd, QR, and QR2values will also be variable and reach a maximum when the sea water temperature is a minimum. These will be employed as a design basis for each component of the installation. In order to know the behaviour of each heat exchanger as a function of sea water temperature, we can obtain t,, from equations (l), (2) and (3). ter = 0.6 - t, + 0.4. t,,,
(6)
which generalises for a percentage x of heat recovered in exchanger Rl t,, = x . t, + (1 - x) * t,.
(7)
This equation is valid when t, < tp. The minimum value of teeris obtained when t,, is a minimum and is a maximum when tsea= t,. For design conditions, t,, = 11°C and t,, = 28”C, we obtain t,, = 21.2”C and, therefore, QR2= 6.8 * m . Sh. If the process temperature is 28°C and the sea water temperature varies sinusoidally between 11°C and 28”C, for a t,, temperature of 24°C we obtain: t,, = 264°C and QRZ= 1.6 * m . Sh. For most of the year, the installation is oversized. This oversize could reach up to 500% in some periods of the year. This excess of exchange area, along with the fact that the temperature of the hot water supply for the boiler can not fall below 80°C using fuel-oils with amounts of sulphur > 1%, or 60°C using gas-oils with amounts of sulphur < 1% and natural gas (Instruction Tecnica IT.IC.17.2. de1 Reglamento de Instalaciones de Calefaccion Climatizacion y Agua caliente sanitaria. Real Decreto 1618/1980), means that the process temperatures, t,, in exchanger R2 could have variations of as much as 30% ( f 8°C) [2]. This is due to the automatic three way valve, which is part of the design constraints and behaves as an all open-shut valve. In order to solve this control problem, it is proposed to use a heat pump system which supplies water at temperatures of 35”C-40°C, rather closer to the process temperature. HEATING
OF REARING
TANKS
BY ELECTRICAL
DRIVEN
HEAT
PUMP
The scheme in Fig. 2 shows the warm water production system using a heat pump. The hot water boiler has been replaced by a heat pump. The heat of the waste return water from the fish tanks is partially recovered by a static heat exchanger, Rl, used to preheat the sea water. An additional recovery of heat takes place in heat exchanger, R2, where the evaporator of the heat pump absorbs heat from the return water. The return water is used as the cold source for the heat pump, such that the evaporation temperature keeps almost constant throughout the year. This second heat
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SYSTEMS IN SPAIN
EVAPORATOR
I Fig. 2. Heat pump system for sea water heating.
recovery permits us to give back to the sea the water of the rearing tanks at similar temperatures as those of extraction. The warm sea water, at temperature t,, is produced in exchanger R3 from the heat released by the condenser of the heat pump. The condensation temperature is variable and depends on the process. A temperature of 40°C in this hot source is sufficient to provide an aquatic system temperature between 18°C and 28°C. Because this source temperature is reduced to 40°C in comparison to the higher operating levels in a fuel boiler, regulating and controlling the temperature is easier. The system works on an open circuit from a hydraulic point of view, whereas it tends towards a closed circuit from an energy point of view. Thus, it recovers much more waste heat than conventional installations. Calculation of the heat pump
A simplified calculation method, according to the process temperatures stated by the biologist, is given below [l], [3]. 1. Total thermal load, ignoring surrounding heat losses Qd = m . Sh * (tP - t=).
(8)
2. Heat recovered in Rl Qa, =x
’
Qt,.
(9)
3. Heat recovered in R2
4. Heat exchanged in R3 Qa3 = Qc =
Qd- Qa, = (1 - x) * Qd.
(10)
5. Heat pump
Qc= QRX= Qra+ E
(11)
Qc = (COP). E.
(12)
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From equations (10) and (12) is obtained E = (1 - x) *Q&COP)
(13)
and, replacing in equation (1 l), results as QRz= [l - l/(COP)] . (1 - x) *Q,+
(14)
In order to determine the numerical value of these equations, it is necessary to calculate the COP of the heat pump. 6. COP calculation Heat pump coefficient of performance COP =
vi
is obtained [l] from the equation * VeI’qrn’(h - h3)/(h - hl)-
(15)
The numerator (hz - hJ) is the condensation heat, and the denominator (hz - h,) is the mechanical energy absorbed by the real thermodynamic cycle described by the refrigerant fluid of the heat pump, as shown in Fig. 3. The indicated efficiency is obtained from the compression ratio p2/pI, given in Refs [l] and [3], including the pressure drop in the condenser and evaporator circuits. 7. Instantaneous
overall efficiency
It is defined as the ratio between the produced compressor
heat and the input electrical energy to the
rj,,, = Qc,/E= (QRI + Qru + EYE. ECONOMIC
(16)
COMPARISON
Cost per KWh is a function of the capital invested, depreciation rate, performance and maintenance costs, variations in the inflation rate, fuel prices, etc. In order to make an economic comparison, it is necessary to study an installation presently burning fossil fuels, and thus producing heat energy, and then compare it with an installation that employs the heat pump proposed in this paper. In this way we can hopefully reach a reliable conclusion. As the determination of some parameters (such as credit interest rates, stock price indices, inflation rates on fuels, etc.) are hardly predictable, the results will be affected by some unavoidable errors.
Bar
hs
hl
t h 3=h,, Fig. 3. Real thermodynamic cycle.
KJ /Kg
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Table I. Comparison of costs Energy source Natural gas Heat pump Gas-oil C Fuel-oil I
Efficiency qor COP
PTA;k Wh
0.80 3.20 0.75 0.70
2.0 9.0 4.4 1.6
Fbw E kWh/m’
PTAFkWh
50.0 .10' 12.5 103 53.3. 103 57.1 . IO)
1.0 1.1 2.4 0.9
Basis: Period = 1 year; flow = 1 m3/h; thermal load = lO’kWh/m’.
Nevertheless, if we ignore these unpredictable factors and focus our attention on fuel prices, which are reliable data, since the cost of fuels have official values, we can obtain interesting conclusions which can direct our efforts in a particular way. In the case of liquid fuels, the cost per produced kWh is easier to obtain applying the following formula Gr = (1 - X) . P,f/(?,f . 09,
(17)
(1 - x) being the heat supplied for the boiler and CV [6] the calorific value of the fuel. If natural gas is concerned, there are different rates depending on gas consumption. For industrial uses, i.e. hatcheries processing sea water flows of 100 m3/h, a contract between the aquaculture facility and the gas supplier would permit a natural gas price not higher than 2.3 PTA/the&e. The cost per produced kWh can be obtained from the following expression Cng= 0.86 . (1 - x) . P&n,.
(18)
An installation with a heat pump will produce energy at a cost Chp= (1 - x) * P,,/(COP).
(19)
As a practical example, we can consider that the plant is designed to increase the sea water temperature to 24°C in winter and 28°C in summer (optimum conditions for a hatchery producing crustaceans), then the heat required to process 1 m3/h of sea water in an installation located at Castellon (Spain) can be evaluated as a function of the sea temperature variation through the year [7] (unpublished), and a result of lo5 kWh/year should be obtained. If we assumed a recovery percentage of x = 0.6 and qnr = 80% for natural gas; qIr = 75% for gas-oil and qIr = 70% for fuel-oil, as is the usual practice, the primary energy burned will be Fbu
=
(1
-
X)
.
Q&r.
(20)
The COP of the heat pump is calculated according to the method given above and to the following real performance conditions: t, = 47°C; t, = 7°C; vapour superheating = 3*C; liquid subcooling = 3°C; condenser pressure drop = 0.5 bar; evaporator pressure drop = 0.5 bar electrical efficiency: 85%; mechanical efficiency: 85%. With this data and the thermodynamic diagram of R22 shown in Fig. 3, the following values are obtained: p2 = 19.5 bar; pl = 5.1 bar; h, = 469.2 kJ/kg; h3 = 627.8 kJ/kg; h2 = 659.2 kJ/kg. A compression ratio r = p2/pI = 3.39 allows us to obtain from Ref. [l] or Ref. [3], r], = 0.75, and applying equation (15), we can obtain a COP = 3.2. The input electricity is given for equation (13). Substituting equations (17) to (20), we obtain the values shown in Table 1. HEATING
AND
COOLING
OF REARING
TANKS
BY A HEAT
PUMP
Aquatic organisms usually respond to higher temperatures of their environment by a faster metabolic rate, increased feed uptake and enhanced growth rate. However, all organisms have an optimal temperature for feed conversion efficiency. At the upper or lower temperatures, the feed conversion efficiency is reduced. On the other hand, certain species exhibit an optimal temperature
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for growth and fattening distinct from reproduction, i.e. sea bass (D. lubrax L.), a much prized marine fish, has an optimal temperature for growth around 20°C [12]. A pilot plant to culture marine species in hatcheries has been designed at the Instituto of Acuicultura de Torre la Sal (Castellon), improving the systems present for thermal conditioning of sea water, temperature control and renewal of sea water in the rearing tanks, with the following programme of temperatures and flows along the year: Species
Temperatures
Renewal
Crustaceans (shrimps) Sea bass and sea bream Sea bass and sea bream reproducers Molluscs
28 + 1°C 20 If: 1°C 13 * 1°C <20 f 1°C
1200 I/h 2420 I.h 600 I/h 480 I/h
Since the temperatures of the sea water employed in the renovation vary as a sinusoid along the year, ranges 11°C to 28”C, it is necessary to have hot and cold production equipment in order to keep the temperature programme proposed. The frigorific machine, by mechanical compression of vapour, supplies two heat sources: a hot source at 40°C and a cold one at 7°C. These temperatures, close to the process temperature, make the regulation and control operation easy. This fact, along with the possibility of obtaining simultaneous economic profits from both sources, is another advantage of the frigorific machine with respect to conventional systems. The return water just employed by the animals is a source of accessible energy, potentially usable, well adapted for its recovery in a refrigeration unit. The cold source of the frigorific system, at lower temperatures than the sea water, permits us to employ a second recovery of waste heat of the return water in such a way that the recovered energy increases with respect to the traditional system. In this way, the plant works on a hydraulic open circuit and tends to an energetic closed circuit, as it returns water at the same temperature it was collected. Another objective that the pilot plant must satisfy is the achievement of these conditions with minimum energetic expenses so that culturing these species can be competitive from a marketing standpoint.
II
11 r--------
32-36
%
1200 I/h ----
----
26* 1-C
‘-2-q 72OI/h2O?l-C ‘I
Fig. 4. Heat pump system for sea water heating and cooling.
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SYSTEMS
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OF THE PLANT
Figure 4 shows a simplified flow sheet of the projected plant. It is composed of two circuits, corresponding to the work regime in the winter or summer period, whose performance is described as follows: (a) 11°C < t,,, < 20°C.
The return water of the rearing tanks, at an average temperature of approximately 20°C is returned to the sea after exchanging heat in the recoverers No. 1 and No. 2, respectively. The heat recovery in the first exchanger has been optimized; the second exchanger recovers the rest by means of the evaporator of the frigorific machine. The primary circuits of conditioners No. 3, No. 4 and No. 5 are connected to the condenser of the frigorific system. The sea water, preheated at the statical recoverer, feeds the water conditioner for crustaceans (heat exchanger No. 3) and the water conditioner for sea bass and sea bream (heat exchangers No. 4 and No. 5). The conditioner of sea bass and sea bream reproductors (heat exchanger No. 6) cools the sea water to 13°C via the water coming from the evaporator of the frigorific machine. In this period, the water for molluscs enters directly from the sea to the rearing tanks because thermal conditioning is not needed. (b) 20°C < t,, < 28°C. The return water, at a mixture temperature around 20°C feeds directly to exchanger No. 3, where the condensation heat is absorbed, increasing the temperature to 32”C-36°C. This water is directly taken to exchanger No. 1 in order to raise the sea water temperature to 28°C and to feed crustaceans tanks. Water conditioners No. 4 and No. 5 for sea bass and sea bream are connected to the evaporator of the frigorific machine. Water for molluscs is obtained from conditioner No. 4. Water for sea bass and sea bream reproducers is obtained in exchanger No. 6 which produces water at 13°C and it is connected to the evaporator of the heat pump. In this performance regime, the dynamical recoverer No. 2 is out of service. Energy production system
The energy production system consists of two refrigeration units connected in parallel and likely to be used separately. The evaporator and the condenser are connected to the thermal sources of the conditioning system by means of heat exchangers, water-water. The design temperatures of these thermal sources has been fixed to 7°C and 40°C. The cold and hot water produced is stored in two tanks of 2000 1capacity, which provide the thermal inertia needed to operate in a transitory regime (rearing tanks cleaning, breakdowns, etc. . . .) for two hours. By means of these thermal sources, any temperature programme can be produced between 10°C and 30°C without any control troubles. In this range of temperature, all sea animals found in our coasts live. CONCLUSIONS ??The control of temperatures between 18 and 28°C with tolerances of f 1°C are difficult in installations which use industrial fuel-oils, since these have a high sulphur (> l%), which forces us to work at a minimum hot source temperature of 80°C. ??Temperature control with gas-oil C and natural gas improves slightly with respect to industrial fuel-oil, since these fuels have smaller amounts of sulphur (< l%), and the minimum temperature of the hot source is reduced to approximately 60°C. These temperatures, however, are still considered to be too far from the process temperature. ??As an electric heat pump produces heat at a temperature of 4O”C, it brings the source temperature nearer to the process temperature, making it easier to control temperatures between 13°C and 28°C with tolerances of & 1°C. ??The heat produced by an electrical heat pump is cheaper than the heat obtained by boilers using
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gas-oil C. Where the fuel-oil 1 and natural gas are concerned, the cost of the heat produced is approximately the same for the heat pump as for the natural gas and the fuel-oil 1. ??A refrigeration unit generates simultaneously a cold source and a hot source. The cold source will be useful to produce chilled sea water at a temperature of 13”C-15°C for induced spawning in periods outside the natural season. The hot source will be useful to produce warm sea water at a temperature of about 40°C for heating rearing tanks. ??The employment of a refrigeration unit allows the return of the sea water at approximately the same temperature as it was extracted. REFERENCES 1. Bernier, J., La pompe de chaleur mode d’emploi. PYC edition, 254, rue de Vougirard, 75740, Paris, 1981. 2. Chapman, A. J., Transmision del calor. Ediciones Interciencia, Costanilla de 10s Angeles, 15, Madrid, 1965. 3. Hemandez, F. and Izquierdo, M., The natural gas-absorption method system as an alternative to the electric power-mechanical compression system in Spain. Energy Conversion and Management, 1993, 34, 663-676. 4. Donaldson, E. M., Fagerlund, U. H. M., Higgs, D. A. and McBride, J. R., Hormonal enhancement of growth, in Fish Physiology, Vol. 8, Bioenergetics and Growth, eds W. S. Hoar, D. J. Randall and J. R. Brett. Academic Press, 1979, pp. 455-537. 5. Harvey, J. and Hoar, W. S., Teoria y prbtica de la reproduccibn inducida en 10s peces. ODCR-TS2ls, Otawa, CIID, 48 il, 1980. 6. Horlock, J. H., Cogeneration: Combined Heat and Power (7’hermodynamics and Economics). Pergamon Press, Oxford, 1987. I. Izquierdo, M., Proyecto de planta pilot0 para el acondicionamiento termico de langostinos, lubinas, doradas y moluscos. Consejo Superior de Investigaciones Cientificas, Madrid, 1987. 8. Klemetson, S. L. and Rogers, G. L., Aquaculture pond temperature modeling. Aquacultural Engineering, 1985, 4, 191-208.
9. Lam, T. J., Environmetal influences on gonadal activity, in Fish Physiology, Endocrine tissues and hormones. eds W. S. Hoar, D. J. Randall and M. Donaldson. Academic Press, 1983, pp. 65-116. 10. Parker, N. C. and Davis, K. B., Requirements of warmwater fish. Bio-Engineering Symposium for Fish Culture (FCS Publ. l), 1981, pp. 21-28. 11. Quinata, I. C., Keeping hatchery design simple. Bio-Engineering Symposium for Fish Culture (FCS Publ. l), 1981, pp. 149155. 12. Zanuy, S., Carrillo, M. and Ruiz, F., Delayed gametogenesis and spawning of sea bass (Dicentrarchus labrax L.) kept under different photoperiod and temperature regimes. Fish Physiology & Biochemistry, 1986, 2, 53-63. 13. Zanuy, S. and Carrillo, M., La reproduction de 10s teleosteos y su aplicacion en acuicultura. Reproduction en Acuicuhura, 1-131, Comision Asesora de Investigation Cientifica y Tecnica, Madrid, 1987.
Pergamon PII: s01%-8904(%)ooo94-5
OPTIMIZATION
OF AQUACULTURE M. IZQUIERDO
Comers. Mgmt Vol. 38. No. 9. pp. 879-888, 1997 0 1997 Elsevier Science Ltd. All rights reserved Printed in &eat Britain 0196-8904/97 $17.00 + 0.00
SYSTEMS IN SPAIN
and M. CARRILLO’
Instituto Ciencias de la Construction Eduardo Torroja, (CSIC), c/Serrano Galvache s/no, 28033 Madrid, Spain and ‘Instituto de Acuicultura de Torre la Sal &SIC), Ribera de Cabanes, 12595 Torre de la Sale, CastelIon, Spain (Received 21 September
1995)
Abstract-An analysis of present heat production systems, using fossil fuel combustion, employed for sea water heating in Spanish hatcheries is given in this paper and compared to a technical solution based on the employment of a heat pump. Price per unit of produced energy is calculated for each system using liquid and gaseous fuels, and then these prices are compared to the price obtained for a heat pump. The heat pump system is also compared, from the point of view of its precision in maintaining temperatures, to the systems used at the present. A project prototype for thermal conditioning and temperature control in aquaculture rearing tanks is described. The waste heat of the return water is recovered: first, by a static recoverer and second, by a recoverer connected to the evaporator of a refrigeration unit. The system should be used in such a way that economic benefits are obtained from its two heat sources of the frigorific system in simultaneous production of warm and chilled sea water. 0 1997 Elsevier Science Ltd Aquaculture
Fuels
Heat recoverer
Heat pump
Heat cost
NOMENCLATURE C = Cost of produced energy (PTA/kWh) COP = Heat pump coefficient of performance CV = Calorific value (kWh/kg) E = Input electrical energy (kWhe) F = Fuel (kWh/m’) h = Enthalpy (kJ/kg) rn = Sea water flow (kg/h) P = Cost of fuel (PTA/m’ year) p = Pressure (bar) Q = Calorific power (kW) R = Recoverer Sh = Specific heat (kJ/kg”C) t = Temperature (“C) x = Waste heat recovered (%) Greek leiters q = Efficiency
T = Compression ratio Subscripts
bu = c= d= e= el = er = hp = i= If = me = ng = ove = p= sea = I. . n =
Burned Condensation Design Evaporation Electric Exit recoverer Heat pump Indicated Liquid fuel Mechanical Natural gas Overall Process Sea water Recoverers; points of cycle
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INTRODUCTION
The increasing demands for proteins by our overpopulated world have forced the search for alternative methods of food production distinct to agriculture of fisheries. Aquaculture, as the husbandry, management, nutrition and breeding of useful aquatic organisms, such as fish, crustaceans, molluscs and algae, has been considered as the above mentioned alternative. The FAO estimates that aquaculture production is capable of considerable improvement by the year 2000 through optimal use of existing resources and the application of science. Intensive culturing of most commercially valuable sea fish and crustacean species requires a continuous water supply in order to maintain their quality and to favour the growth and reproduction physiological processes of these organisms [l l] in hatcheries. A detailed knowledge of these parameters, such as oxygen demand or fish weight as a function of temperature, enables the calculation of water flow requirements. These are critical [9] when it is intended to define production capacity in an aquatic medium. In large masses of water, optimum temperature must be maintained for every species; larval stage, year period or geographic latitude. Every cultured sea species, given adequate nourishment conditions, have temperature as a basic parameter controlling its growth. In addition, each species has an optimal development temperature [lo]. An important implication of this is that growth cycles of each species vary as a function of average water temperature, being shortened or prolonged as temperature is raised or lowered. On the other hand, the seasonal variation of temperature is predisposed to stop production in winter periods when the temperature is lower, since growth and nourishment parameters are sharply reduced [ 111. The employment of sexual, thyroid and pancreatic hormones in aquaculture [4] could maximize production per cost unit in sea hatcheries. In spawning, induced production of egg and semen by the administration of gonadotrophic hormones is widely used [5], [13]. Nevertheless, when reproduction cycles are modified by environmental manipulation leading to induced spawning out with the natural season, the larvae often suffer serious defects. These manipulations of photoperiod and temperature could be a major cause of these defects [ 1l] because the role played by temperature in the reproduction process has not been clearly identified, due to its difficulty to control and its need to be differentiated from photoperiod effects. Its influences on gonadal, spawning and post-spawning development [9], has been applied to induced spawning of sea bass (Dicentrarchus lubrux L.) out of season in a combined action of photoperiod and temperature (and eliminating hormone administration). This verified a spawning thermal threshold at a temperature around 16°C. For these reasons, it is necessary to develop a technology applicable to hatcheries such that it is possible to reproduce optimum temperatures at every stage of marine life (reproducer maintenance, egg spawning, larval and alevinal development) in these installations. Prefattening and fattening stages, however, will not be included in these studies [8]. To obtain these temperatures according to the tolerances demanded by the biologist and, in turn, to achieve them with the minimum expense of energy and production costs are the main aims of this work. The studies to be conducted and the ideas to be developed in the near future will be summarized below. This paper will analyse the present situation with respect to the technology employed in thermal energy production and temperature control of hatcheries installed in Spanish aquaculture facilities. This technology consists basically of heat production by means of the burning of liquid fuels. As an alternative to this technology, it is proposed to employ heat pumps which use mechanical compression to produce thermal energy. The advantages of this system, are analysed from an economic stand point as well as from its precision in temperature regulation and control. A prototype was designed by the CSIC at the Instituto de Acuicultura de Torre la Sal, Castellon (Spain). This prototype was designed according to energy saving criteria and employs a refrigeration unit in which mechanical compression of vapour is used to obtain economic benefits from both cold and hot (around 7°C and 40°C) thermal sources simultaneously. With the heat obtained from the cold source, chilled sea water (13°C) is produced that is employed in treatment of spawning induction tanks for fish, e.g. sea bass (D. lubrux L.) and sea bream (Spurus uurutu). The heat obtained from the hot source is utilised to produce warm sea water (18”C-28°C) that is
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employed in spawning induction for crustaceans (e.g. Pennaeus keraturus) as well as in obtaining an optimal temperature for larval and alevinal development of crustaceans, fish and molluscs. To keep this parameter throughout the year in the optimal ranges determined and with minimal energy costs is a task that overcomes biologists functions and results in the need for collaboration among biologists and engineers in order to find technical solutions which permit optimal conditions of life for these animals to reproduce in hatcheries. The project depicted in this paper is the result of this collaboration. OF REARING
HEATING
TANKS
USING FOSSIL
FUEL COMBUSTION
Figure 1 shows a simplified flow diagram of heating sea water in Spain. Preheating, using the residual heat of the waste water, takes place in heat recoverer, Rl. The required process temperature (28°C) aided by the boiler producing hot water at 8O”C-90°C is obtained in heat exchanger, R2. Sea water temperatures on Spanish coasts are approximately sinusoidal and range from 8°C to 20°C in the North and 11°C to 28°C in the South and East. In the North, it is usual to culture oysters or mussels with the optimal temperatures being around 20°C. In the South, shrimps are cultured at temperatures of about 28°C. In either case, it is necessary to heat the sea water in order to obtain these temperatures. This task is accomplished by the installation shown in Fig. 1. The problem, however, in these installations, employing a hot source of 8O”C-90°C is the difficulty in controlling process temperatures to 18”C-28°C with a tolerance of + 1°C. Energetic analysis
The difficulty in controlling temperatures mentioned above can be understood after taking an overall energetic analysis of the system and studying each one of its components. The thermal behaviour of these systems can be summarised as follows: As can be seen in Fig. 1, these systems work on an open circuit with a constant renewal of the water to the rearing tanks, such that the same flow of water extracted is returned to the sea. In heat recoverer Rl, the heat is partially recovered from the waste stream before it is discharged to the sea. In the heat exchanger R2, the water is conditioned in order to reach the required temperature for the rearing tanks. Accordingly, and assuming that there are no heat losses to the surroundings, the heat Qd required to obtain the process temperature, t,, can be calculated, Q,, = m . Sh . (tp - txa).
(1)
Since the sea temperature is variable, Q,, will be a maximum when t,, is a minimum, and this will be the design value for the installation.
to sea
IL.. FJ
F
month I
WLa Fig. 1. Fossil fuel system for sea water heating.
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Heat recoverer RI
There are different criteria for energy optimization in the design of this exchanger, and these must be applied to each given installation. For an operating temperature about 3O”C, a result, compatible with a reasonable exchange area, is that which permits us a maximal residual heat recovery from the return water of about 60%. According to this criterion, the heat recovered QR, will be
QRI= 0.6. Q,,.
(2)
QR, = m . Sh * (t,, - tsea).
(3)
This heat can also be expressed as
Water conditioner of process R2
The heat needed to be delivered to sea water in this exchanger will be QR2which, in turn, will be the energy that must be produced by the boiler
QRl= Qci- QRI
(4)
which, expressed as a function of mass flow rate, will be QR2= m * Sh * (tp - t,,).
(5)
With a variable sea water temperature, the resulting Qd, QR, and QR2values will also be variable and reach a maximum when the sea water temperature is a minimum. These will be employed as a design basis for each component of the installation. In order to know the behaviour of each heat exchanger as a function of sea water temperature, we can obtain t,, from equations (l), (2) and (3). ter = 0.6 - t, + 0.4. t,,,
(6)
which generalises for a percentage x of heat recovered in exchanger Rl t,, = x . t, + (1 - x) * t,.
(7)
This equation is valid when t, < tp. The minimum value of teeris obtained when t,, is a minimum and is a maximum when tsea= t,. For design conditions, t,, = 11°C and t,, = 28”C, we obtain t,, = 21.2”C and, therefore, QR2= 6.8 * m . Sh. If the process temperature is 28°C and the sea water temperature varies sinusoidally between 11°C and 28”C, for a t,, temperature of 24°C we obtain: t,, = 264°C and QRZ= 1.6 * m . Sh. For most of the year, the installation is oversized. This oversize could reach up to 500% in some periods of the year. This excess of exchange area, along with the fact that the temperature of the hot water supply for the boiler can not fall below 80°C using fuel-oils with amounts of sulphur > 1%, or 60°C using gas-oils with amounts of sulphur < 1% and natural gas (Instruction Tecnica IT.IC.17.2. de1 Reglamento de Instalaciones de Calefaccion Climatizacion y Agua caliente sanitaria. Real Decreto 1618/1980), means that the process temperatures, t,, in exchanger R2 could have variations of as much as 30% ( f 8°C) [2]. This is due to the automatic three way valve, which is part of the design constraints and behaves as an all open-shut valve. In order to solve this control problem, it is proposed to use a heat pump system which supplies water at temperatures of 35”C-40°C, rather closer to the process temperature. HEATING
OF REARING
TANKS
BY ELECTRICAL
DRIVEN
HEAT
PUMP
The scheme in Fig. 2 shows the warm water production system using a heat pump. The hot water boiler has been replaced by a heat pump. The heat of the waste return water from the fish tanks is partially recovered by a static heat exchanger, Rl, used to preheat the sea water. An additional recovery of heat takes place in heat exchanger, R2, where the evaporator of the heat pump absorbs heat from the return water. The return water is used as the cold source for the heat pump, such that the evaporation temperature keeps almost constant throughout the year. This second heat
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EVAPORATOR
I Fig. 2. Heat pump system for sea water heating.
recovery permits us to give back to the sea the water of the rearing tanks at similar temperatures as those of extraction. The warm sea water, at temperature t,, is produced in exchanger R3 from the heat released by the condenser of the heat pump. The condensation temperature is variable and depends on the process. A temperature of 40°C in this hot source is sufficient to provide an aquatic system temperature between 18°C and 28°C. Because this source temperature is reduced to 40°C in comparison to the higher operating levels in a fuel boiler, regulating and controlling the temperature is easier. The system works on an open circuit from a hydraulic point of view, whereas it tends towards a closed circuit from an energy point of view. Thus, it recovers much more waste heat than conventional installations. Calculation of the heat pump
A simplified calculation method, according to the process temperatures stated by the biologist, is given below [l], [3]. 1. Total thermal load, ignoring surrounding heat losses Qd = m . Sh * (tP - t=).
(8)
2. Heat recovered in Rl Qa, =x
’
Qt,.
(9)
3. Heat recovered in R2
4. Heat exchanged in R3 Qa3 = Qc =
Qd- Qa, = (1 - x) * Qd.
(10)
5. Heat pump
Qc= QRX= Qra+ E
(11)
Qc = (COP). E.
(12)
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From equations (10) and (12) is obtained E = (1 - x) *Q&COP)
(13)
and, replacing in equation (1 l), results as QRz= [l - l/(COP)] . (1 - x) *Q,+
(14)
In order to determine the numerical value of these equations, it is necessary to calculate the COP of the heat pump. 6. COP calculation Heat pump coefficient of performance COP =
vi
is obtained [l] from the equation * VeI’qrn’(h - h3)/(h - hl)-
(15)
The numerator (hz - hJ) is the condensation heat, and the denominator (hz - h,) is the mechanical energy absorbed by the real thermodynamic cycle described by the refrigerant fluid of the heat pump, as shown in Fig. 3. The indicated efficiency is obtained from the compression ratio p2/pI, given in Refs [l] and [3], including the pressure drop in the condenser and evaporator circuits. 7. Instantaneous
overall efficiency
It is defined as the ratio between the produced compressor
heat and the input electrical energy to the
rj,,, = Qc,/E= (QRI + Qru + EYE. ECONOMIC
(16)
COMPARISON
Cost per KWh is a function of the capital invested, depreciation rate, performance and maintenance costs, variations in the inflation rate, fuel prices, etc. In order to make an economic comparison, it is necessary to study an installation presently burning fossil fuels, and thus producing heat energy, and then compare it with an installation that employs the heat pump proposed in this paper. In this way we can hopefully reach a reliable conclusion. As the determination of some parameters (such as credit interest rates, stock price indices, inflation rates on fuels, etc.) are hardly predictable, the results will be affected by some unavoidable errors.
Bar
hs
hl
t h 3=h,, Fig. 3. Real thermodynamic cycle.
KJ /Kg
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Table I. Comparison of costs Energy source Natural gas Heat pump Gas-oil C Fuel-oil I
Efficiency qor COP
PTA;k Wh
0.80 3.20 0.75 0.70
2.0 9.0 4.4 1.6
Fbw E kWh/m’
PTAFkWh
50.0 .10' 12.5 103 53.3. 103 57.1 . IO)
1.0 1.1 2.4 0.9
Basis: Period = 1 year; flow = 1 m3/h; thermal load = lO’kWh/m’.
Nevertheless, if we ignore these unpredictable factors and focus our attention on fuel prices, which are reliable data, since the cost of fuels have official values, we can obtain interesting conclusions which can direct our efforts in a particular way. In the case of liquid fuels, the cost per produced kWh is easier to obtain applying the following formula Gr = (1 - X) . P,f/(?,f . 09,
(17)
(1 - x) being the heat supplied for the boiler and CV [6] the calorific value of the fuel. If natural gas is concerned, there are different rates depending on gas consumption. For industrial uses, i.e. hatcheries processing sea water flows of 100 m3/h, a contract between the aquaculture facility and the gas supplier would permit a natural gas price not higher than 2.3 PTA/the&e. The cost per produced kWh can be obtained from the following expression Cng= 0.86 . (1 - x) . P&n,.
(18)
An installation with a heat pump will produce energy at a cost Chp= (1 - x) * P,,/(COP).
(19)
As a practical example, we can consider that the plant is designed to increase the sea water temperature to 24°C in winter and 28°C in summer (optimum conditions for a hatchery producing crustaceans), then the heat required to process 1 m3/h of sea water in an installation located at Castellon (Spain) can be evaluated as a function of the sea temperature variation through the year [7] (unpublished), and a result of lo5 kWh/year should be obtained. If we assumed a recovery percentage of x = 0.6 and qnr = 80% for natural gas; qIr = 75% for gas-oil and qIr = 70% for fuel-oil, as is the usual practice, the primary energy burned will be Fbu
=
(1
-
X)
.
Q&r.
(20)
The COP of the heat pump is calculated according to the method given above and to the following real performance conditions: t, = 47°C; t, = 7°C; vapour superheating = 3*C; liquid subcooling = 3°C; condenser pressure drop = 0.5 bar; evaporator pressure drop = 0.5 bar electrical efficiency: 85%; mechanical efficiency: 85%. With this data and the thermodynamic diagram of R22 shown in Fig. 3, the following values are obtained: p2 = 19.5 bar; pl = 5.1 bar; h, = 469.2 kJ/kg; h3 = 627.8 kJ/kg; h2 = 659.2 kJ/kg. A compression ratio r = p2/pI = 3.39 allows us to obtain from Ref. [l] or Ref. [3], r], = 0.75, and applying equation (15), we can obtain a COP = 3.2. The input electricity is given for equation (13). Substituting equations (17) to (20), we obtain the values shown in Table 1. HEATING
AND
COOLING
OF REARING
TANKS
BY A HEAT
PUMP
Aquatic organisms usually respond to higher temperatures of their environment by a faster metabolic rate, increased feed uptake and enhanced growth rate. However, all organisms have an optimal temperature for feed conversion efficiency. At the upper or lower temperatures, the feed conversion efficiency is reduced. On the other hand, certain species exhibit an optimal temperature
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for growth and fattening distinct from reproduction, i.e. sea bass (D. lubrax L.), a much prized marine fish, has an optimal temperature for growth around 20°C [12]. A pilot plant to culture marine species in hatcheries has been designed at the Instituto of Acuicultura de Torre la Sal (Castellon), improving the systems present for thermal conditioning of sea water, temperature control and renewal of sea water in the rearing tanks, with the following programme of temperatures and flows along the year: Species
Temperatures
Renewal
Crustaceans (shrimps) Sea bass and sea bream Sea bass and sea bream reproducers Molluscs
28 + 1°C 20 If: 1°C 13 * 1°C <20 f 1°C
1200 I/h 2420 I.h 600 I/h 480 I/h
Since the temperatures of the sea water employed in the renovation vary as a sinusoid along the year, ranges 11°C to 28”C, it is necessary to have hot and cold production equipment in order to keep the temperature programme proposed. The frigorific machine, by mechanical compression of vapour, supplies two heat sources: a hot source at 40°C and a cold one at 7°C. These temperatures, close to the process temperature, make the regulation and control operation easy. This fact, along with the possibility of obtaining simultaneous economic profits from both sources, is another advantage of the frigorific machine with respect to conventional systems. The return water just employed by the animals is a source of accessible energy, potentially usable, well adapted for its recovery in a refrigeration unit. The cold source of the frigorific system, at lower temperatures than the sea water, permits us to employ a second recovery of waste heat of the return water in such a way that the recovered energy increases with respect to the traditional system. In this way, the plant works on a hydraulic open circuit and tends to an energetic closed circuit, as it returns water at the same temperature it was collected. Another objective that the pilot plant must satisfy is the achievement of these conditions with minimum energetic expenses so that culturing these species can be competitive from a marketing standpoint.
II
11 r--------
32-36
%
1200 I/h ----
----
26* 1-C
‘-2-q 72OI/h2O?l-C ‘I
Fig. 4. Heat pump system for sea water heating and cooling.
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OF THE PLANT
Figure 4 shows a simplified flow sheet of the projected plant. It is composed of two circuits, corresponding to the work regime in the winter or summer period, whose performance is described as follows: (a) 11°C < t,,, < 20°C.
The return water of the rearing tanks, at an average temperature of approximately 20°C is returned to the sea after exchanging heat in the recoverers No. 1 and No. 2, respectively. The heat recovery in the first exchanger has been optimized; the second exchanger recovers the rest by means of the evaporator of the frigorific machine. The primary circuits of conditioners No. 3, No. 4 and No. 5 are connected to the condenser of the frigorific system. The sea water, preheated at the statical recoverer, feeds the water conditioner for crustaceans (heat exchanger No. 3) and the water conditioner for sea bass and sea bream (heat exchangers No. 4 and No. 5). The conditioner of sea bass and sea bream reproductors (heat exchanger No. 6) cools the sea water to 13°C via the water coming from the evaporator of the frigorific machine. In this period, the water for molluscs enters directly from the sea to the rearing tanks because thermal conditioning is not needed. (b) 20°C < t,, < 28°C. The return water, at a mixture temperature around 20°C feeds directly to exchanger No. 3, where the condensation heat is absorbed, increasing the temperature to 32”C-36°C. This water is directly taken to exchanger No. 1 in order to raise the sea water temperature to 28°C and to feed crustaceans tanks. Water conditioners No. 4 and No. 5 for sea bass and sea bream are connected to the evaporator of the frigorific machine. Water for molluscs is obtained from conditioner No. 4. Water for sea bass and sea bream reproducers is obtained in exchanger No. 6 which produces water at 13°C and it is connected to the evaporator of the heat pump. In this performance regime, the dynamical recoverer No. 2 is out of service. Energy production system
The energy production system consists of two refrigeration units connected in parallel and likely to be used separately. The evaporator and the condenser are connected to the thermal sources of the conditioning system by means of heat exchangers, water-water. The design temperatures of these thermal sources has been fixed to 7°C and 40°C. The cold and hot water produced is stored in two tanks of 2000 1capacity, which provide the thermal inertia needed to operate in a transitory regime (rearing tanks cleaning, breakdowns, etc. . . .) for two hours. By means of these thermal sources, any temperature programme can be produced between 10°C and 30°C without any control troubles. In this range of temperature, all sea animals found in our coasts live. CONCLUSIONS ??The control of temperatures between 18 and 28°C with tolerances of f 1°C are difficult in installations which use industrial fuel-oils, since these have a high sulphur (> l%), which forces us to work at a minimum hot source temperature of 80°C. ??Temperature control with gas-oil C and natural gas improves slightly with respect to industrial fuel-oil, since these fuels have smaller amounts of sulphur (< l%), and the minimum temperature of the hot source is reduced to approximately 60°C. These temperatures, however, are still considered to be too far from the process temperature. ??As an electric heat pump produces heat at a temperature of 4O”C, it brings the source temperature nearer to the process temperature, making it easier to control temperatures between 13°C and 28°C with tolerances of & 1°C. ??The heat produced by an electrical heat pump is cheaper than the heat obtained by boilers using
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gas-oil C. Where the fuel-oil 1 and natural gas are concerned, the cost of the heat produced is approximately the same for the heat pump as for the natural gas and the fuel-oil 1. ??A refrigeration unit generates simultaneously a cold source and a hot source. The cold source will be useful to produce chilled sea water at a temperature of 13”C-15°C for induced spawning in periods outside the natural season. The hot source will be useful to produce warm sea water at a temperature of about 40°C for heating rearing tanks. ??The employment of a refrigeration unit allows the return of the sea water at approximately the same temperature as it was extracted. REFERENCES 1. Bernier, J., La pompe de chaleur mode d’emploi. PYC edition, 254, rue de Vougirard, 75740, Paris, 1981. 2. Chapman, A. J., Transmision del calor. Ediciones Interciencia, Costanilla de 10s Angeles, 15, Madrid, 1965. 3. Hemandez, F. and Izquierdo, M., The natural gas-absorption method system as an alternative to the electric power-mechanical compression system in Spain. Energy Conversion and Management, 1993, 34, 663-676. 4. Donaldson, E. M., Fagerlund, U. H. M., Higgs, D. A. and McBride, J. R., Hormonal enhancement of growth, in Fish Physiology, Vol. 8, Bioenergetics and Growth, eds W. S. Hoar, D. J. Randall and J. R. Brett. Academic Press, 1979, pp. 455-537. 5. Harvey, J. and Hoar, W. S., Teoria y prbtica de la reproduccibn inducida en 10s peces. ODCR-TS2ls, Otawa, CIID, 48 il, 1980. 6. Horlock, J. H., Cogeneration: Combined Heat and Power (7’hermodynamics and Economics). Pergamon Press, Oxford, 1987. I. Izquierdo, M., Proyecto de planta pilot0 para el acondicionamiento termico de langostinos, lubinas, doradas y moluscos. Consejo Superior de Investigaciones Cientificas, Madrid, 1987. 8. Klemetson, S. L. and Rogers, G. L., Aquaculture pond temperature modeling. Aquacultural Engineering, 1985, 4, 191-208.
9. Lam, T. J., Environmetal influences on gonadal activity, in Fish Physiology, Endocrine tissues and hormones. eds W. S. Hoar, D. J. Randall and M. Donaldson. Academic Press, 1983, pp. 65-116. 10. Parker, N. C. and Davis, K. B., Requirements of warmwater fish. Bio-Engineering Symposium for Fish Culture (FCS Publ. l), 1981, pp. 21-28. 11. Quinata, I. C., Keeping hatchery design simple. Bio-Engineering Symposium for Fish Culture (FCS Publ. l), 1981, pp. 149155. 12. Zanuy, S., Carrillo, M. and Ruiz, F., Delayed gametogenesis and spawning of sea bass (Dicentrarchus labrax L.) kept under different photoperiod and temperature regimes. Fish Physiology & Biochemistry, 1986, 2, 53-63. 13. Zanuy, S. and Carrillo, M., La reproduction de 10s teleosteos y su aplicacion en acuicultura. Reproduction en Acuicuhura, 1-131, Comision Asesora de Investigation Cientifica y Tecnica, Madrid, 1987.