Modeling Design and Construction of a Solar Space Conditioning System for Drying- VC2006-Baltazar

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Original Article Proceedings of Virtual Concept 2006Playa Del Carmen, Mexico, November 26 th December 1st, 2006

Modelling, Design and construction of a Solar Space

Conditioning System

Martin E. Baltazar-Lopez, Juan Carlos Bahena-Bustos, Rafael Castillo-Rincon, J. Jasson Flores-Prieto, David Perez-Salmeron

Centro Nacional de Investigacin y Desarrollo Tecnolgico, CENIDET

Prol. Palmira Esq. Apatzingan, Cuernavaca, Mor. 62240, Mexico.

Phone/Fax 52 777 312 76 13

E-mail : {baltazar, jasson}@cenidet.edu.mx,[email protected],{rafascast,sayaman5010}@hotmail.com,

Abstract: A design methodology based on abstraction,critical parameter identification, and questioning was used for

the conceptual design of a virtual prototype of a solar spaceconditioning system. It was implemented in order to have a

fine control on space conditioning variables such as

temperature and relative humidity. A case study is presented

for space conditioning for drying, where the virtual prototypes

obtained were tested and the information obtained was

successfully implemented in the construction of the realprototypes.

Key words: design methodology, solar space conditioning.

1- Introduction

On of the problems associated to ceramic industry in the

Mexican state of Morelos, is that the energy used to dry their

plaster molds is too expensive. Most of the times to avoid that

cost of using gas furnaces, the ceramic manufacturers prefer to

dry by direct exposure to the sun and natural air flows. This

technique is useful in dry sunny days. The problem arises when

the weather is rainy and the humidity of the air is high, making

the natural drying process very inefficient, taking several

weeks in order to have the molds acceptable for another set of

production.

Paradoxically at times when the sales are higher and

production volume increases, the weather does not help in

natural drying of molds making necessary an artificial way to

dry. Some times an additional recycled heat produced byfurnaces for cure ceramic is used also to dry the molds. The

process can take up to 12 days though.

As shown in Baltazar-Lopez, et al [BF1] a design methodology

based on abstraction, critical parameter identification and

questioning was applied by neophyte engineers to get quick

innovative solutions. Based on that experience, the practitioner

research engineers used the same methodology to designprototypes of solar conditioning space conditioning systems.

The idea is to heat water and with a high thermal mass, which

is easy to handle, it is possible to use it to heat and dry the

later air used for drying. The final application of these solar

systems being the drying, in this case of plaster molds for

ceramic industry.

2- Drying

The drying or dehydration is an operation in which it takes

place heat transfer and mass transfer. The drying is a process

of physical separation, which objective is to remove a liquid

phase of a solid one by means of thermal energy; this processhappens when hot air contacts with a humid solid, its surface

is warmed up and the transmitted heat is used like latent heat

of evaporation, thus the contained water passes from liquid

phase to gas phase. The water steam, that crosses by

diffusion the air layer in contact with the solid, is dragged out

by the moving air, being generated a zone of low pressure

and between the air and the solid a gradient of steam

pressure. This gradient provides the impelling force thatallows for elimination of water, in steam form.

3- Modelling

A modelling of the solar system was carried out in order to

know the physical parameters necessary for the design. Themodel was implemented through a computer program known

as SCADES.

This software development (SCADES) for the modelling

and dimensioning of flat solar collectors and systems ofwater heating for space conditioning could be for industrial

applications or for domestic use, taking advantage of the

solar energy. It works for natural convection and forced

convection, in addition it counts on data bases of

environmental and material conditions in Mexico. The

energy gained is obtained considering balance of energy in

the thermal tank using the multi-node model, and for the

design and dimensioning of the collector It was used the

methodology reported by Duffe and Beckman[DB1].

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Virtual Concept 2006 Modelling of a Solar Conditioning System

Figure 1: Main window of SCADES

Figure 2: Plots of Database Model

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The solar radiation that affects SCADES is obtained by

using the model of transmitted direct radiation through

cleared atmospheres and the room temperature is obtained

from the data bases. The verification and validation of the

code of the program, was made using works reported and

with obtained experimental data of SCADES to natural

convection. Comparing the results of the program to those

reported by Olarte[O1] the following differences were

obtained: 4% between the temperatures average of the

water, 13% with respect to the energy stored in the thermal

tank (thermotank) and a difference of 16% in the

efficiencies.

For modelling it is possible to specify different materials,

as well as data specifications like geographic conditions,

collector orientation, etc.

The way this model works is by obtaining the solar

radiation and the room temperature by means of the use

and access to experimental data bases. These data bases

are of different places from Mexico, the data to select are

specific for a designated date. It is possible to clarify that

the shown solar radiation and room temperature are per

hour in the course of one day (1 to 24 hours).

It is possible to visualize graphically the solar radiation

data and room temperature data and hour by hour

information. Typical plots of data are shown in Figure 2.

The model and dimensioning of the collector is carried out

through the main window of the program. Figure 3 shows

the main window for design of the collector. Here it is

possible to specify material of the fin, the calibre of the

same, the tube and absorbent surface, capitation area , kindof encapsulation, thermal isolation, etc.

The other part of the system is to model the thermal tank,Figure 4, where the volume of the tank, thickness of the

insulator, temperature income flow and global loses of the

tank, are specified.

Once the data set is complete it is possible to model a

complete system by natural or forced convection as shownin figure 5.

Fig. 3. Main Window for Modelling of Solar Collectors



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Fig. 4. Model of Thermal Tank

Fig. 5 Model of system with Instant Forced Convection



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Radiacion Solar

n, ,hora, ,L,A

Gon=Gsc [1+0.33*Cos(360*n/365)] =23.45*Sin[360*(284+ n)/365]

1 =15*(hora-12)

3 Cos z=Cos Cos + Sin Sin

a*0=0.4237-0.0082*[(6-A)*(6-A)]

a*1=0.5055-0.00595*[(6.5-A)*(6.5-A)]

k*=0.2711-0.01858*[(2.5-A)*(2.5-A)]

1

2a0=a0**r0

a1=a1**r1

K=k**rk

b=a0+a1*exp[-K/Cos z]

Gb=Gon * b*Cos z Gd=Gon * d*Cos z 22 3

Rb=Cos z/Cos Rd=(1+Cos )/2

Rf=[(1-Cos )/2]*g

1 4 5

7

6 8

n1/n2=Sin 2/ Sin 1

r=Sin2 (2-1) / Sin2(2+1) r= tg

2 (2-1) / tg2 (2+1)

rII=(1-r) / (1+r) rI=(1-r) / (1+r)r=0.5*(I+II)

=r*aa=exp[-K*L / Cos z]

g=a- = / [1(1)d]

Id=2

1Gddt

Ib=21Gbdt

S=Ib*Rb()b+Id*Rd()d+(Ib+Id)*()Rf

8

7

6

FIN

Figure 8: The algorithm used in SCADES to provide the solar radiation



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4- Solar Collector System

The solar collector system was designed and constructed

based on the results of SCADES. It consists of five plane

collectors, built and detailed in CENIDET, It is placed on a

movable structure. It is observed in Figure 9 that the

collectors are connected in parallel allowing for easy

access for maintenance.

The thermal tank, Figure 10, used in conjunction of solar

collectors was also designed after the virtual model with

SCADES. It consisted of a 5000 lt. stainless steel tank with

a layer of fibreglass isolation covered with laminated

galvanized steel.

Figure 9: Solar collectors system

Figure 8: Thermal Tank



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5-Design of the drying chamber

When a wet mold is placed in a forced hot air dryer, the

evaporation begins more quickly. This initial evaporation

maintains the mold with lower temperature than the

temperature of the air in the dryer. The water at the interiorof the mold moves towards the surface tending to replace

the evaporated humidity. The drying process is completed

when the centre of the mold reaches the temperature of the

surrounding air. The used energy to evaporate the water

was calculated, for each evaporated kilogram of water it is

required a minimum of 512 kcal, Incropera[I1]. The use of

dryers with hot and forced air accelerates and controls the

drying process. The plaster molds hardly are dried to the

100% without the use of this kind of dryers. The main

physical limitation when drying a plaster mold is the

maximum temperature to which it can operate before

burning of the mold; the recommended temperatures are in

the interval from 45 to 50 C.

With all the previous information the next step is to apply

the design methodology as follows.

Need Statement: To dry plaster molds

Main function: To remove water of plaster molds

Main constraints:

Using an air flow with lower humidity. Working Temperature lower than 45C.

Design Conditions:

Drying of molds using solar energy and/orexhausted heat.

Having a drying capacity normally used in adraining table of 6 meters long typically used in

the ceramic industry.

Ability of the dryer to move from one drainingtable to another one.

To reduce the drying time to not more than 4days.

To reduce the handlings of molds in the processof manufacture;

by reducing the loading and unloading of worktables.

that is feasible of being reproduced by personnelnot highly enabled

easy operation.

that can operate of continuous wayCritical parameters:

Uniform distribution of temperature and airhumidity.

Temperature of the air non greater than 45C.

Optimal energy consumption.Functional alternatives:

Use of air extractors for provision and extractionof the air,

Use of internal diffusers and valves forhomogenous distribution of the air

Use of a low cost monitoring system and controlof temperature and humidity

Air heating provided by hot water Water heating by solar energy and exhausted heat

from furnaces

Implementation of a controlled environmentchamber (air temperature, humidity and air flow)

with similar dimensions to the typical draining

table with ability to be transportable in shortroutes.

6- Construction of the drying chamber

The construction of the drying chamber is based on thetypical dimensions in the industry of ceramics. As seen

in Figure 9, it was used structural steel for the frame of

the drying chamber. At the interior of the chamber was

instrumented with several temperature and humidityprobes in order to monitor the process, Figure 10. The

tests were done using the full surface capacity of the

draining table as shown in Figure 11. As shown on

Figure 12, the main frame of the drying chamber wasprovided with wheels to give mobility along the drying

tables, so that while the drained is made in one table, the

drying process is carried out in another one. The air flow

is forced by blowers. Doors at respective ends of the

chamber, will allow moving it on the longitudinal axis of

the draining table. The air flow inside the chamber willbe distributed from a duct on the internal superior part of

the chamber. The draining table was instrumented with a

hydraulic system for mass monitoring.

Figure 9: Construction of the drying chamber



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Figure 10: Interior of Drying Chamber

Fig. 11. Plaster molds to be dried.



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Fig. 12. Drying chamber showing wheels for mobility over draining table, which was instrumented with a hydraulic

system for mass measuring.

7- Construction of air condi tioner for drying

As noted before, one of the critical parameters in the

drying process is the humidity contain in air. Theconstruction of an air conditioner for drying, Fig. 13, will

allow us to have a control over of the humidity parameter

taking in consideration the temperature of dry bulb and the

percentage of existing humidity in the environment; by

means of the use of the psicrometric chart we are able to

determine the temperature necessary to take the

environment air to a 100% humidity and thus to approach

to a wished degree of humidity for drying.

The reached temperature when coming out of the extractor

once stabilized the hot water flow through all the pipe of

the condensers was of 41C, with 20% humidity contain.

This air was introduced by the upper part to the dryingchamber. For testing purposes, plaster molds were placed

and monitored by around 2 days obtaining acceptable

results.

The heat exchanger and recirculation pumps used in the

system were automated by using timers, contactors andphoto cells.

8- Conclusions

With a SCADES Modelling system it was possible to

dimension the solar collector, the storage tank, it has been

specified the water flow of the water pump. The energy in

the system was known an also the possible temperatures in

the thermal tank and difference in heights of components

in a natural convection system. This information was

integrated in a virtual prototype of a solar space

conditioning system. The virtual prototype was then used

for the design of the space conditioning system used fordrying process. The design methodology employed proved



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Figure 13: Heat exchanger used in the drying system.

to be an efficient way to get innovative solutions. Further

improvements are necessary at the time of this publication

in order to optimize drying times and energy use.

9- References

[BF1] Baltazar-Lopez M, Flores-Porras J.D., Zenteno-

Cardoso E., Functionally efficient conceptual design andinnovation tools, International Conference Virtual Concept

2006.

[DB1] Duffie, J. A., Beckman, W. A., Solar Engineering

of Thermal processes, Wiley, New York, 1991.

[O1] Olarte J., Desarrollo de un programa de computo

para el diseo de colectores solares planos y sistemas de

calentamiento de agua, Tesis, Centro Nacional de

Investigacin y Desarrollo tecnolgico, Cuernavaca, Mor.Mexico, 2005.

[I1] Incropera, F., Fundamentos de transferencia de calor,

4 ed. Editorial Prentice Hall., 1999.


Documents

Modeling Design and Construction of a Solar Space Conditioning System for Drying- VC2006-Baltazar