Category Archives: EuroSun2008-2

The heat transfer medium water

Since 2003 PARADIGMA has been using water as heat exchange medium with the so called AquaSystem. Frost protection is ensured by unutilized low-temperature heat from the installation. Because of the low heat losses of the CPC vacuum tube collectors only a small amount of energy (about 2 to 4 % of the yearly gain in energy) is required for frost protection and this is more than compensated by the advantages of using water and high operating temperatures. This concept has proven successful more than 30,000 times so far.

The advantages of the AquaSystem over conventional solar collector systems are obvious. The operation with water:

allows easy, direct connection to the in-house heating network,

requires less equipment such as heat exchangers, de-aerators, valves, pumps and controllers,

is the precondition for the use of efficient and small storage tanks,

eliminates the high costs for antifreeze and the associated running costs,

reduces considerably the costs and time for commissioning and repairs,

ensures a long operating life with almost constant performance,

removes all risks associated with thermal stagnation and

minimizes running costs (e. g. maintenance).

The use of state of the art CPC vacuum tube collectors ensures the maximum harvesting of solar energy with a system that has a long operating life and is virtually maintenance free.

It is absolutely unscientific that the power investigations according to DIN EN 12975 (see chapter 7 “power performance…”) were carried out with water without any consideration of the heat transfer fluid propylene glycol which is normally used. Glycol has with 40 °C compared with water only 88 % of the heat capacity, 3.8 times the viscosity, 62 % of the heat conductivity, only a quarter the Reynolds number (therefore these collectors must work at unfavourable, laminar flow conditions), 75 % of the heat-transfer coefficient and up to 3.85 times higher pressure losses. And the deeper the temperatures are the worse the glycol conditions. A water-glycol heat exchange requires up to 3 times the heat exchange area than a water-water heat exchange to get the same NTU (number of thermal units). Nearly all simulations programs for solar harvest and almost the whole solar literature disregard this physical context, e. g. by use of too simple models.

Sun — gas system functioning routine

The control system is based on the comparison of a reference temperature (ref. T) and that of the water circulating in the pipes of the bed, 30 — 35°C. The reference temperature is that fixed in the sensor placed on one of the collectors. During the day, provided there is radiation (ref. T > 32 °C), the control system (electrovalves) orders the water circulation through the collectors and the hot water tank until it reaches the beds. In case there is no radiation (at night or cloudy days), water does not circulate through the collectors but goes directly to the hot water tank where it takes the

required temperature. A sensor inside the hot water tank records the switching on and off which allows to determine the time spent for the operation and the energy consumed.

Подпись:Подпись: 270 220 170 О4 ш 120 70 20 Подпись: 0 12 24 36 48 60 72 84 96Подпись:image262о

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a

a

E

и

&

a

и

H

Fig.3. Hygrothermal behavior of the propagation chamber during winter.

2. Experimental results

Подпись: Fig. 4 shows the outdoor conditions during the days when the trials were conducted, the first three and the fifth day were clear with a horizontal radiation between 830 and 900 W.m-2, while the fourth and sixth day it was between 270 y 350 W.m-2 respectively by midday. The temperature outside (RT.out.) was between 8 °C and 35 °С. Time [hs ]

To analyze the functioning of the system, several measures were taken during six consecutive days: on the first two days, the system functioned automatically according to prefixed ranges; on the third and fourth day, the water pass was closed by the collectors in order to evaluate the hot water tank switching on and off time. The following days, the system worked automatically.

Fig.4. Outdoor temperature and radiation during the trials

Fig. 5 shows the water temperature when water comes out of the solar collectors and the hot water tank. On the first days, when the system functioned combining sun — gas, the hot water collectors contribution started at 11 am and lasted until 6 pm. During this time, the sensor placed in one of

the collectors allowed water passing from the beds. When the temperature (32 °С) recorded by the sensor decreases, it closes the electro valve, and water passes only to the hot water tank, previously regulated to maintain the temperature required by the rooting process. On the third and fouth day, the water flow was operated manually, closing it for the collectors so as to be able to compare the energy consumption of the hot water tank. On the last two days, the system was again regulated automatically: on the fifth day (clear sky) water was provided by the collectors for six hours. The last day, with a maximum solar radiation of 350 W. m-2, the collectors provision was null; for this reason, the hot water tank worked all day.

The records about the hot water tank switching on (1) and off (0) time make it possible to determine the operating time and the consumption.

Подпись:5

45

image265

40

Fig.5. Water outflow temperature from the collector and from the hot water tank during the trials

Подпись: Fig. 6 shows the water in the beds inflow and outflow temperature values. The solar collectors contribution are observed during the first, second and fifth day with values as high as 37°С; whereas the other days and during hours without sunlight, temperatures ranged from 32°С to 35°С corresponding to the hot water tank thermostat cut off. Besides, this was the prefixed temperature for the stems rooting. The water flow in the pipes was 0.4 m3.h-1. « Water outflow temperature from the bench Water temperature when accessing the bench 40 38 36 О 34 32 30 28 26 24 22 20

0 8 16 2 4 32 40 48 56 64 72 80 88 96 104 112 120 128

Time [hs]

Fig.6. Water inflow and outflow temperature on the benches and water outflow temperature from the hot

water tank

Agronomic results Walnut:

The walnuts (seeds) were sown on the bed with a 22°C basal heating. Germination occurred 30 days after sowing, with a 70% of success. 45 days after germination, the vegetal material was transplanted to black plastic bags which were placed on the same bed. When the plants were about

0. 30 m high and had a 0.05 stem diameter, the grafting process started.

The pots with the grafted plants were put on the benches with basal heating, and two pipe lines were added to both sides of the grafting zone, 0.05 m away from the grafting area while water circulated through them heated between 28°C and 31°C. This temperature was maintained during the cicatrization process until buds came out about 21 days later.

Though the relative room humidity values were adequate, transparent plastic bags were used to cover the cicatrization zone and the graft to ensure the maintenance of the recommended humidity values (Fig. 7). The stem taking results, obtained from 100 grafted walnuts using four different grafting treatments (25 grafts per treatment), were 48%, 84%, 16% y 56%.

image267

Fig.7. Inside view of the chamber with the grafted walnut young plants and the carob stems

Carob:

Stems from the basal part of the branch were taken with a length of 0.25 to 0.30 m and a diameter of 0.0025 a 0.0030 m. Each stem had 4 to 5 buds and some leaves cut in halves to diminish transpiration. At the base of the stem, a scraping was made to leave the bark exposed and with a wider contact surface with the hormone solution to be used.

5. Conclusions

The hygrothermal behavior of the chamber used simultaneously for walnut grafting and carob rooting was highly satisfactory, obtaining temperaturas and humidities appropriate to the requirements for the production of both species.

Two trials were carried out using three types of IBA concentrations (6000, 8000 y 10.000 ppm). Fr each concentration, 50 stems were placed on the bench with the plastic tunnel with a substrate temperature of 32°C.

The results elicited showed a direct relation with the type of treatment applied, obtaining a 40% callus formation and a 20% rooting of the stems treated with 8.000 ppm concentrations. The stems which took roots were the ones to maintain the leaves with rooting callus formation 25 days after the treatment. Better results were obtained with the stems treated with 10.000 ppm, where the

Подпись: Fig. 8. Carob stems for rooting in the heated Fig. 9. Carob stems rooted in the bioclimatic benches. chamber.

percentage of callous stems was 60%, with a 30% of rooting. The rooting of stems which did not lose their leaves occurred 20 days after the treatment (Fig. 8).

From the agronomic point of view, the results were those expected because the chamber could be maintained within the limits of the required environmental conditions. Callus formation was accelerated by the heat treatment on the cicatrization zone, a controlled process to avoid the live tissue destruction due to possible excess temperature levels.

The inclusion of the plastic tunnel significantly improved the hygrothermal room conditions of the carob stems. Such tunnel enables the independent work of the hydrocooling humidity and the surrounding environment, so that the humidity in the tunnel basically depends on the irrigation micro sprinklers contribution.

The results also give the means to analyze the sun — gas system behavior for the rooting benches heating. From the thermal point of view, the achievement of the expected results makes it possible to have an adequate resource for stems propagation.

6. References

[1] Moraldi M. y Lanzi, P. (1993). Il riscaldamento localizzato dell’ innesto nella produzione vivaistica del noce. Rivista di Frutticoltura, N° 1, pp 53 — 556.

[2] Iriarte A., S. Bistoni, L. Saravia. "Modelo de prediction del comportamiento de colectores solares plasticos para calentamiento de sustratos en invernaderos". Revista Avances en energias renovables y medio ambiente, Vol. 6, N° 2, pp. 02.37 — 02.42, 2002.

[3] Felker, P. 1980. Niitrogen cycling-water use efficiency interaction in semi-arid ecosystems in relation to management of tree legumes (prosopis). IN International Symposium on Browse in Africa. Pp.214-222.

[4] Garcia V., Iriarte A., Lesino G,, Flores S., Matias C. (2003). Comportamiento termico de una camara para microinjertacion de plantines de nogales. Avances en energias renovables y medio ambientes. Vol. 7, pp. 2.19 — 2.24.

Reference Buildings

The reference building has a floor area of 1000 m2 (height: 6 m). It is facing south and has a flat roof. The floor slab consists of 20 — 60 cm concrete with integrated underfloor heating system and 10 cm of insulation underneath. Four different reference cases were defined corresponding to typical insulation and usage scenarios for industrial buildings. The used building parameters can be found in Table 1.

Case 1

Case 2

Case 3

Case 4

Poorly insulated, high air exchange rate

Poorly insulated

Standard

High internal gains

Building construction

U-Value Walls

W/(m2 K)

0.584

0.584

0.233

0.233

U-Value Roof

W/(m2 K)

0.350

0.350

0.184

0.184

U-Value Floor

W/(m2 K)

0.307-0.364

0.307-0.364

0.307-0.364

0.307-0.364

Area (Windows and Doors)

m2

88

88

88

88

g-value Windows

0.589

0.589

0.589

0.589

U-value Windows

W/(m2 K)

1.4

1.4

1.4

1.4

Internal Gains

People (8-18 h), Mon-Fri

15

15

15

15

Light

W/m2

5

5

5

5

Machine Operation (8-18 h), Mon-Fri

kW

0

0

0

8

Air Exchange Rate

h-1

0.6

0.3

0.3

0.3

The so-called “Standard” reference building (Case 3) in the table is a relatively well insulated building with wall sections consisting of 160 mm of mineral wool insulation and 2 mm of sheet metal on both sides, for the roof 200 mm of mineral wool insulation was assumed. The windows are standard double-glazed insulating windows.

However, in retrofit situations, industrial buildings are often not very well insulated (sometimes not insulated at all). Therefore, a poorly insulated reference building was also defined with only 60 mm of mineral wool insulation in the walls and 100 mm in the roof (Cases 1 and 2). The air exchange rate in industrial buildings is especially difficult to estimate. It varies strongly depending on the number and duration of door openings. For this study, a standard air exchange rate of 0.3 h-1 was assumed. Case 1 includes an increased air exchange. In addition, another case with 8 kW of waste heat (e. g. of machine operation) during working hours was considered (Case 4).

Calculation of the Number of Collectors per Row

The first step in the calculation of the number of collectors per row is to estimate the mass flow rate per row. To guarantee an appropriate heat transfer coefficient inside the absorber tube a turbulent flow is necessary. Taking into account that the conditions of the pre-design are good ones in terms of available solar energy, it is chosen a Reynolds number high enough to have turbulent flow even at any other solar radiation conditions — like in winter, for example-. It means to work with a Reynolds number around 600.000, since the useful thermal power in winter can be, in the worst cases, 30% lower than the one collected in summer, inducing a reduction of the mass flow rate to a third of the value in summer.

The mean fluid velocity, v, in the absorber tubes is calculated from the expression of the Reynolds number. A first estimation of the mass flow rate per row, m0, is given by

m0 = vp At (eq.1)

where p is the fluid density and At is the inner cross section of the steel absorber tube.

The number of collectors required in every row depends on the conditions set at the inlet and outlet of the solar field and the thermal power supplied by every collector. The increase of temperature between the inlet and outlet of every row, ATrow, should consider that the fluid temperature at its outlet has to be higher (7-10°C) than the temperature of the steam generated in the heat transfer fluid to water heat exchanger, in order to compensate the thermal heat losses through the pipe lines which connect the rows in the solar field, the efficiency of the heat exchanger and the pinch point of the latest. The energy given by a collector is assumed to be the same for every collector, independently what the position of the collector in the row is, i. e., the heat capacity of the heat transfer fluid, cP, and thermal losses are assumed to be constant in the range of working temperatures in a row.

To calculate the temperature increment in every collector, AT°kc, an energy balance is done

Подпись: Ed C°s(^) Popfi Kfa)Fe Ac - Qloss Подпись: m0c AT0, r p colec Подпись: Q'Util,colc. Подпись: (eq.2)

considering that the solar thermal power absorbed by the collector minus its heat losses to ambient is the useful thermal power, Qutii, coic, used to increase the temperature of the working fluid mass flow. Mathematically,

where Ac is the aperture area of a collector and T^s and Qloss are the absorber tube temperature and thermal losses, respectively.

Подпись: N image225 Подпись: (eq.3)

Thus, the number of collectors, N, is given by the rate of the temperature increase needed per row, ATrow, and the temperature increase in every collector, AT°kc,

The obtained number of collectors per row, N may not be an integer. If so, this number has to be rounded to an integer number, Npre_design from which the new temperature increment in a collector in the pre-design, ATcol, is obtained from (eq.3) considering N=Npre-design and the new pre-design mass flow rate, mr, comes from (eq.2) considering AT°lec = ATcolec.

Industrial sectors and processes

The second noteworthy outcome of this survey is the definition of the most suitable industrial sectors, where solar thermal heat could be fruitfully used. In these sectors, the heat demand is remarkable and more or less continuous throughout the year. Furthermore, as described above, the temperature level required by some of the processes is compatible with the efficient operation of solar thermal collectors.

The key sectors are food (including wine and beverage), textile, transport equipment, metal and plastic treatment, and chemical. The areas of application with the most suitable industrial processes include cleaning, drying, evaporation and distillation, blanching, pasteurisation, sterilisation, cooking, painting, and surface treatment. Finally, among the most promising applications space heating and cooling of factory buildings should be included as well [4].

The relevance of each sector regarding the solar thermal market development also depends on the local industrial profile; for example, breweries represent an important industry in Austria and Germany, while dairies are important in Italy and Greece.

. Modellization

Подпись: MR =
Подпись: MC - MCeq MC0 - MCeq Подпись: exp(-K•t) Подпись: (1)

The main purpose of the drying tests is to obtain a mathematical model that describes the performance of the dryer under any given condition. The drying curves can be modelled using theoretical or semi-theoretical equations [8]. The model expressions concern the simultaneous heat and mass transfer equations that describes the process. Moisture ratio equation is the common theoretical expression to model the drying process, described in Equation 1:

Where MC is the moisture content at any time, MC0 is the initial moisture content and MCeq is the equilibrium moisture content obtained as the asymptotic value of the weight of the sample when it remains constant [9].

Many researchers have described the solar drying process for common products like crops, fruit, leaves, etc. using different mathematical expression, all based on equation 1. The Page model equation, equation 2, resulted in a simple expression, similar to the theoretical expression that employs two constants: k and n, to describe with high degree of precision the woodchip drying performance [9]. Thus each drying test is described by two constants as it is shown in the table 2.

MR = exp(-ktn ) (2)

Table 2 shows the values of the constants k, n for 5 tests, described before in Table1, selected to build a global model that describes the drying process for woodchip. The statistical values, correlation coefficient, R, and mean square of deviations, X2, shows the good agreement between data and modelled values for the Page model.

Table 2: Constant values к and n for 5 selected tests.

k(min )

n

R2

X2

test1

0.068747

1.285319

0.999008

0.00008690

test2

0.176961

1.158643

0.995203

0.00036985

test3

0.123004

1.323546

0.99714

0.00024686

test4

0.516446

1.13697

0.995961

0.00021541

test5

0.572723

1.045855

0.998157

0.00008684

The effects of temperature and drying velocity on the moisture ratio were investigated using a multiple regression analysis to account for the drying variables on the Page model constants. The values of constants к and n were regressed against those of drying air temperature and air velocity using multiple regression analysis. All possible combinations of drying variables were included and tested in the regression analysis [10]. The multiple combinations of different parameters which gave the highest R[7] [8] were finally included in the model. The model equation was as follows:

MR = exp(-(0.1288 • V + 0.0008) • (-0.0009 • T[9] [10] [11] + 0.5559 • T + -2.0808) • t(-9-5259V+3-3147V+Ы768)) (3)

Validation of the Page model was confirmed by comparing the estimated or predicted moisture ratio at any other particular drying condition. The validation of the Page model at different air temperatures and air velocities is shown in Figure 3, where the experimental data of 4 tests is compared with the predicted values obtained from the model giving a good fitness.

image248

Fig 3: Comparison of experimental and predicted MR for 4 new tests.

on the transpired plate type: the collector was a wooden box that comprised a perforated absorber plate made of 1.6 mm thick Aluminium. The area of the collector is 1.80 m2 has been drilled forming a distribution of 2 mm diameter holes spaced 20mm apart. The lower section of the collector frame had 35 holes of 20 mm diameter for air inflow. At the rear backing plate on the top, the fan has been mounted to deliver the air into a 150 mm flexible duct. The gap between the bottom of the collector and the absorber plate was 110 mm. In addition, considering the operating temperatures predicted at higher levels of irradiance, the channelled transparent polycarbonate cover is held 4 cm from the absorber plate to minimise convective heat loss. [5]

System design

The system design follows two different approaches. The first one is the “Compact System” for very low fresh water capacities between 100 and 500 l/day. The second one is a “Two-Loop System” for higher capacities.

Compact System

The low thermal capacity of MD-modules enables a quick change of feed volume flow and operation temperature without instabilities in the desalination process. Thus the MD-modules can directly be connected to a corrosion free solar thermal collector without any heat storage. Intermittent operation is possible without reasonable efficiency losses.

image165

Fig. 4: Compact System — sketch of the principle set up (left), demo-system installed in December 2006 in

Tenerife, Spain (right)

The main components of the Compact System are a 500 l feed storage (not represented in the sketch of Fig.4, left side), 1 MD-module, a 6- 7m2 sized solar thermal collector, a pump and a PV module for the electrical power supply of the pump and control system. While the feed storage is mounted above the collectors, most of the hydraulic components are installed in a closed housing covered by the back site of the collectors. However, the cold feed water is pumped into the condenser channel. Afterward the pre-heated feed water leaves the condenser outlet and enters the solar thermal collector which is directly connected to the condenser outlet. The feed temperature is increased by about 5 to 10 K and is re-circulated to the evaporator inlet. The advantage of this configuration is its simplicity and the good efficiency because there are no additional heat losses by a heat exchanger. The disadvantage is that no common flat plate collectors can be used, but the collectors must have sea water resistant riser and header tubes.

The distillate flows to a fresh water tank. The brine rejected from the evaporator outlet of the MD module is recirculated to the feed storage. So the salt concentration as well as the temperature of feed water in the feed storage increases over the day while the content of water decreases due to distillate production. The feed storage is refilled automatically when a certain level or temperature is reached.

Six Compact Systems were constructed, installed and operated since December 2004 in Pozo Izquierdo (Gran Canaria), Alexandria (Egypt), Irbid (Jordan), Kelaa (Morocco) and Freiburg (Germany). In December 2007 an improved compact system with a maximum daily capacity of 120 litres was installed in Tenerife, Spain (Fig. 4, right hand side).

Function control

Among other disadvantages with conventional solar collector systems there is the problem that nobody realizes if they work badly or not at all or even if a forthcoming failure is near. It is

suspected that at least one forth of all solar collector systems (in Germany) is practically out of order or has even more heat losses than harvest (gain). Installations without antifreeze must work nearly error-free. Otherwise their producer will be bankrupt soon. Therefore sophisticated and reliable diagnosis software is essential. The solar controller of the Paradigma AquaSystem has these functions since 2003 and works with the same software for single family houses as well as for industrial installations of any size. It is alarming but characteristic for the German solar market that these tools were not developed until they were technically necessary whereas both the producers of conventional solar installations and the distributors of subsidies have been making do with the high failure rate for many years.

Development of process heat collectors for Solar Heat for Industrial Processes (IEA-SHC Task 33 SHIP)

Matthias Rommel

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany

matthias. rommel@ise. fraunhofer. de

Abstract

A large share of the energy which is needed in commercial and industrial companies for production processes is below 250°C. Solar collectors for the temperature level up to 80°C are already on the market. But new and advanced collector technologies are needed for the higher temperature levels between 80°C and 250°C.

Several new development activities on process heat collectors have been started in the last years. Many, but of course not all of them were somehow linked to Subtask C of the IEA-SHC Task 33/Solar PacesIV "Solar Heat for Industrial Processes" which was carried out under the Solar Heating and Cooling Programme SHC of the International Energy Agency IEA. The paper gives an overview on these collector developments.

Also Fraunhofer ISE is involved in different collector development activities for process heat applications. In order to carry out high quality R&D work in this field together with the solar industry, a new collector testing facility was set up which allows to measure efficiency curves for collector operating temperatures up to 200°C.

Keywords: process heat collectors, industrial process, IEA SHC Task 33, SHIP, collector testing

1. Introduction

As reported by Weiss et al. in [1], the solar thermal collector capacity in operation worldwide in 2006 equalled 127.8 GWth corresponding to 182.5 million square meters at the end of the year 2006. Of this, 102.1 GWth were accounted for by flat-plate and evacuated tubular collectors and 24.5 GWth for unglazed plastic collectors. Air collector capacity was installed to an extent of 1.2 GWth.

The solar heat is mainly used in the household sector for domestic hot water and room heating. In contrast to that, the use in commercial and industrial applications is very limited up to now. On the other hand, the industrial sector in the OECD countries has the highest share of the total energy consumption, at approximately 30%. Solar heat for industrial processes is therefore an important field with a high potential of conventional energy savings, reduction of CO2 emission and economical interest for the solar thermal industry in Europe and world-wide.

A large share of the energy which is needed in commercial and industrial companies for production processes is below 250°C. Solar collectors for the temperature level up to 80°C are already on the market. But new and advanced collector techniques are needed for the higher temperature levels. Especially collectors for operating temperatures between 80°C and 250°C are of interest.

Insulation Layer Underneath the Floor Slab

All cases shown above were calculated with an insulation layer of 10 cm underneath the floor slab. However, industrial buildings are often constructed without an insulation layer underneath the floor. The concrete floor is put directly on some kind of gravel to prevent the concrete from frost damage.

Table 2. Comparison of parameters of heavy and wet soil and light and dry soil.

Soil Parameters

Space Heating Demand

Conductivity

Density

Heat Capacity

with insulation layer underneath the floor

without insulation layer underneath the floor

X

P

c

QSH, w/insulation

QSH, w/o insulation

W/(m K)

kg/m3

J/(kg K)

kWh/(m2 a)

kWh/(m2 a)

Heavy soil (wet)

2,42

3200

840

76

173

Light soil (dry)

0,35

1442

840

69

129

How much energy is lost through the floor slab of such a building depends on the physical parameters of the soil underneath. To illustrate the influence of an insulation layer underneath the floor slab and to show the influence of the soil characteristics, simulations were carried out with two extreme sets of soil parameters: very light and dry soil such as sand that doesn’t conduct the heat well and heavy wet soil such as clay that is much more dense and conducts heat well (see Table 2). As expected, the influence of the soil parameters on the heat demand of the building is much larger if there is no insulation layer underneath the building. However, the most important result is that the space heating demand is roughly doubled without an insulation layer underneath the building.