Category Archives: EuroSun2008-11

Numerical simulation of fluid dynamics

It is of major importance to determine the influence of the fluid medium and the fluid dynamics inside the collector on the heat transfer. The heat transfer through a surface depends not only on the thickness and the heat conductivity of the material but also — and in some cases even predominantly — on the heat transfer coefficient between the fluid and the wall which is determined by the fluid dynamics in the vicinity of the surface. Thus one of the main objectives of this work was to study in detail the behaviour and the characteristics of the fluid flow by means of numerical simulations. By using computational fluid dynamics methods (CFD) it was possible to calculate the fluid flow in the absorber, the heat distribution in the solid materials and the radiation interaction between the internal and external surfaces.

In order to develop a systematic way to compare the mechanisms and the performance of different collector types, calculations for a simple flat plate collector were first made and the influence of various parameters was examined.

Подпись: Fig. 1. Mesh consisting of approx. 100,000 elements For symmetry reasons, the numerical model was reduced to a stripe of 1cm width and symmetric boundary conditions were set for both sides. The model consists of approx. 100’000 finite volumes (Fig. 1). CFD simulations were performed in order to quantify how the heat transfer depends on

• the length of the collector.

• the position of the absorber (above, in between or below the carrier fluid).

• the height of the fluid channel.

• the mass flow rate.

Through series of systematic calculations the following dependencies were discovered:

Подпись: • AT ~ L • Ap ~ L • AT ~ 1/d • Ap ~ 1/d • AT = const. • Ap ~ Фm The temperature difference between inlet and outlet is proportional to the length (Fig. 2).

The pressure drop in the fluid channel is proportional to the length.

The temperature difference increases if the height d of the channel decreases.

The pressure drop increases reciprocally with the channel height d.

AT is constant if the mass flow rate Фщ is held constant for different collector geometries.

The pressure drop is proportional to the mass flow rate.

Furthermore, the simulations showed that the position of the absorber does not influence the outlet temperature of the carrier fluid. However the temperature of the surrounding solid materials depends on the position of the absorber. Thus this parameter is of relevance for the design of the absorber for material or stability reasons. This topic is discussed in Section 4.

Ray-tracing and Variation of Parameters

In order to carry out investigations and to describe an established collector we developed an input mask in which all relevant geometrical and material data can be entered. A macro is generated from this information that completely describes the collector and that can be executed by OptiCAD.

For instance, different types of CPC-reflectors can be easily generated just by entering the designated half acceptance angle and the reflector arc length in such a parametrical model. Every other parameter of the reflector geometry such as angle of truncation, concentration ratio and height of the reflector are automatically determined. For more information on CPC-collectors, see [2] and [3].

The next step after fixing the geometrical and optical properties of the different collector components is to define the simulation mode. Simulations in the transversal or longitudinal plane are possible or also for all other angles (3D-IAM).

Two different alternatives exist for the determination of the IAM-values over the hemisphere (3D — IAM):

image312Either, the simulation can be carried out in polar coordinates (0 — theta, ф — phi) or in the projections of the incident angle (0t — theta_t, 0l — theta_l). The latter are used e. g. in different solar simulation programs such as TRNSYS. Figure 3 shows the angle conventions for the projected incidence angles theta_t and theta_l of solar radiation incident from an optional direction.

Transversal

Подпись: Fig. 3. Projected angles of incidence in the longitudinal and transversal plane [4]

Longitudinal

Direction

When OptiCAD is started, the macro which also contains the description of the ray-tracing calculation is imported and executed. The interpretation of results (e. g. plot of 3D-IAM) happens automatically within the file of the input mask. Thus, it is assured that simulation settings and the output of a simulation as well as the analysis of results are documented together in one file.

The Temperature Field in a Tube’s Coating

To describe the temperature field in a tube’s coating we choose a system of polar coordinates with O as the initial point and write Laplace’s equation in the polar coordinates assuming that the process is stationary [7]:

image079

(12)

 

image080

(13)

(14)

(15)

(16)

(17)

 

image081

d© 2jf>1,pP = Q (p <ф<п-ф1); dp

d©2d(/?1,p) = 0 {n + p1 <p< 2п-ф1);

dp

 

image082

image083

image084 Подпись: (18)

Applying the Fourier method of separation of variables [8] one obtains:

where k is a positive integer.

Variations in measured hot water use

The time-use data is only available for one weekday and one weekend day per household, and therefore the model only generate average profiles for one day of each type, not depending on season

or day of the week. To enable further model development the time dependent variations in load measured in the 24 apartments are investigated below.

Подпись: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Figure 3. The measured daily hot water consumption per average household member in the 24 apartments for the different months of the year.

The energy use for hot water for different months, expressed as average daily demand per average household member, is found in Figure 3. The lowest hot water consumption is found during the summer months and the highest consumption during winter. The average daily energy use for hot water is 5.4 and 4.4 kWh/person for 2005 and 2006 respectively, with a variation of the maximum and minimum consumption of ±17-26 % around the average. The results correspond well to the monthly relative variations found in [9].

Подпись: Figure 4. The energy use for hot water different weeks during the half-year including summer. A decrease can be observed for the vacation time in July for both measured years.

The half-year including summer is investigated more in detail in Figure 4, showing the weekly variations in hot water demand in April to October. A clear minimum is found in July and August, which is the usual vacation period in Sweden.

Assessing the effect of a new approach to the incidence angle modifier

A more fair and uniform approach is being proposed by Horta et al.(2008 a, b), to be applied independently of the underlying optics. This approach allows using the test data currently collected on trials and is based on absorber irradiance values, Gabs = B cos 0 K(0) + Kd D + Kr D (where Kd and Kr are computed assuming an isotropic diffuse field) and a certain a posteriori correction of the efficiency, that includes an estimate of the diffuse fraction of the radiation during performance trials, fD — unknown but by norm in the range 10% to 30%.

The work examined how these new algorithms can be implemented in practice and what is their influence on thermal solar system (not just collector) performance, using a solar simulation software with quasi-stationary thermal balances, SolTerm 5.

Results with a typical DHW system for Lisbon can be found at Table 1, using data for three flat-plate and one 2D commercial collectors (flat, CPC), coupled to an oversized storage (to ignore dissipation and other effects in a first phase). The energy supplied by the system ES (kWh/year) and the respective solar fraction fS, computed with the previous and proposed approaches (ref, new) are compared.

Table I — Sensitivity analysis (Lisbon climate).

Load: 45

°C, 2037 kWh/year

Load: 60

°C, 3056 kWh/year

fD = 15%

fD =

25%

fD = 15%

fD

= 25%

collector

type

approach

Es

fs

A

Es

fs

A

Es

fs

A

Es

fs

A

CPC

ref

1486

73%

1486

73%

1493

49%

1493

49%

EW

6%

7%

5%

6%

2D

orientation

new

1609

79%

1638

80%

1633

53%

1670

55%

CPC

ref

1464

72%

1464

72%

1467

48%

1467

48%

NS

6%

8%

5%

6%

orientation

new

1589

78%

1618

79%

1609

53%

1646

54%

ref

1546

76%

1546

76%

1555

51%

1555

51%

Flat #1

new

1595

78%

2%

1618

79%

4%

1611

53%

2%

1640

54%

3%

ref

1718

84%

1718

84%

1719

56%

1719

56%

1D

Flat #2

new

1751

86%

2%

1761

86%

2%

1836

60%

4%

1854

61 %

4%

ref

1485

73%

1485

73%

1489

49%

1489

49%

Flat #3

new

1528

75%

2%

1547

76%

3%

1538

50%

2%

1560

51 %

2%

Conclusions

For the range of climates and collector types surveyed, the new approach to the incidence angle modifier meant a 2% to 4% rise in estimated performance for flat plate type collectors and a 6% to 8% rise for a CPC type collector.

This is very significant, and justifies that further results for realistic storage sizes, other climates, collectors, load profiles, and ranges of solar fraction must be obtained, aiming the elaboration of recommendations for the use of the new algorithms.

References

Horta, P., M. J. Carvalho, M. Collares-Pereira and W. Carbajal (2008a). “Incidence angle modifiers for beam, diffuse and reflected radiation: a general approach for energy calculations”, submitted to Solar Energy, May 2007.

Horta, P., M. J. Carvalho, M. Collares-Pereira, and W. Carbajal (2008b). Long term performance calculations based on steady state efficiency test results: analysis of optical effects affecting beam, diffuse and reflected radiation”, submitted to Solar Energy, May 2007.

EINSTEIN: Expert-system for an Intelligent Supply. of Thermal Energy in Industry

Christoph Brunner 1, Hans Schnitzer 1 2, Bettina Slawitsch 1,

Hans Schweiger 3, Claudia Vannoni 4

1JOANNEUM RESEARCH Forschungsgesellschaft mbH — Institut fur Nachhaltige Techniken und Systeme

Elisabethstrasse 16, A-8010 Graz
Tel.: +43-316 / 876 2413, Fax: +43-316 / 8769 2413
E-Mail: christoph. brunner@joanneum. at

2Institute fur Ressourcenschonende und Nachhaltige Systeme, Technische Universitat Graz

Inffeldgasse 21, A-8010 Graz

3energyXperts. BCN

Creu dels Molers 15, 2-1, Barcelona, Spain
4Dipartimento di Meccanica e Aeronautica, Univerity of Rome, Sapienza

Abstract

Thermal energy (heat and cold) demand in industry constitutes about 20 % of the total final energy demand and produces about 21 % of the CO2 emissions in Europe. Even if energy efficiency in industry in Europe has improved the last decades, there remains a large unexploited potential for reducing energy demand that could be achieved by the intelligent combination of existing solutions and technologies.

The present project aims at contributing to a widespread implementation of integral energy- efficient solutions for thermal energy supply in the industrial sectors with a high fraction of low and medium temperature heat demand, especially the food, wood processing and metal treatment sectors that will be first addressed within this project.

For optimising thermal energy supply, a holistic integral approach is required that includes possibilities of demand reduction by heat recovery and process integration, and by an intelligent combination of existing affordable heat (and cold) supply technologies, under the given economic constraints. The presented Intelligent Energy Europe — IEE project uses as a basis available methods and tools that address some parts of these topics, adds missing elements and brings them together into a complete EINSTEIN tool kit for thermal energy auditing, which is used in all the project’s activities.

Most of the tools have been developed within the IEA Task 33/IV “Solar heating for industrial processes (SHIP)”. The proposed methodology focuses at first on the optimization of the given process to reduce the absolute energy demand before further integrating solar heat in the technical and economical most suitable way.

The optimization of the process is achieved by the integration of energy efficiency measures and heat exchange. The analysis for this optimization is done with the mathematical model of the Pinch Analysis. With the help of the Pinch Analysis the minimal heat and minimal cooling demand of a process can be calculated and the theoretical possibilities of heat recovery are shown.

After having gained a proposed solution for optimizing the given process in terms of energy efficiency, a strong information tool is used that allows for further optimization that might lead to changes of the process, changes of the energy distribution system or changes of the energy supply system. This developed tool, the so called “matrix of indicators” will be described in the following. It was designed as a decision support tool for solar experts and facilitates work with industry and the identification of suitable solar applications.

With the help of the “Matrix of Indicators” the ideal solution to implement energy efficiency measures and integrate solar process heat can be found. The final steps prior to implementation include the solar simulation, the economical evaluation and the analysis of the practicability of the proposed solution.

The realisation of the methodology in form of a complete auditing tool-kit including an expert system software tool makes it easy to use, easily distributable, and helps reducing time (and therefore cost) and increasing standardisation (and therefore quality) of energy audits. The EINSTEIN tool-kit will be available in Czech, English, German, Italian, Polish, Spanish and Slovenian language. Within the project, 90 industrial companies will be audited, starting from autumn 2008. It is assumed that in addition to the thermal audits carried out within this project; at least 100 companies will have used this self-assessment at the end of the project and 100 reports to be available. Energy auditors and consultants are trained in training courses. At least about 200 trained auditors (of the 240 people attending the training courses) will be available at the end of this project as multipliers, who in their everyday practice are working in the field of energy auditing and therefore in the future will induce further improvements in energy efficiency. A Europe-wide training-of-trainers course will complete the training program and help to expand its impact also to countries not directly involved into the project’s activities.

1. Introduction

Thermal energy (heat and cold) demand in industry constitutes about 20 % of the total final energy demand and produces about 21 % of the CO2 emissions in Europe. Even if energy efficiency in industry in Europe has improved the last decades, there remains a large unexploited potential for reducing energy demand that could be achieved by the intelligent combination of existing solutions and technologies.

The present project aims at contributing to a widespread implementation of integral energy — efficient solutions for thermal energy supply in the industrial sectors with a high fraction of low and medium temperature heat demand, especially the food, wood processing and metal treatment sectors that will be first addressed within this project.

2. Methodology

The present project aims at overcoming the barriers mentioned above (and described in details in b) and at contributing to a widespread implementation of integral energy-efficient solutions for thermal energy supply, especially in SMEs and in general in all industrial companies where the really used energy supply systems are still far from best practice. The project focuses on the food, wood processing and metal treatment sectors but can be easily extended to other industrial sectors

with high fraction of low and medium temperature heat demand such as chemical, textile industry, pulp and paper industry.

The specific objectives in order to achieve this are:

• the reduction of the time and cost required for thermal energy audits and the development of a self-assessment tool for thermal energy supply, in order to reach a wider market penetration of energy-efficient technologies

• a wider diffusion of knowledge on integral energy-efficient solutions for thermal energy supply in industry among the actors who can act as future driving forces for improving energy efficiency

• the execution of a thermal energy auditing campaign in industrial companies and the creation of best practice examples for an efficient heat and cold supply

• the improvement of the technical skill of the relevant actors (auditors, industrial technical staff, etc.) by means of training activities and of an assisted thermal energy auditing campaign

• the improvement of quality and reliability of thermal energy audits by means of the standardisation of the procedure and decision support tools

EINSTEIN contributes to the IEE priorities and in particular answers to the need of setting “tools to help industries, in particular SMEs, to develop an intelligent energy approach, including training of energy auditors”. The project is structured into the main blocks below:

• Standardized Audit — the EINSTEIN Expert System: methodology that works out energy efficient solutions for your production process based on energy saving and renewable energy sources

• Audit Campaign: A limited number of energy audits

• Self-Assessment Tool: Check your Performance

• High Quality Training: Training workshops for energy managers, technicians and consultants

• Information Workshops for decision makers

The EINSTEIN methodology can be divided into 4 main steps. During the pre-audit the important data for the future calculation will be collected and a preliminary evaluation will be done. If a potential will be identified the analysis of the status quo will form the main step during the audit phase. By using the EINSTEIN tool kit the conceptual design of saving options and preliminary energy targets will be defined. At the end an economical feasible solution will be proposed.

.

Most of the tools of the EINSTEIN audit tool kit have been developed within the IEA Task 33/IV “Solar heating for industrial processes (SHIP)”. The proposed methodology focuses at first on the optimization of the given process to reduce the absolute energy demand before further integrating solar heat in the technical and economical most suitable way.

The complete thermal auditing tool kit is formed by a software tool (expert system), questionnaires for data acquisition and thermal auditing and system design guidelines. The EINSTEIN tool kit for the day-by-day auditing practice is based on the information on current auditing practices, tools and user needs.

1st International Congress on Heating, Cooling, and Buildings " ‘ 7th to 10th October, Lisbon — Portugal *

image25

Fig: 2: Screen-shot of the EINSTEIN thermal audit software tool

The optimization of the process is achieved by the integration of energy efficiency measures and heat exchange. The analysis for this optimization is done with the mathematical model of the Pinch Analysis. With the help of the Pinch Analysis the minimal heat and minimal cooling demand of a process can be calculated and the theoretical possibilities of heat recovery are shown.

After having gained a proposed solution for optimizing the given process in terms of energy efficiency, a strong information tool is used that allows for further optimization that might lead to changes of the process, changes of the energy distribution system or changes of the energy supply system. This developed tool, the so called “matrix of indicators” will be described in the following. It was designed as a decision support tool for solar experts and facilitates work with industry and the identification of suitable solar applications.

With the help of the “Matrix of Indicators” the ideal solution to implement energy efficiency measures and integrate solar process heat can be found. The final steps prior to implementation include the solar simulation, the economical evaluation and the analysis of the practicability of the proposed solution.

As stated above the Matrix of Indicators was developed as decision support system for solar experts, to facilitate the ideal integration of solar process heat into industrial processes.

On the ordinate for solar application relevant industry sectors are listed. Although the above discussed processes occur in practically all industry sectors, some distinguish themselves by a greater potential than others. On the abscissa different unit operations, which have been identified as relevant for solar applications are listed.

Further on the ordinate there is a button for general information about the unit operations, about competitive technologies for this unit operations and a column for possible solar integration schemes.

The unit operation is generally described by the functioning of the unit operation, the implementation possibilities and the area in which it can be used.

Following information will be found in the description:

• general objectives

• influencing parameters (temp, pressure, humidity, medium)

• temperature levels

• fields of application

• descriptions of techniques, methods and equipment

• environmental issues

The idea of the competitive technologies is described generally and an overview of alternative technologies and possible application fields is given. Competitive technologies are technologies that might constitute an alternative option for the specific requirements. Solutions how else the required problem could be solved and how (dis)advantageous other options are compared to the proposed unit operation are shown.

The solar integration schemes give a general overview of the solar installations for different applications. Additionally, specific integration schemes are proposed for each problem, as solar integration is possible in several ways and the ideal methods depend on the industrial processes which are sector — and plant-specific.

The availability of a new methodology that allows for realising more energy audits in future at a lower cost, delivering at the same time high quality and innovative recommendations will be the main outcome of the present project, even more important as the impact of the directly induced improvements of energy efficiency in industry described below.

The realisation of the methodology in form of a complete auditing tool-kit including an expert system software tool makes it easy to use, easily distributable, and helps reducing time (and therefore cost) and increasing standardisation (and therefore quality) of energy audits. The EINSTEIN tool-kit will be available in Czech, English, German, Italian, Polish, Spanish and

Slovenian language. With the available EINSTEIN tool-kit it will be possible to realise high quality fast-audits including company visit, proposal and approximate energetic and economical evaluation of an alternative system within about 24 to 40 hours of a trained auditor. The alternative proposals will include all the available technological options including cogeneration and polygeneration, process integration technology, heat pumps, solar thermal energy and biomass.

There will be also a simplified version of the software tool for web-based self-assessment of the companies, so that even a larger number of companies can be reached by the proposal, especially those companies who are not willing to spend money for energy-auditing due to other priorities. This web-page for self-assessment will be widely disseminated via the different dissemination activities foreseen in the project. It is assumed that in addition to the thermal audits carried out within this project, at least 100 companies will have used this self-assessment at the end of the project and 100 reports to be available.

Energy auditors and consultants will be trained in training courses. At least about 200 trained auditors (of the 300 people attending the training courses) will be available at the end of this project as multipliers, who in their every day practice are working in the field of energy auditing and therefore in the future will induce further improvements in energy efficiency. A Europe-wide training-of-trainers course will complete the training program and help to expand its impact also to countries not directly involved into the project’s activities. This training and the guidance towards proposals generated by the EINSTEIN expert system decision support tools will induce that energy auditors increase their horizon towards innovative, non-conventional solutions using the holistic approach of EINSTEIN. At the same time they will have the appropriate tool in order to correctly evaluate these options. Some of these auditors at the end of the project will be able to act as future trainers using the EINSTEIN training material.

From the dissemination activities, that range from workshops and conferences oriented to specific target groups, to distribution of the methodology and software tool via internet, further public will be reached. It is expected, that at least 10,000 persons (technicians and decision makers in industry, energy auditors, consultants, policy makers and local authorities) will be reached and 1,000 persons will assist to these activities while 2,500 copies of the EINSTEIN tool-kit will be distributed all over Europe by the end of the project.

Standardisation of the auditing methodology and comparability of energy supply alternatives also will help public bodies and energy agencies to evaluate the present state (situation with respect to current best practice) and the achieved improvements by energy efficiency measures, and therefore lead to a better control of the real impact of energy policies. The tool could become a reference for public support programmes in the field of industrial energy efficiency.

Within the project, 90 industrial companies will be audited. For all these industrial companies a proposal for improvement of energy efficiency and use of renewable energy for heating and cooling will be available and several best practice examples will be selected. Energy demand for heating and cooling in industrial companies may vary from 50 MWh/year for small enterprises up to several 100,000 MWh for big companies

Assuming a mean primary energy demand for heating and cooling of 5000 MWh/year for the studied companies, and a mean potential for saving of 30 %, a total saving potential of 150.000 MWh/year (about 2.5 M€/year in terms of energy costs based on natural gas prices) will be identified. It is expected that decisions for realisation of at least 10% of this savings potential will

be taken during the project and another 20% during the first year after the project leading to a real energy saving of 45,000 MWh/year (0.7 M€/year in terms of energy costs).

Furthermore it is assumed that this direct impact of the project is at least multiplied by a factor of two by energy efficiency measures and renewable energy projects indirectly induced by the project and while the project is ongoing, due to the training and dissemination activities.

3. Conclusions

Energy efficiency measures often have benefits that extend beyond energy savings and include pollution prevention, process efficiency, and increased productivity. Total benefits for industrial companies are therefore on the focus of the proposed action. Among such benefits, the implementation of the recommended measures for energy saving will lead to a reduction in the energy bill and therefore to an increased competitiveness of the companies. Apart from the economic savings on a short term, due to reduced demand and the use of renewable energy sources (RES) the companies will have a higher security of supply and more stable and foreseeable energy costs on the long term.

Moreover the thermal energy auditors will benefit directly from the project by improving their qualification, by learning about innovative technological options and by therefore being able to offer a better service.

The EINSTEIN tool will form a very innovative and strong tool which will change the energy audit methodology radically in order to identify the maximum of energy saving and the ideal integration of renewable energy into industrial processes in a very short time.

References

[1] Muller T., Begander U., Schnitzer H., Brunner Ch., Weiss W. (2003), PROMISE — Produzieren mit Sonnenenergie — Potenzialstudie zur thermischen Solarenergienutzung in Gewerbe — und Industriebetrieben in Abhangigkeit der Produktionsprozesse,

[2] Energieverwertungsagentur, www. eva. ac. at/enz/efluss. htm vom 8.6.2004

[3] Brunner CH. Schweiger H. Vannoni C. IEA Task 33 — Solar heating for industrial Processes — SHIP

[4] Faninger G. (2004), Entwicklung des Solarmarktes in Osterreich 1975 — 2003, Verband AUSTRIA SOLAR und Dachverband Energie-Klima, Marz 2004

[5] Schnitzer H., Brunner C., Gwehenberger G. (2006): Minimizing greenhouse gas emissions through the application of solar thermal energy in industrial processes. Journal of Cleaner Production,

doi: 10.106/j. jclepro.2006.07.023

System Concept Description

The following HPS system concept has been selected for the investigations. It may be described by the characteristics given below:

• Unglazed Collectors: Inexpensive, uncritical operation mode (no stagnation, no steam generation), uncomplicated opportunities for building integration

• In order to realize an uncomplex system: Solar heat only directed to the evaporator side of the heat pump and: No option for direct use of solar heat for domestic hot water or space heating foreseen.

• Limitation of the investigations: Heating systems for small buildings (< 30 MWh/a heating demand)

• Thermal soil regeneration as the main target: A permanent increase of the yearly mean HPS source temperature if compared to a non-solar BHE system is the aim, but a significant seasonal storage effect is not planned. Thus, even with SC the ground is mainly serving as a heat source.

All these characteristics and restrictions form a reasonable unit. As for small buildings the seasonal storage effect may not be achieved due to the high thermal losses, the solar heat is mainly used for the regeneration of the soil. This leads to lower source temperatures in winter. Under these condi­tions unglazed SC find an ideal application temperature level, due to the fact that they are operated nearby or even below ambient air temperature.

1. Подпись: Fig. 1. System concept and operation modes. A common heat pump system with BHE is extended by an unglazed SC. Only the heat source side of the heat pump is shown. BH discharging ►

2. BH charging

…………………………. ►

3. Serial-operation

The system (Fig. 1) is operated in three modes: Charging, discharging and serial operation. While operating in the discharging mode the BHE delivers with its own ground heat to the heat pump, whereas in the serial-operation mode the BHE is supported by the SC as additional heat source. In the operation mode “charging”, the collector is heating up and thus regenerating the soil around the BHE, while the HP is turned off. This concept realizes a simple hydraulic connection of the com­ponents and therefore allows different volume flow rates in the collector and BHE circuit.

Validation of the Storage Tank Model

After finishing the code, the storage model was validated in two steps. In a first step, the simulation behaviour was compared to measured data from previous system tests using measured hot water draw off curves and storage heat loss curves from several different testing days (Figure 3).

image221

Fig. 3. Comparison between a measured and a simulated storage behaviour a) temperature increase and energy draw off at night, b) temperature drop caused by heat loss within a period of 18h

In a second step, the thermal behaviour of the double mantle heat exchanger store was compared with CARNOT’s existing simple water store model — insulated cylindrical hot water storage without any additional heat exchangers — validated by Hafner et al [4]. In order to compare both storage types, the gap between the inner and outer mantle of the double mantle storage model was assumed to be zero.

Both types of validation tests showed a deviation between simulation and measurements of less then 5%.

3. System Simulation

Подпись: Fig. 4. CARNOT simulation model of a thermosiphon solar water heater.

In the next step a complete thermosiphon system composed of solar collector, the necessary piping, the so-called thermosiphon pump (a block setting the thermosiphonic flow rate according to pressure differences in the system) and the newly developed double mantle heat exchanger storage was built up (Figure 4). The implemented technical data is identical to a system which was tested according to ISO 9459-2 at the CENTRE OF EXCELLENCE FOR SOLAR ENGINEERING in Ingolstadt (Table 1).

Table 1. Technical characteristics of the measured and simulated thermosiphon system.

Collector aperture area

1.89m2

Collector cover material

Prismatic tempered glass, 3.2mm thickness

Collector hydraulics

Diameter header: 18x1mm; diameter riser: 8×0.5mm; number of risers: 10

Collector slope

38°

Heat transfer fluid

Water

Storage

Horizontal double mantle heat exchanger storage, volume: 180l, diameter: 0.48m, length: 1.46m

Tank insulation

30mm polyurethane

Heat exchanger

Double mantle, volume: 8.5l

Connecting pipes

Well insulated tube, 22x1mm, total length: 2.64m

Location

Ingolstadt, Germany

All blocks in the system are connected via data vectors according to their position in the system. The most important vector is the so called thermo hydraulic vector (THV), in which all relevant values are bundled. It includes the fluid identity, e. g. water or water-glycol mixture, fluid pressure, pressure drop (calculated in the block before), fluid temperature and density. Due to this THV, a realistic system behaviour can be achieved.

In the following, two different thermosiphon systems were simulated according to data measured at Ingolstadt University, using irradiation on the collector plane, the thermosiphonic flow rate and temperatures at collector / storage in — and outlet. The corresponding simulation runs show a good correlation between measured and simulated data, as can be seen in Figure 5 showing the flow rate and the accumulated flow through collector and storage over a whole day.

Подпись: 08:00 10:00 Подпись: 12:00 14:00 time M Подпись: 16:00 Подпись: 18:00

Fig. 5. Measured and simulated flow rate in a thermosiphon solar hot water heater

The energy management

The combination of an existing heat source, a solar thermal plant and a thermal storage tank makes it necessary to develop an ‘energy management’ as a fundamental component (Fig. 3). This energy coordination has to guarantee, that

• The overall system operates in economic efficient way, i. e. a minimum of fuel is used to meet the demand of the district heating grid

• Boundary conditions will be fulfilled. These general conditions are e. g. physical limits of the storage tank or the combined gas and steam cogeneration plant.

image279 image297,image299,image300

The necessary heat supply of the cogeneration plant can be calculated at any time instant from the heat demand of the heating grid, reduced by the actual power of the solar thermal plant and the contribution of the storage unit (which can be either positive or negative — depending on charging or discharging operation of the storage tank). Changing the contribution of the storage unit results in a changed operating point of the cogeneration plant and it associated degree of efficiency.

Fig. 3: Structure of the solar assisted district heating grid

The energy management has then to decide, if — at a certain heat demand — the storage tank has to be charged or discharged. This decision is based not only on actual values of the overall system but can also depend on future assumption of the plant behaviour. That means e. g., that the storage tank is not

charged, if the solar thermal plant can provide enough energy to the district heating grid in the next hours. To predict such an event, a weather forecast has to be implemented in the energy management tool. Since the heat demand is correlated strongly with the weather conditions, the weather forecast can also be used to predict the heat demand.

The concept of such an energy coordination unit is typically based on an enormous number of ‘lingustic’ rules, like

• “… if the heat demand is low, the ‘state of charge’ of the storage tank is low and the temperature forecast predicts 15°C in the next hour, then storage unit is charged with 780 kW heat power…”

Within this project, a different approach for the design of the energy management was used. It is based on an analytical procedure [2, 3], where the development of the energy coordination unit is the result of an optimization task

T

min [ ф(и(т),…)ёт = min^ф(ик,…)

u (t) uk

s. t. boundary conditions are fulfilled,

where ф(-) is the instantaneous fuel consumption of the cogeneration plant and u(t) represents the charging/discharging level of the thermal storage tank. Boundary conditions have to guarantee, that the storage tank is not overcharged of discharged completely.

The basic principle of the ‘load management’ and the development of the underlying mathematical methods have been shown in [2]. The used optimization horizon was set to 1 year. Since one can not know exactly the weather conditions and solar radiation for such a long forecasting horizon, the optimization problem was split up into two tasks

• The long-term scheduling problem (with assumed average values within the optimization horizon) and

• The short-term scheduling problem (works as a correction of the long-term problem with actual values).

Reference geometry and comparisons

For the comparison the geometry of the greenhouse-environment system shown in figure 1 was taken into consideration.

image42

Fig. 1. Sunspace-environment system and schematization of solar heat gains. Te external air temperature, Ta air temperature in the airconditioned environments, Ts air temperature in the sunspace.

The sunspace, measuring 6^3×3 m in dimension, borders with the outside by means of three vertical walls and the roof, and is separated from the airconditioned environment by a wall that is part glazed and part opaque; furthermore it borders with an underlying environment that is also airconditioned. The glazed surfaces are formed by a clear double glass 4-12-4 with thermal transmittance Ug = 2,88 W/m2K and solar gain coefficient gi = 0,747. The internal opaque wall and the floor of the sunspace have a respective thermal transmittance of Uw = 0,55 W/m2K and Uf= 0,57 W/m2K. The other walls of the airconditioned environments are considered internal walls, adiabatic in corrispondence with the symmetry axis.

The overall heat transfer coefficients by ventilation between the greenhouse and the outside, and between the airconditioned environment and the greenhouse were placed equal to 18 W/K and correspond to an air change equal to 1 volume/h. The coefficient of surface thermal exchange of the sunspace floor hf was set equal to 10 W/(m2K) and that of the opaque dividing wall hw = 7,7 W/m2K [7]. In the evaluation of the thermal coupling coefficient between the airconditioned volumes and the sunspace, through the vertical wall and the floor, the effects of thermal bridges were not taken into consideration. Furthermore for the glazed surfaces corrective factors due to shading, to curtains and to the frame were considered unitary.

With reference to figure 1, the total solar contribution through the vertical separation wall QW and through the greenhouse floor Qf were obtained by adding the respective direct Qd and indirect Qt contributions

Qw = Qd, g + Qd, w + Q w (1)

Qf = Qd, f + Qf (2)

Indirect contributions require the evaluation of the net energy absorbed by the sunspace Qa

Qa = Qw + QaJ — Qd, w — Qdf (3)

Подпись: Qi,w image039 image040

and are calculable with the relations

with Hw and H f heat loss coefficients between the airconditioned spaces and the greenhouse, through the mixed vertical wall and the floor, and He loss coefficient between the greenhouse and the external environment [8].

With the considered outlined conditions, the average monthly temperature within the greenhouse is given by the relation

Подпись: (6)Qa

(Hw + Hf + He )At

with Ta the temperature of the adjacent airconditioned environments, set at 20°C during the heating period and at 26°C during the cooling period, and At the number of seconds in a month.

With reference to two Italian localities characterised by different climatic conditions, Milan (Lat. 45°27’, Long 9°11’) and Cosenza (Lat. 39°18’, Long 16°15’), direct, indirect and global solar contributions were compared with a variation: of the glass fraction f of the greenhouse — environment separating wall f = 20%, 60%, 100%); of the orientation of the greenhouse (South, East, West); of the optical properties of the opaque surfaces, the absorption coefficient of the solar band was presumed to be identical for the different surfaces of the greenhouse and the environment, and was made to vary between a=0,2 (highly reflecting environment) and a=0,5 (moderately absorbing environment). Furthermore, the solar contribution obtained in the airconditioned environment in the absence and presence of the greenhouse was evaluated.

2. Comparison results

image43 image44

The monthly direct, indirect and total solar heat gain, through the separation wall and the floor obtained with the simulation code and the Standard were compared. The direct contribution through the glazed surface for Milan and Cosenza, with a variation of the glass fraction f, for a = 0,20 and for its Southern exposure are reported in figures 2 and 3.

The results of the comparison depend on the glass fraction, in particular for glass fractions of 20% the Standard underestimates with greater deviation in the summer period (23% for Cosenza, 17% for Milan) and is more contained during winter months (less than 10%). Instead, for 100% solar fractions, the Standard leads to a significant overestimation in winter months for both locations, with deviations of 43%. During summer months such a percentage is reduced to 13% for Cosenza and to 20% for Milan. For Eastern exposure, the previous trends remain confirmed with reduced deviations which do not exceed 17% for f = 20%, and 33% for f = 100%. For an absorption coefficient value of the greenhouse-environment system equal to 0,50, the Standard provides evaluation in good agreement with the code, with deviations which does not exceed 17%.

image45 Подпись: f=20% S f=20% C f=60% C f = 100% C image46 Подпись: —•—f=20% S f=20% C —*—f=60% S f=60% C —ж—1=100% S ....... |=100% C

For the comparison of the indirect contribution through the separation elements between the greenhouse and the airconditioned environments, the comparison of the temperature increase of the air within the sunspace is significant, as a consequence of solar absorption and exchange through the shell. In figures 4 and 5, for Milan and Cosenza, the monthly trends of the increase in temperature in the greenhouse relative to its Southern exposure are reported for environments with a = 0,20.

Fig. 4. Increase in sunspace air temperature 4 Fig. 5. Increase in sunspace air temperature consequent to consequent to solar absorption, varying the glass 4 solar absorption, varying the glass fraction f. Cosenza, fractionf. Milan, South, a=0,20. S Standard, C code. South, a=0,20. S Standard, C code.

The Standard underestimates the increase in temperature with variations between 1,0 °C in January and 4,5°C in July for Milan, and between 2,5°C and 5,3°C for Cosenza, for the different glass fractions. The increase in the absorption coefficient (a = 0,50) gave rise to a more contained underestimation of the increases, with deviations that for Milan do not exceed 3,0°C in July for f= 100%, and 3,7°C for Cosenza. For Eastern exposure similar variances were found.

image47 image48

The solar heat gains obtained as the sum of direct contribution through the opaque surfaces and the indirect contributions consequent to the heating of the greenhouse, evaluated with the Standard, are compared with the contributions in the environment adjacent through the separation wall, evaluated with the code. It can be observed that the Standard gives rise to a systematic underestimation of such contributions. In figures 6 and 7 the values of the previous contributions for a = 0,20, southern exposure, for the considered localities are reported.

The Standard provides trends which vary slightly with the glass fraction, since, with an increase in the glass surface, the thermal coupling coefficient between the greenhouse and the adjacent environment increases while the energy absorbed by the opaque walls diminishes as demonstrated by the equation (4). Significant variations produced by the different evaluations of the increase in temperature between the greenhouse and the environment are highlighted, which increase with the glass fraction and are between 33% (f = 20%) and 84% (f = 100%) inclusive. For more absorbent environments, it is ascertainable that for contained glass fractions, the Standard gives rise to an overestimation, attributable to the evaluation of the direct contribution through the opaque wall, with deviations varying between 10% and 33%. Instead, for glass fractions f = 100%, the Standard leads to an underestimation with deviations between 23% and 55% inclusive. For Eastern exposure, similar trends are recorded with more contained deviations. The comparison of the contributions through the sunspace floor leads to considerations similar to the previous ones, in particular for a = 0,20 the Standard underestimates in a proportion increasing with the glass fraction, with variations between 24% and 58% inclusive.

For a = 0,50 the Standard overestimates in the summer months and underestimates in the winter months with deviations that do not exceed 24%. Finally, the comparison of total solar heat gains for the adjacent environment, the sum of the direct and indirect contributions, is reported for case a=0,20 and for Southern exposure in figures 8 and 9. For such configurations, the Standard underestimates the solar contribution with deviations which diminish with the increase of the glass fraction, and do not exceed 28%. Fore more absorbent environments and reduced glass fractions (a=0,50 ed f=20%), for both the localities the Standard underestimates the contribution in a measure less than 18%, while for the glass fractions it is higher (60%, 100%) [9].

image49 image50

The results provided by the two calculation procedures appear to be in agreement, as an effect of the compensation between the underestimation of indirect contributions and the overestimation of direct contributions.

3. Considerations

In light of the simplifications hypothesised by EN ISO 13790, the results obtained with the code were used for quantitative evaluations on the direct and indirect solar heat gains for the adjacent airconditioned environment. Furthermore, the solar contribution of the sunspace — environment system obtained with the code was compared with those calculated by applying the Standard to the environment lacking a greenhouse in front of it.

image51 image52

The indirect contribution was generally lower than the direct contribution, but they can also result as being greater in the case of absorbent environments with reduced glass fractions. Their relationship is a function of the glass fraction, of the absorption coefficient of the spaces, of the exposure and of the locality. In figures 10 and 11 the results obtained for Southern exposure for Milan and Cosenza are reported.

It should be immediately highlighted that the indirect contributions are not negligible in any case compared to the direct contributions, even when the increases in the temperature of the greenhouse are modest. With the same glass fractions, the indirect solar heat gain increases in a significant manner with the absorption coefficient since, in comparison with a modest increase of the direct contribution, the indirect contribution increases in a considerable way due to the greater absorption of the sunspace. It should be highlighted that the greater total contribution is obtained by configurations which ensure maximum direct and indirect solar heat gain, with prevalence over
direct contribution (f=100%). The previous trends with a lesser monthly variability remain confirmed for Eastern exposure.

In figures 12 and 13 the total solar heat gains for the sunspace-environment system, with a = 0,20 and Southern exposure, calculated with the code are compared with those evaluated by the Standard for the same windowed environment.

In Milan the sunspace gives rise to increases in contributions that, for a glass fraction of 20%, result as being hardly variable with the absorption coefficient and being between 25% and 35% inclusive during summer and between 11% and 19% during winter.

For Cosenza the percentages are between 21%-43% during summer, and between 12%-20% during winter. If the glass fraction increases, the solar contributions undergo variations during winter which differ to those during summer. In particular, during the winter months the presence of the sunspace gives rise to a reduction of the contributions, which results as being less than 17% for f = 60%, and less than 49% for f = 100%. The reduction of the direct solar heat gains linked to the presence of the greenhouse does not appear to be compensated by the indirect contributions.

image53

During summer, viceversa, the greenhouse determines an increase in contributions which do not exceed 21%. For environments which are more absorbent the previous variances undergo an attenuation during winter, while during summer they remain substantially unaltered. For Eastern exposure, poor monthly variability was observed for f = 20%, with values between 24% and 31% inclusive for both the localities and optical configurations. For higher glass fractions an invariance in solar contribution in the case of f = 60% was observed, and a significant reduction for f = 100% that is equal to 28% for Milan and a = 0,20. More contained variations were recorded for Cosenza.

Fig. 13. Solar heat gain for sunspace-adjacent environment systems and for windowed environments, varying the glass fraction f. Cosenza, South, a=0,20.

4. Conclusions

For a system comprising of a sunspace bordering airconditioned environments, the solar heat gain calculated by the simplified calculation procedure of the EN ISO 13790 Standard was compared with those obtained by means of a dynamic calculation code that accurately calculates the interaction between solar radiation and the glazed spaces adjacent to airconditioned rooms. The analysis, carried out for different climatic conditions, geometries and optical configurations, permitted the evaluation of direct solar contribution, through glazed and opaque surfaces, and indirect due to heating of the greenhouse, and the total contributions.

The different modelling of the absorption of solar radiation leads to different increases in air temperature in the greenhouse which lead to considerable underestimation of the indirect contributions on behalf of the Standard. For configurations with a solar fraction of 100% the variances reach 80%. Furthermore, it is possible to observe that, for such configurations, the direct contributions result as being overestimated by 40%. With regards to total contributions, the results provided by the two calculation procedures appear to be in greater agreement: deviations of less

than 28% are recorded for environments with a = 0,20 and lower than 18% for more absorbent environments.

With the aim of evaluating applicability, for the climatic conditions considered, the simplified hypothesis proposed by EN ISO 13790 (do not compute the indirect contributions, ignore the presence of a glass shell) the direct and indirect contributions obtained with the code were compared, and the solar heat gain for a windowed environment was calculated with the Standard. The indirect solar contributions, compared to direct contributions, result as being significant in percentage and variable according to the locality, the glass fraction, the absorption coefficient of the environments and the exposure. Therefore, the indirect contributions are in no case negligible even in the presence of contained increases in the sunspace temperature.

To the South, for reduced glass fractions (f=20%), the greenhouse gives rise to an increase in solar heat gain that is more accentuated in summer, and which reaches 35% for Milan and 43% for Cosenza. For greater glass fractions the greenhouse leads to a reduction in solar gain in winter, for f=100% it results as being equal to 49%, and increases of less than 21% during summer. For Eastern exposure the effects of the greenhouse appear to be more contained, for f=100% the increase of solar contribution does not exceed 31% for both localities.

References

[1] De Simone M. , Oliveti G. , Ruffolo S. , " EVALUATION OF THE ABSORPTION COEFFICIENT FOR SOLAR RADIATION IN SUNSPACES AND WINDOWED ROOMS". Solar Energy, Vol. 82, pp. 212-219, 2008.

[2] EN ISO 13790. Energy performance of buildings. Calculation of energy use for space heating and cooling, 2008.

[3] UNI EN 832. Thermal performance of buildings. Calculation of energy use for heating. Residential buildings, 2001.

[4] DEROB-LTH v1.0. User’s Manual. Division of Energy and Building Design, Lund Institute of Technology, Sweden, 2004.

[5] Arumi-Noe, F. The DEROB System, Volume II, Explanatory Notes and Theory. Numerical Simulation Laboratoty, School of Architecture, University of Texas Austin, 1979.

[6] UNI 10349. Heating and cooling of buildings. Climatic data, 1994.

[7] UNI EN ISO 6946. Building components and building elements. Thermal resistance and thermal transmittance, 1999.

[8] EN ISO 13789.Thermal performance of buildings. Transmission and ventilation heat transfer coefficients. Calculation method, 2007.

[9] De Simone M. , Oliveti G. , Arcuri N. , Bruno R. , " RADIAZIONE SOLARE NEGLI AMBIENTI ADIACENTI A SPAZI SOLEGGIATI. UNA VERIFICA DELLA PROCEDURA DI CALCOLO PREVISTA DALLA NORMATIVA". Book proceedings of the "46° International Conference AICARR", Milan, Vol. I, pp. 1009-1023, 12-13 March, 2008.