Category Archives: EuroSun2008-7

Heat and mass transfer characterisation for thermochemical heat transformers

G. Rambaud*, N. Mazet, S. Mauran

1 CNRS-PROMES (Processes, materials and solar energy laboratory — UPR 8521) Rambla de la
thermodynamique Tecnosud 66100 Perpignan — France.

Corresponding Author, guillaume. rambaud@univ-perp. fr

Abstract

For safety and environmental reasons, thermochemical processes involving solid gas reactions with water as reactive gas are very promising, but the working pressure is rather low (103 — 104 Pa). Investigations have to focus on mass transfer coefficients (permeability, k, and the Klinkenberg’s coefficient, b) and heat transfer coefficients (effective thermal conductivity, X, and the heat transfer coefficient at the exchanger wall, hsw). The whole set of transfer coefficients will be identified on the same sample in a single characterisation apparatus. First, we analyse the transfer coefficients according to the characteristics of the composite. The mass transfer coefficients was analysed in transient state, by comparing the experimental pressure evolution versus time and simulations using Comsol®. In steady state, the heat transfer between the upper side of the composite and the heat exchanger is simulated. The experimentation shows that the reactive bed pressure is stabilized in less than 10 seconds, and depends on the transfer parameters k and b, on the bed parameters (p and wl), and on operating parameters as the initial pressure drop and the sample temperature. The ranges of heat and mass transfer parameters are consistent with previous experiments on similar porous samples: 10-11 to 10-16 m2 for the permeability, nearly 500 Pa for the Klinkenberg’s coefficient and about 1 W. m-1.K-1 for the effective thermal conductivity.

Keywords: thermochemical heat transformer, mass transfer, heat transfer, porous media

1. Introduction

Thermochemical heat transformers can be used for solar air-conditioning for individual dwelling. For safety and environmental reasons, solid gas reactions involving water are very interesting. Numerous solid reactants have been tested by the laboratory and only few of them have the thermochemical properties required. The selected solid reactant is strontium dibromide.

The salt reacts with 5 moles of water during synthesis phase and releases these 5 moles during the decomposition phase. The working pressure is between 1000 and 10000 Pa and that leads to a strong mass transfer limitation. The solution would be to increase the porous volume around the salt. That’s why the reactive salt is mixed with expanded natural graphite (ENG) which is a very porous material and a good heat conductor. The thermal conductivity increases with apparent density of ENG but on the other hand the permeability decreases. The accurate determination of these two parameters is very important for chemical reaction with steam at low pressure.

2. Experimentation

A single apparatus was developed to identify both mass and heat transfer coefficients on the same composite block. To reduce the handling of the composite, the compression and the reactions with steam were realised in a single apparatus as well as the characterisations.

Description of the Multi Solar (MSS) Technology

image276

The Multi Solar system is an innovative (PATENT NO 5522944) Solar PV/Thermal System that makes it possible to convert solar energy into Electrical energy (PV) and Thermal energy at the same time using a single integrated flat plate collector system. The MSS collects the sun’s irradiation full spectrum and uses air and water pipes to cool the PV cells in order to increase the relative efficiency of the electric system and at the same time produces hot water and hot air which can be channelled for further thermal use. The cooled PV cells can provide up to 30% higher annual electrical production than the usual PV system. This is accomplished by preventing the efficiency degradation of regular PV caused by excessive heat (negative heat coefficient of half percent per degree of heat in any normal photovoltaic panel). The thermal efficiency of the MSS collector system reaches up to 70% thermal energy (35% hot water and 35% hot air). With the additional 15% efficiency of the PV electrical production the MSS reaches to 85% efficiency: The Most Efficient Solar Collector in the World.

Electrical Properties Millennium mss air conditioning components

Max

Power

[W]

Percentage

Operating

Hours

Average

Energy

Consumption

[kWh/year]

Water Pumps

80

20%

140

LiCl Pumps

80

100%

701

Control System

10

100%

88

Total

170 W

931 kWh/ year

90 to 170 W is needed for the machine during normal operation and the average power is 106 W which comes from the MSS PV Cells. Full load amps are 6 A only.

Current status and conclusions

As of September 2008 the cold box, reject heat exchangers and ice bank are complete and the other components are all under construction. Laboratory testing with a dummy (electrical) heat source is scheduled for October and shipping for solar tests in November.

References

[1] R. E. Critoph, Z. Tamainot-telto, S. J. Metcalf, "Advanced regenerative adsorption air conditioning unit", International Heat Powered Cycles Conf. 2006 (Ian W. Eames, eds), Newcastle, UK, Paper 06123,

(Sep 2006)

[2] R. E. Critoph, "Heat Exchanger", P113950WO, (Sep 2007)

Comparison Of Control Strategies Of Solar Absorption Chillers

Annett Kuhn*, Jose Luis Corrales Ciganda, Felix Ziegler

Technische Universitat Berlin, KT2, Marchstrasse 18, 10587 Berlin, Germany
* Corresponding Author, annett. kuehn@tu-berlin. de

Abstract

The choice of the control strategy of a solar cooling system is essential for the system performance. Control strategies should not only have the focus on increase of cooling capacity but also on reduction of electric energy consumption of the overall system. Normally, electric power consumption of the cooling tower fan is the major fraction of power requirement in such a system. Therefore, a new control strategy which uses the cooling water temperature as a control parameter has been developed. This strategy is based on the method of the characteristic equation which describes a linear relationship between driving, cooling and chilled water temperature and cooling capacity. Chilled water temperature can be kept constant even at such unsteady driving temperatures as in case of solar cooling. Simulations of a cloudy day in Berlin showed that full cooling capacity can be provided nearly an hour earlier compared to the conventional control strategy. This offers the possibility to save a short-time storage tank. If irradiation is high as in the case of a sunny day in Iran up to 50% electricity consumption of the cooling tower fan can be saved applying the new control strategy.

Keywords: solar cooling, absorption chiller, control strategy, energy saving

1. Introduction

Cold water distribution systems often require a constant chilled water temperature or chilled water temperatures within a certain range. Chilled water entering chilled ceilings must not be to cold to prevent condensation of water on the panels. But chilled water entering fan-coils has to be cold enough to ensure air dehumidification. So, the chilled water temperature must be controlled. The control of a solar cooling system must be carefully chosen and designed in order to maximise efficiency and minimise electric energy consumption. This is especially important as experiences with solar cooling applications revealed several mistakes in control and hydraulic scheme design.

There are different strategies to control the chilled water temperature of an absorption chiller. The most common strategy is to control the hot water inlet temperature using a three-way valve to mix the return flow from generator with the supply flow from the hot water source [1]. The result is a decrease of the hot water inlet temperature and a decrease in cooling capacity. Consequently, the chilled water temperature rises. Nevertheless, this widespread control strategy is not the optimal solution, as it can only be used to prevent the chilled water temperature from too low values. An increase of the cooling capacity at low irradiation is not possible. Another disadvantage is the high power and water consumption of the cooling tower as the cooling water temperature is kept constantly low.

A newly developed control strategy uses the cooling water temperature as a control parameter.

This strategy shows special advantages for solar cooling. If the hot water temperature is not high enough for a given cooling load, the cooling water temperature can be lowered in order to reach the desired chilled water temperature. On the other hand, if solar radiation is high, cooling water temperature can be increased to save power and water consumption of the cooling tower.

The exergy-topological method

The exergy-topological method also called exergy graph method is a new approach in exergy studies based on the arrangement or mapping of elements (links and nodes) of a network. In this method, the components: turbines, pumps, heat exchangers, etc linked by the pipes, valves and other devices are the major elements. Different steps involved in the application of the topological methodology are summarized below:

■ Step 1: drawing of the flow sheet of the system,

■ Step 2: establishment of the exergy flow graph,

■ Step 3: determination of the matrix of incidence,

■ Step 4: determination of the flow parameters,

■ Step 5: determination of the thermodynamic characteristics and

■ Step 6: analysis of the results.

The useful parameters for the analysis of the system are as follows:

V = E°ut / E” (Degree of thermodynamic perfection of i th-element) (1)

l = Ei” — E°ut (Exergy loss of i th-element) (2)

rfx = Ei /Ea (Exergy efficiency of i th-element) (3)

в = Ef / El (Coefficient of influence of i th-element) (4)

Vj, = ElU / El” (Degree of thermodynamic perfection of the system) (5)

n

= ^ili (Total exergy loss of the system) (6)

i=1

Пі = Ej / Ej (Overall exergy efficiency of the system) (7)

Where, E“ and E°ut are the sum of exergy flows at the inlet and outlet of the element i; Eut and Ef are the sum of useful and available exergy of i th-element and the subscript E refers to the system. The exergy rate of a j-flow is given by

Ej = mej (8)

ej is the specific exergy flow and m represents the mass flow rate. Specific exergy can be calculated by the following equation for any flow [8]:

ej = (hj — h0) -T0(Sj — s0) + ef (9)

Where hj and sj are specific enthalpy and specific entropy at the point under consideration. T0, h0 and s0 are the temperature, specific enthalpy and entropy at the restricted dead conditions. ef is the chemical exergy.

The specific exergy transfers by heat and by work are respectively given by Eqs. 10 and 11.

ej = q} (1 — TJ Tj) (10)

ej = Wj (11)

Where qj is the specific amount of heat, Tj is the average thermodynamic temperature of the

working body at which heat is added or removed, w3 is the specific work.

The methodology of calculation of useful and available exergy rates is described in Ref.[8]. A description and exemplification of this approach can be found in Mago et al. [4] and Nikulshin et al. [5-7].

Monitoring of both systems

Both subprojects are monitored following a common monitoring scheme within the Polysmart project (with common measuring and estimating procedures) for later comparison of results and evaluation of the best combination between components, fuels chosen and hydraulic design.

Two different type data acquisition loggers will be used in each subproject to register the values registered by the several sensors.

The main performance figures to be evaluated in order to assess the energy balance are: thermal energy consumed by the CHP, electric energy consumed by each part of the CHCP system, net electric energy production of CHP, Cold and Heat distributed to each end use, running time and number of starts of each part. CO2 as well as NOx emissions, of the CHP will also be evaluated.

2. Conclusions

The importance of the work done in the framework of this project relies on the possibility of exploiting the knowledge and experience of the most important agents on the markets that are relevant to the future dissemination of the polygeneration systems, through the erection, operation and evaluation of a set of small demonstration energy systems in different European countries, with different combinations of the main equipment. This permits to test under real conditions new technologies and products that are (some of them) close to the market, and to compare them, improving the system design as well as system control and operation experience.

The final performance figures that will be achieved will be of crucial importance to demonstrate the premises behind this type of energy system conception. The software tools that will be also developed, combining a simulation of the different technologies involved, will improve the knowledge and economic studies needed for implementation in large scale of polygeneration systems.

The successful demonstration of these trigeneration projects will be an opportunity to show a possible direction for implementation of energy systems in buildings intending to attain the category of a net zero balancing energy building. The legislation and political interest already existing in Portugal, push towards that objective and we hope that main agents in the market will find in the results of these subprojects, the reasons and arguments in favour of it.

References

[1] “POLYSMART — Polygeneration with advanced small and medium scale thermally driven air­conditioning and refrigeration technology”. Integrated Project, FP6-2004-TREN-3. Contract No. 019988.

[2] Miguel Morgado, “Projecto Regional de Produqao Distribuida no Baixo Alentejo”. Energia Solar, Revista de Energias Renovaveis & Ambiente da SPES, No. 53, 2003.

[3] Estatiscas da Consttuqao e Habitaqao 2007, Statistics Portugal, www. ine. pt

[4] Estatiscas do Turismo 2007, Statistics Portugal, www. ine. pt

[5] SENERTEC Dachs HKA F 5.5 technical data, www. senertec. de

[6] Costa, A., Collares-Pereira, M., Adao, P., “Pre-commercial development of a cost effective solar-driven absorption chiller”, EUROSUN 2008 , Lisboa, Portugal

[7] TRNSYS — a Transient System Simulation program. Solar Energy Laboratory, University of Wisconsin-Madison.

Operation concept of the solar cooling plant

The cold water temperature of the cold water supply network is driven as function of the ambient air temperature as shown with the diagram in Figure 2.

Подпись:
cold water
temperature [°C]

The cold water temperature can be differing from the target temperature in the range of ±1.5 K, which is depicted in the diagram by the red lines (upper and lower lines). If the temperature of the cold water return flow (means the cold water back flow from the cold consumer) drops under the target cold water temperature more than 1.5 K, the chiller is switched off. This leads to a timing operating of the absorption chiller especially in part load.

Furthermore, the operation mode of free cooling is realised. If the wet bulk temperature drops under the cold water temperature with more than 8 K, the operating mode of the solar cooling plant changes into free cooling. This means, that the cold capacity of the plant is not generated by the absorption chiller anymore, but directly by the cooling tower via an additional heat exchanger connecting the cold water cycle with the re-cooling water cycle. Because this operating mode often occurs in spring and autumn, the heat generated by the solar collector field at this moment can be provided as surplus heat to the heating system of the institute via the heat exchanger WT 2.

In the case of low solar irradiation but cold demand, the absorption chiller can also be driven by heat from the heating system of the institute. The heating system of the institute consists of a gas fired boilers and a micro-gas turbine. Especially in summer time, when there is no space heating demand, exclusively the micro-gas turbine is used to provide the necessary heat for the absorption chiller. At this moment, the system is operating as combined heat, cold and power generation plant.

2. Operational experiences

Solar Refrigeration. System Experimental Design

A thermal/solar panel unit was designed and connected to a refrigerator unit, as shown in Figure 2. In this project the electric source for a bubble pump was replaced by the solar thermal system. The system consisted of two parts; the cooling unit and a deep freezer that used an ammonia refrigerant for cooling. The other part is the solar thermal system. The main function of the system was to heat the water by utilizing solar energy.

image091

Figure 2: A thermal-solar panel system used in the present work

The design of the purchased solar unit was altered in order to increase the heat absorbed by the system. The straight panel pipes (Figure 3), that carry the water used to collect heat from the sun, were replaced with a more efficient design. A copper piping system, in the form of a spiral (Figure 3), was designed to transport water around the system. The original piping system had less surface area and thus less heat absorption capability. The newly designed spiral copper pipe had a length of 30m, and was painted black to give it the black body characteristics. The length of the pipe was increased in order to increase the residence time of water in the pipe, and hence can absorb more heat from the sun. The heater was removed and water hoses were connected to the bubble pump.

image092
image093

Figure 3a: Old piping design Figure 3b: New designed spiral piping system

Подпись: Figure 4: Full solar unit during operation

During operation, hot water runs through the spiral and gets heated by the solar energy (the maximum water temperature obtained by the new design was 80oC). The resulting heat from the water was then used to heat the NH3-H2O mixture. Welding skills acquired from a previous welding course were used to weld and braze the pipe. The copper pipe was then connected to pipes that were connected to the bubble pump and water tank. Thermocouples were installed in several locations, with the most important ones being those measuring the inlet and outlet pipe temperatures. In order to increase the heat absorption efficiency of the system, four mirrors were also installed along each side of the solar panel. The angle of the mirrors can be adjusted such that a maximum amount of heat, at any time of day, can be absorbed. Figure 4 shows a photograph of the thermal solar system during operation.

image095

Figure 5: Combined system consisting of thermal unit and photovoltaic cell

Product testing

The newly designed system was tested during the hot summer months, and it showed that it is possible to absorb heat from the sun, heat the running water and cool the refrigeration system by more than 20oC.

The minimum temperature achieved by the present system design was +5oC. However, since the desired refrigeration temperature is < 0oC degrees, some modifications thought to be needed to possibly achieve the target temperature. Since the system showed a good potential in not only absorbing heat, but also cooling down a deep freezer by about 20oC, the next step of the project was to combine the present solar system with a photovoltaic system. The function of the photovoltaic is to operate the 50 Watt pump and store electrical energy for night use. A photovoltaic unit was purchased. The system has a capacity of 450-Watt and consists of three photovoltaic cells (150 Watt each), AC-DC inverter, and a 12-volt converter. These specifications are expected to operate the system for 4-6 hours at a maximum capacity. Unfortunately, the purchased photovoltaic cells never made through customs, and after a long time waiting, the idea of using photovoltaic cells had to be abandoned. The encouraging results obtained from the refrigeration system enticed us to explore using the system to cooling the space around it.

ENERGY PERFORMANCES OF A RADIANT CEILING SYSTEM SUPPLIED BY SOLAR THERMAL COLLECTORS AND ABSORPTION CHILLER

G. Oliveti, N. Arcuri, M. De Simone, R. Bruno,

Department of Mechanical Engineering — University of Calabria
87036 — P. Bucci 44/ C — Rende (CS) — ITALY
M. De Simone, marilena. desimone@unical. it

Abstract

The energy performance of a radiant ceiling system used for the heating and cooling of an open space environment with a surface area of 800 m2 and an air-conditioned volume equal to 3200 m3 is presented. In winter, the radiant ceiling is supplied by an integrated field of solar collectors with an auxiliary source. During summer, the solar collectors are used to supply the generator of a closed cycle simple effect absorption machine which produces, along with a traditional auxiliary heat pump, the refrigerated flow rate for radiant ceilings. The control system regulates the temperature and the supplied flow rate of the radiant ceilings with different logics during winter and summer periods, intervening directly on the solar circuit. The entire building-plant system was simulated with the TRNSYS code using climatic data for the location of Cosenza (Lat. 39.18N) in order to identify the energy performance varying the collectors surface and storage volume.

Keywords: Thermal solar collectors plant, Absorption Chiller, Control strategy

1. Introduction

The energy performance of a plant which permits the heating and cooling of a building which uses a solar radiation capturing field, a storage tank, a cooling tower and a simple effect absorption machine using a water-lithium bromide mix was evaluated. From the storage tank a flow rate of hot water feeds the radiant ceiling in winter, and the generator of the absorption machine in summer. The existing absorption machine has a nominal power of 35 kW, and requires a cold water flow rate to the condenser and absorber obtained by means of a cooling tower. A three way valve system permits the regulation of the temperature of the flow rate used during winter heating, and of the flow rate used by the generator of the absorption chiller. The plant is equipped with an auxiliary heater which is only active in winter, linked parallelly to a storage tank, which supplies the flow rate at the required temperature when the temperature of the water in the tank is insufficient [1].

For the distribution of thermal energy, a radiant ceiling plant was chosen, characterised by limited thermic inertia and which requires moderate surface temperatures [2].

The building-plant system was studied in winter and in summer with the TRNSYS dynamic simulation code [3], which permitted the modelling of various plant components and the implementation of a specially created control procedure. The building considered is situated in Cosenza, a location in southern Italy with Mediterranean type weather, characterised by not too severe winters and hot, relatively humid summers [4]. In order to simulate the effective climatic variability, a procedure of hourly values of solar irradiation and external air temperature were used, beginning from respective average monthly data [5].

2. Description of the building-plant system

The plant considered represented in Figure 1 is formed by a primary solar circuit and by a secondary circuit which supplies the radiant ceiling. The primary circuit is formed by a field of

Подпись: Primary Circuit Secondary Circuit Fig. 1. Plan of the plant considered for the air-conditioning of the environment

heat pipe solar collectors, by an extraction pump (a) from the storage tank, and by a thermal relieve valve which guarantees safety conditions in the plant. The solar collectors are inclined at 20°, in that with such an incline the solar irradiation recorded during summer is at the maximum and represents the minimum limit in order to guarantee the correct functioning of the heat pipe solar collectors.

The secondary circuit is formed by elements involved both in winter and summer functioning, by elements which function only in heating or cooling applications and by three three way valve systems. During winter, the first three way valve system (1) permits, by means of the activation of a pump (b), the mixing of the flow rate extracted from the tank with the flow rate exiting the radiant ceiling plant, for the regulation of the inlet temperature in the case in which the water temperature within the storage tank is higher than that required by the control system [6]. The flow rate exiting the absorption chiller coming from the circuit with the second three way valve system (2) is only active in summer. In such a period the two valves (1), by means of the activation of a pump (d), mix the flow rate extracted from the storage tank with that exiting the absorption chiller generator in the case in which the tank temperature is higher than that required by the control system of the absorption machine, in such a way as to provide the required refrigerating power [7]. The third three way valve system (3) completes a parallel circuit, which only functions in winter, which activates the auxiliary system in order to provide the entire supplied flow rate at the desired temperature in the case in which the temperature within the tank is lower than that required by the control system.

The hot or cold water flow rate produced in the plant supplies a radiant ceiling by means of a group of plastic parallel pipes, with thermal conductivity equal to 0.35 Wm"1K"1, embedded in the covering plaster of the upper floor having a thickness of 2 cm. The internal diameter of the pipes is 8 mm and their pitch is10 cm; in each pipe a flow rate of 36.9 kg/h circulates which guarantees a velocity of about 0.2 m/s. In such conditions the mass flow rate regime is laminar. The radiant ceiling system is situated in an open-space environment with an air-conditioned volume equal to 3200 m3, parallelepiped in shape with four vertical dispersant walls, and by an upper delimitation flooring that is also dispersant. In Table 1 the structural and thermo-physic properties of the walls are listed. The vertical windowed surfaces cover an area equal to 50% of the total surface area of the opaque wall. They are made of a metallic frame equipped with a thermal cut and double glazed system, with a transmittance of 2.8 Wm’2K_1 and a solar gain gx of 0.75. In order to limit the incidence of loads during the summer, the South, East and West facing windows are equipped with external shading devices which minimize the incident solar radiation by 50%. The activation of the shading devices is illustrated in Table 2. During the day within the building the endogenous loads produced by the presence of people, a number varying between 48 and 96 with a pro-capita generation of 65 W of sensible heat and 55 W of latent heat were taken into consideration. Furthermore, 96 personal computers were taken into consideration which provide a sensible unitary power of 140 W and an artificial illumination system, lit from 08.30 to 18.30 which delivers a sensible heat flux of 5 Wm-2.

Table 1. Physical and structural properties of the simulated building opaque walls

Walls

Surface area [m2]

Exposure

Transmittance

[Wm’2K1]

Solar absorption coefficient as

Vertical1

160

South

0.447

0.3

Vertical2

160

North

0.447

0.3

Vertical3

80

East

0.447

0.3

Vertical4

80

West

0.447

0.3

Ceiling

800

Horizontal

0.443

0.35

Floor

800

0.458

0.3

Table 2. Activation and deactivation times of the solar shading devices for three exposures

Exposure

Activation time

Deactivation time

South

10:30

14:30

East

8:30

12:30

West

14:30

18:30

According to Italian standards [8], for this type of building a crowding index of 0.12 people per surface unit and a pro-capita external air change of 11 ls-1 is foreseen, which gives rise to a total external air renewal load equal to about 1.2 Volh-1. The task of removing latent loads and regulating the specific humidity within the environment was assigned to the external air flow rate.

Third step : decision scheme

This step is aimed through a decision scheme at answering the following question : which solar cooling technology is the most adapted to the project? The decision scheme is based on the method developed in the framework of IEA Task 25 by Fraunhofer ISE [2]. Basically, the use is selecting a path through several technical questions to identify the best technology among solar thermal driven sorption systems (absorption, adsorption, desiccant cooling) with or without a back up system. The adaptation of this method to a user friendly web interface is one of the challenge remaining to do. This step would be made of the following parts :

• general presentation of the scheme : what? how?

• questionnaire to fill in with consequences on the tree creation

• results of the path and final tree

• advice on which technology for a solar cooling project