C. A. Cruickshank1* and S. J. Harrison1

1 Solar Calorimetry Laboratory, Department of Mechanical and Materials Engineering, Queen’s University,

Kingston, ON, K7L 3N6, CANADA

Corresponding Author, cruickshank@mc. quccnsu. ca

Abstract

The thermal response of a series-connected multi-tank thermal storage was studied under variable charge conditions. Tests were conducted on an experimental apparatus consisting of three 270 L storage tanks, connected in series and each charged through a coupled, side-arm, natural convection heat exchanger. Both energy storage rates and temperature profiles were experimentally measured during charge periods consisting of two consecutive clear days or combinations of a clear and overcast day. Of particular interest was the effect of rising and falling charge loop temperatures and collector-loop flow rate on storage tank stratification levels. Results of this study show that the series connected thermal storage reaches high levels of temperature stratification in the storage tanks during periods of rising charge temperatures and limits destratification during periods of falling charge temperature. This feature is a consequence of the series connected configuration that allows sequential stratification to occur in the component tanks and energy distribution according to temperature level. A further aspect of the study investigated the effect of increasing charge — loop flow rate on the temperature distribution within the storage system during a simulated charge sequence. Temperature distributions measured during high flowrate tests were found to be similar to those obtained during parallel charging of the storage system. This final result is consistent with expected theory. A disadvantage for both the high-flow series — connected and parallel connected multi-tank storage is that falling charge-loop temperatures that normally occur in the afternoon tend to mix and destratify the storage tanks.

Keywords: Solar energy, thermal storage, stratification, multi-tank

1. Introduction

This paper builds on previous studies undertaken to investigate the operation of medium capacity, multi-tank thermal storages connected in both series and parallel configurations. Multi-tank storage schemes have been previously investigated [1-5]. A multi-tank thermal storage with immersed-coil heat exchangers was originally proposed by Mather et al., [1] and showed to sequentially thermally stratify during charging. More recently, a series-connected thermal storage, Fig. 1, in which the individual storage tanks are charged through natural convection heat exchangers (NCHE’s), has been studied [2-5]. In this arrangement, each NCHE can charge its associated storage tank in a fashion similar to a traditional thermosyphon [2]. As well, the charging of the individual storage tanks is self-regulating (i. e., each acts as a thermal diode) and self­distributing (i. e., the energy from lower temperature charge periods is directed to lower temperature downstream tanks). This sequential stratification maintains higher temperatures in the thermal storage by avoiding destratification during cooler charge periods or the removal of energy from the upstream tanks to the downstream storage tanks (i. e., “thermal carry-over”). Under ideal

operation, maximum thermal stratification is obtained in the storage system. In real systems, however, non-ideal conditions exist including: a finite number of storage tanks; limited heat transfer rates in the heat exchangers; and non-adiabatic storage tank walls and piping, etc. In addition, reverse thermosyphoning due to adverse temperature gradients in the heat exchanger or thermosyphon loop may result in destratification. Thermal carry-over may occur during periods of falling charge temperatures, i. e., in the afternoon or during periods of low solar input.

To investigate these effects, a multi-tank thermal storage, Fig.2, was assembled in a laboratory environment to allow tests to be conducted under controlled charge conditions representative of combinations of clear and cloudy days. Data obtained included temperature profiles within the storage tanks (recorded as a function of charge duration) and energy transfer rates in the individual heat exchangers. The description of the system, the test apparatus and results are presented below.

Two peaks appeared on DSC solidification curve of a Composite 3 (Fig.1), as a simple signal of inhomogeneous composite or chemical products generated during crystallization process. The re­crystallization of the Composite 3 was made with DSC at a cooling/heating rate of 0.50C/min and temperature range from 150-2200C. Comparable FTIR spectral study was performed on Composite 3 for examination of the possible chemical degradation or chemical interaction (Fig.2).

Relative intensities of the absorption peaks were almost the same for the two samples of salt/eutectics graphite composites Fig.2, (1), (2), but lower as compared to the single NaNO3/KNO3 eutectics (3). Evaluations of the spectra provided with an evidence of a maximum characteristic absorption band at 1379 cm-1, displayed for vibrations of the nitro functional group of NaNO3 and KNO3 compounds. Products of chemical degradation or chemical reaction were not identified from anion frequencies recorded for all samples.

M. Bakker1*, W. G.J. van Helden1, and A. Hauer2

1 ECN, Energy Research Centre of the Netherlands, P. O. Box 1, 1755 ZG Petten, The Netherlands
2 ZAE Bayern, Walther-Meissner-Strasse 6, D-85748 Garching, Germany
Corresponding Author, m. bakker@ecn. nl

Abstract

A new IEA SHC/ECES Joint Task will be started on January 1, 2009. The objective of this new task, designated SHC Task 42 and ECES Annex 24, is to develop advanced materials for compact thermal energy storage, which can be used for renewable heating and cooling as well as for energy conservation. The task focuses on thermochemical materials, including sorption and composite materials, as well as on phase change materials, and includes work on material engineering and development, characterisation, numerical modelling, demonstration, and dissemination.

Keywords: thermal storage materials, renewable heating and cooling.

Background

Thermal energy storage is an important technology for renewable energy systems. By improving the effectiveness of thermal storage, the effectiveness of all renewable energy technologies that supply heat can be improved.

Particularly for solar thermal systems, thermal energy storage is essential. To reach high solar fractions, it is necessary to store heat (or cold) efficiently for longer periods of time. Until now, no cost-effective compact storage technologies are available to do this. For high solar fraction systems, hot water stores are expensive and require very large volumes of space. Alternative storage technologies, such as phase change materials (PCMs) and thermochemical materials (TCMs) are available on a laboratory scale. However, more research and development is needed before these technologies can be developed into commercial solutions.

In several IEA annexes, both ongoing and completed, it was concluded that materials are the main bottleneck for finding effective solutions for compact thermal energy storage, and that there is a need for new storage materials with a higher specific energy storage density and lower material cost.

Around the world, several groups are working on either thermal energy storage materials or applications. However, these activities are not sufficiently linked. The current activities are either limited to specific applications, or to specific materials. What is needed, and what can be provided by this new task, is a way to bring the ongoing work on materials and applications together.

In the literature, several intermediates have been identified during the dehydration of MgSO4.7H2O. However, the published results are at some points conflicting on which intermediates are formed [2-6]. The dominant natural occurring magnesium sulfates on earth are Epsomite (MgSO4.7H2O), Hexahydrate (MgSO4.6H2O) and Kieserite (MgSO4.H2O). Since it is not clear which other intermediates are formed, it was decided to measure the dehydration of MgSO4.7H2O anew. The dehydration of MgSO4.7H2O was studied by means of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA involves the measurement of the mass change as function of time and DSC involves the measurement of heat as function of time, in which both are subjected to a predefined temperature program. The experimental results indicated that dehydration of MgSO4.7H2O proceeds in three discrete steps as illustrated in Figure 1:

The above Figure shows that the first dehydration reaction occurs at low temperature (<50°C) and involves the loss of one water molecule. The largest number of water molecules is dehydrated during the second reaction, which also allows storing the largest amount of energy of the three dehydration reactions: almost 10 times more energy compared to water (~2.3 GJ/m3 compared to 0.25 GJ/m3 for water in the temperature range 25-85°C). Additionally, the reaction occurs between approximately 60°C and 200°C: a temperature range which can be almost completely covered using a vacuum tube solar collector (Tmax=150°C). For these two reasons the second dehydration reaction is most interesting

for seasonal solar heat storage. Figure 1 also shows that the last water molecules are dehydrated at a high temperature (~275°C). In contrast to the first two dehydration reactions, the third dehydration reaction involves an exothermic process (indicated by the positive DSC signal). Ruiz et al [3] suggests that the final transition to MgSO4 includes an exothermic reaction due to recrystallization of an amorphous precursor. This suggestion was further investigated by performing X-ray diffraction (XRD) experiments as shown in Figure 2:

When the temperature of the sample is increased from 25°C to 55°C, the XRD measurement shows that MgSO4.7H2O is converted to MgSO4.6H2O. This result is in agreement with the results obtained by the TGA-DSC experiments (see also Figure 1). No peaks are observed in Figure 2 between 80°C and 276°C, indicating that an amorphous state is formed when MgSO4.6H2O is dehydrated further. This observation is in agreement with the findings of Ruiz et al [3].

The XRD measurements shows that the material only recrystallizes at T>276°C. It should be noted that recrystallization only occurs when a small amount of water is present (0.2 water molecules per MgSO4 molecule, see Figure 1) after which an exothermic reaction is observed. This could be explained by the hypothesis that below the critical water content of 0.2 water molecules per MgSO4 molecule, a spontaneous exothermic crystallization occurs that expels the last water that is present in the material. The XRD measurements confirm the observations from TGA-DSC experiments that the material is completely dehydrated at 300°C.

L. H. Godinho1*, M. P.F. Gra^a 1,2

1) Prirev, Lda, Zona Industrial de Vagos, Lt.61, 3840-385 Vagos — Portugal
2) Physics Department, I3N — Institute of Nanostructures, Nanomodelling and Nanofabrication,
Aveiro University, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal
* Corresponding Author, lhalgodinho@gmail. com

Abstract

The main purpose of a greenhouse is to create and maintain a controlled artificial environment that will favour the crop production with the maximum profit. Late increase on fuel prices, together with colder than normal seasons, make heating costs a significant burden on greenhouse operations. Therefore, the use of renewed energy systems, namely solar thermal systems, to control the inner environment of agricultural greenhouses becomes an economical and technological topic of unquestionable interest. This publication is related with the analytical analysis of the possibility of using a solar thermal system to control the climate environment of a greenhouse. Preliminary calculations show that, in certain climate conditions, a solar greenhouse can collect sufficient solar energy to feed, at least, another standard thermally optimised greenhouse of the same size. The implementation of this project, which is based on Portuguese Utility Model n°10218 — “Thermal Solar System for Collection, Storage and Distribution of Heat at Low Temperature”, considers the construction of a prototype and, later, of a larger industrial greenhouse, to verify the technological and economical viability of the patented idea.

Keywords: greenhouse, energy, solar, agriculture,

1. Introduction

Since the beginning of the XX century the world has been suffering from rising exploitation of its natural resources, with the resulting consequences in pollution and degradation. Oil, for example, considered a traditional energy source, have been extracted in such huge amounts that oil-wells already started to be depleted less than 100 years after the beginning of its effective use.

The “Lisbon strategy”, set in March 2000 by the European leaders, assumed the commitment of the UE to become, up to 2010, an economy based on the most dynamic and competitive knowledge of the world, capable to guarantee a sustainable economic growth, with more and better jobs, bigger social cohesion and respect for the environment. One of the main goals will be the ambient sustainability, that is, to develop and to spread out the echo-innovations and to build the leadership in the echo-industry; to adopt policies to generate long term improvements supported in increased productivity through echo-efficiency. Economic growth will be supported by the echo-innovations leading to a decrease in pollution and to a more efficient management of the resources. Many examples of these echo-efficient innovations already exist in several areas, agriculture and energy included.

The conversion of traditional agriculture, which is strongly climate dependent and characterized by extremely hard work conditions, in a more technical agriculture in controlled environment, will have an enormous impact in the reduction of the risk and in the increase of productivity and quality of the cultures, providing more appealing work conditions.

Basically, greenhouses are solar collectors with poor heat storage capacity. From local weather conditions in the last years we know that in late autumn, winter and beginning of spring, night temperatures can be quite low for long periods. Temperature represents a critical factor to the survival and growth of the plants. In fact, keeping the greenhouse at the most suitable temperature for the development of a given plant will significantly improve its productivity and quality.

Also in summer we can have in greenhouses a phenomena called “inversion” whenever the inside temperature drops dramatically at dawn when the sun rises, due to the evaporation of water condensed during the night.

Traditional Portuguese greenhouse consists of a galvanized iron pipe structure, covered with single layer of polyethylene (PE) film. This type of greenhouse implies enormous heat losses with consequent high fuel consumption. As a result, an increasing number of farmers have just switched off the heating and given up some greenhouse cultures in the last years due to the rise in energy costs.

The main objectives of this project are the thermal efficiency analysis of the greenhouse (type of covering, heating system, thermal curtains, etc.) and how much of its heating by traditional energy sources can be replaced by solar energy. The economic viability of its construction, the use of low cost (and yet efficient) polymeric solar thermal collectors together with simple water heat storage tanks — made from expanded polystyrene (EPS) blocks with an inner bag of reinforced polyvinyl chloride (PVC) film — and the way to transport and release the thermal energy inside the greenhouse, will also be studied.

During the cold night periods, water from the storage tanks will flow through small polymeric tubes inside the greenhouse and will heat the ground (if buried), or the bottom of the vases (if covering the growing tables).

At the energy level, the phases of this project are:

• pre-study of thermal properties in the design phase of the project;

• design of energy monitoring systems;

• data analysis and presentation of results.

Intended results are:

• substantially improved production conditions (quantity and quality);

• increased effectiveness of the heating using thermal efficient greenhouses;

• lower fuel consumption and, consequently, less air pollution;

• promotion of specialized agriculture, more competitive and with better work conditions.

In this paper we describe the analytical analysis that supports this solar thermal system project.

The ANZEH is a two storey, wood-frame, detached building, with 210 m2 of inhabitable area. The insulation values selected for the building envelope are higher than conventional Canadian homes: walls with 5.6 RSI (U = 0.18 W/m2K), ceiling with 12 RSI (U = 0.08 W/m2K) and floor insulation of 4.6 RSI (U = 0.22 W/m2K). This house relies heavily on passive solar design to satisfy its

[7] The hygroscopy increases in the order: dry, slightly hygroscope, hygroscope, strongly hygroscope.

[8] O = Oxidizing agent, Xn = Harmful, N = Dangerous for the environment, T = Toxic

[9] No data available, dry to hygroscope

[10] Introduction

The main findings from the simulation are as follows:

The heat exchanger effectiveness increases with

• Smaller sand grain size

• Air inflow velocity

• Heat exchanger dimensions (height and width)

With the air and sand inflow temperatures of 800 °C and 200 °C, the obtainable average sand and air output temperatures are in the range TSand= 720-740 °C, TAir= 250-270 °C and heat exchanger effectiveness in the range sHX = 85-90 %.

Further results are

• For sand grain sizes < 3 mm and heat exchanger dimension of 50 cm height and 5 cm width (as given in the heat exchanger set-up described below) a smaller grain size has limited effect on the heat transfer

• Dynamic behaviour: short-term sand blockage with blocking period below 20 s are uncritical with respect of air outflow temperatures increase due to the sand bed heat capacity

• To minimize pressure drop and to increase specific heat flux per volume (dimension of 1 MW/m3) a sand grain size dSand =1-3 mm is favourable

In the methods listed above, effectiveness is principally equated in terms of the total exergy content of an experimental TES, with comparison to an ideal and/or mixed system. Although this is useful when assessing the overall performance of a complete simulation, the indices are not particularly suited to highlight not only the effect of thermal mixing within a TES, but more importantly, clearly indicate at which point turbulent mixing effects become negligible, if at all. They also offer a varying degree of applicability for different TES simulations. Panthalookaran’s method, for instance, does not seem to apply easily to variable temperature inflow conditions. [7]

In support of the existing indices, a new characterization index has been derived and is proposed in eq. (4). The index, deemed the Exergy Charge Response (nCR), examines the rate at which the exergy stored within a real TES increases, and compares it to that of an ideal tank. The index is derived principally from eq. (3) above, which describes the exergy stored within a TES at a given time.

The applicability of the Exergy Charge Response is limited by the understanding that, given sufficient energy input with high inlet temperatures, a fully-stratified and a fully-mixed tank will eventually reach similar exergy levels:

да да со

J£stored, strat(t) £stored, real (t) stored, mix (t) if! iS Estored (t) > 0 (6)

0 0 0

It is therefore possible that at some point towards the end of a simulation with high energy availability, n^CR must become greater than 100%. This will only occur, however, when the energy response efficiency of the tank, tfER, begins to decrease from 100%. Therefore, so long as (nER > 0), hcR will be affected primarily by internal mixing, particularly at the early stages of charging.

The particular advantage of the Exergy Charge Response applied to TES simulations is the ability to effectively monitor the transient relationship between the thermocline and the fluid motion within the TES itself. It is clear that when п^ск ~ 1, the inlet plume is not thermally mixing the TES, and oppositely so when ncR ~ 1.

Measurements have shown a maximum storage capacity of approximately 30 kWh for operation with external melting. Otherwise, a maximum storage capacity of 45 kWh is possible. In this case no liquid phase is left in the ice store. At a charging temperature of -10°C, the ice storage can be loaded with System A at a rate of 4 kW, on average (Figure 4). System B provides an average charging rate of 2.5 kW. Fluctuations at the beginning of the charging are caused by adjustment of the thermostat. Figure 5 compares the charging times of both systems at -12 and -10°C.

Figure 5 compares the charging times of System A and B at two different charging temperatures.

As expected from the different charging rate System A charges the ice store faster than System B. Charging the ice store at -10°C with System A takes 421 minutes and with System B 657 minutes. Table 1 offers an overview about all results for charging.

The innovation with the High-Combi system, examined in this paper, is the achievement of high solar fractions in combination with relatively small SST. The original design of the system under investigation includes the solar heating and cooling plant and the SST coupled with ground heat exchangers (GHE) to minimize heat losses from the storage tank. The main system components of the installation are the solar thermal collectors (180 m[26] [27], selective flat plate), the heating/cooling elements (a combination of floor heating and fan coils2), the absorption cooling machine (35 kWc), the SST (200 m3) and the GHE. The simulated total building area is equal to 700m2 having space heating and cooling energy demands of about 40 and 45 kWh/m2 respectively.

A schematic of the High-Combi plant’s operation is illustrated in Figure 1. This configuration corresponds to one of the five pilot plants (the one to be constructed in Greece) in the framework of the High-Combi project.

The cooling machine is driven by the solar thermal energy (Qh or Qs) at a high temperature (i. e., 70- 90°C) from the SST3, and provides useful cooling by extracting heat (Qc) from the building. To keep the absorption process going, the latent heat from the absorber and the heat from the condensation of the desorbed refrigerant in the condenser, is rejected to the ground heat exchangers, positioned in the earth, around and concentric to the hot water storage tank. In this way, the earth surrounding the tank

will be heated, thus reducing the tank’s thermal losses. Currently, the ground heat exchangers (GHE) are not included in the simulated plant’s configuration and the heat rejection is accomplished through a cooling tower. The use of GHE will be investigated at a follow-up stage.

The plant’s operation can be described as follows: during summer, whenever an excess of solar energy is available, this excess heat is delivered directly into the water storage tank. The water in the tank may be heated up to almost 100°C in the summer. The combination of a good conventional thermal insulation and an additional low cost thermal insulation (composed of low cost materials, for example, empty plastic bottles and chipped tires[28]) will reduce the heat losses from the SST, extending its use into the winter months and potentially cover a substantial amount of the heating loads (reaching even 100%). During winter, part of the heating load will be covered directly by the solar heat gains.

At a first glance, it seems contradictory to make a small SST, compared to the conventional ones that are of the order of at least some thousand cubic meters. Thermal losses from the heat storage are proportional to its envelope surface to volume ratio; this ratio increases as the volume becomes smaller. Thus, a smaller SST is expected to have higher thermal losses that need to be accounted for. On the other hand, the construction of a “small” sized SST is attractive since it will permit solar heating and cooling applications at a different scale.

Simulations made in this paper, show that there are still advantages against what might initially be regarded as unfavourable scale application. The use of a relatively low-temperature heat distribution system will allow using the stored heat at a wider range of supply temperatures. This will result to a higher amount of usable energy accumulated in the SST for the same storage volume, and to lower overall heat losses as the mean storage temperatures are reduced.

An overview of the simulation methods and results is presented in the following section.