Category Archives: EUROSUN 2008

Brick cubicles

The structure was done using 4 mortar pillars, also with reinforcing bars, one in each edge of the cubicle. The base consists of a mortar base of 3×3 meters with crushed stones and reinforcing bars. The walls consist of perforated bricks (29x14x7.5 cm, Fig. 2) with an insulating material (depending on the cubicle) on the external side and plaster on the internal side. The external finish was done with hollow bricks and a cement mortar finish. Between the perforated bricks and the hollow bricks there is an air chamber of 5 cm. The roof was done using concrete precast beams and 5 cm of concrete slab. The internal finish is plaster. The insulating material is placed over the concrete, protected with a cement mortar roof with an inclination of 3% and a double asphalt membrane.

Three cubicles using different insulating materials are compared:

1. Reference cubicle: This cubicle has no insulation.

2. Polyurethane cubicle: The insulation material used is 5 cm of spray foam polyurethane.

3. PCM cubicle: The insulation used is again 5 cm of spray foam polyurethane and an additional layer of PCM. CSM panels (Fig. 3) containing RT-27 paraffin are located between the bricks and the polyurethane (in the southern and western walls and the roof).

The most important properties of the insulation materials and the PCM are shown in Table 1 and Table 2.

Fig. 2 Hollow brick

Fig. 3 CSM panel containing the PCM.

Fig

4 Demonstration cubicle built with brick.

Fig. 6 Demonstration cubicle built with brick, RT-27 and polyurethane.

Table 1 Physical properties of polyurethane.

Polyurethane

Thermal conductivity (W/mK)

0.o28

Density (kg/m3)

35

Maximum temperature (°С)

80

Table 2 Physical properties of RT-27.

Units

RT-27

Melting point

°C

28

Congealing point

°C

26

Heat Storage Capacity From 19 °C to 34 °C

kJ/kg

179

Density solid (at 15 °C)

kg/L

0.87

Density liquid (at 70 °C)

kg/L

0.75

Heat capacity

Solid

kJ/kgK

1.8

Liquid

2.4

Heat conductivity

W/mK

0.2

Fig. 5 Demonstration cubicle built with brick and polyurethane.

From Fig. 4 to Fig. 6 it is shown the demonstration cubicles built with brick, polyurethane, mineral wool, polystyrene and with brick, RT-27 PCM and polyurethane.

Modelling of the House and its Systems

A thermal network based on a control volume finite difference discretisation of the house was used. The model used was implemented in MATLAB [4]. Two zones were used: a basement zone and the rest of the building. MATLAB subroutines were written to model the performance of the BIPV/T system, the heat exchanger/heat pumps group, the ground source/HPs interaction, and the TES reservoir.

The BIPV/T model [1, 3] was based on dividing the channel lengthwise in a given number of sections, assuming uniform temperatures at the top and bottom surfaces. An exponential profile is assumed within each section; temperatures and heat transfer rates are found iteratively using a 1-D thermal network between the air and the surfaces. The exit temperature of each section is used as the entrance temperature of the next section.

A simple model based on the variation of effectiveness of the heat exchanger with varying flow rates and manufacturer’s data was used to model the performance of the HX and one or both HPs

[2] . This model was modified to use an adjusted temperature of the ground as an input.

Finally, a 4-node simple model, as described in [5], was used to model the TES reservoir. In this model, the water in the TES is divided into four nodes (a convenient model since there are also four internal divisions in the tank). The water entering the tank (either from the radiant floor heating system or the HPs) is assumed to go to the node having the highest temperature below that of the incoming water. Simple water exchanges are then assumed to take place between the nodes to guarantee mass flow rate balance. According to [5], the use of 3 or 4 nodes is a good compromise between an accurate model and a conservative design (which would assume only one node).

SIMULATION METHOD OF SOLAR POND HEAT EXTRACTION 2.1 Energy balance on solar pond

(1)

pC

dT

dt

_d_

dz

к

дт

dz

+ q — q

loss

Fig.3.The physical structure of the coordinate system of the solar pond model

Подпись: (1)

The system being considered is the one dimensional heat transfer problem through a parallel piped whose base is a unit surface area of the pond, and its height is the depth of the pond. To obtain better results from the model developed, it is more convenient to address each zone in respect to the boundary conditions. For this purpose, the pond is considered to have three zone, as in Fig 3. The UCZ and LCZ are considered as single grid points which have a thickness of Z0 and ZA, respectively. The total depth of the pond is Z. Conservation of energy is then applied on each zone. For NCZ, the energy equation ca be written as follow;

For LCZ, the energy equation is also written as follow;

dT

PCpADL^7 = QR — Qup — Qs — Qg — Ql (2)

dt

By assuming that the pond is well insulted, heat loss around the pond is small comparative with amount of heat extraction. The equation (2), thus, can be written as follow;

AT

Qsolar = Qs + mCp — (3)

Qsolar

(4)

Q thermosyphon

+ mcp

AT

At

Подпись: Qsolar Подпись: (4)

From the fact that extracted heat from the solar pond is equal to the heat extracted by thermosyphon. Substitute (3) into (2), we got;

Estimation of solar pond’s heat absorption is expressed by Rubin et al.(1984), the heat from solar radiation of any location can be often.

Where ф is the rate of the solar radiation at any wave length pass though the pond in any depth z, can be obtained.

(6)

Подпись: (6)ф = ф0 n exp(-M z /c°s<92)

J =1

Substitute ф0and constant value from (6) into (5). Therefore, it follows that

4 Mj — p. z/cos# (7)

Ч(А = ф0 ^ exp J

0J=1 cos#

Hence, the calculation of energy balance at LCZ can be obtained from (4).

The Role Of Thermal Energy Storage In Helping. Solar Thermal Reduce The Heating, Cooling. And DHW Demands Of The Housing Stock. In A Northern European Country

E. Ampatzi[5]*, I. Knight1, M. Rhodes1 and F. Agyenim1

1 Welsh School of Architecture, Cardiff University.

* Corresponding Author, AmpatziE@cf. ac. uk

Abstract

This paper investigates the applicability of solar thermal systems for domestic hot water preparation and thermal comfort satisfaction across the Welsh housing stock. It analyses the role of thermal energy storage technologies (TES) in achieving significant solar fractions with these systems.

Twelve house types, considered as representative of the entire Welsh Housing stock are modelled and the thermal energy demand for space heating and cooling and domestic hot water preparation for each house type is calculated. The share of the total thermal energy requirement that can be met by solar energy, instantaneously and by means of thermal energy storage techniques for each house type is predicted. The results of this work reveal the relative importance of the use of TES in solar thermal applications for the climate of Cardiff and the specific housing types. The analysis will show for each house type the trade off between storage capacity and collector’s area, to achieve solar fractions of 50%. The effect of warmer than average weather conditions is also revealed, with the use of actual measurements of year 2007 for the city of Cardiff.

Keywords: Thermal Energy storage, space heating and cooling, DHW, TRNSYS.

Storage using chemical reactions

The high density of storage with chemical reactions makes the topic attractive. However many difficulties must be overcome to get to a commercial solution. The choice of an adequate reaction has kept attention at ECN, The Netherlands. A promising material is magnesium hydroxide seven hydrates which could theoretically store 777 kWh/m3 at 122 C.

Fig. 13. Sketch of a future chemical heat store system with its three vessels, at ECN, the Netherlands (for a 12.2 MWh capacity, tanks volume could be: A 14 m3, B 5 m3, C 16 m3)/

This is a temperature that high performance solar collectors can achieve in summer time. The principle is to dry the material in summer with solar heat and in wintertime rehydratation can deliver back the energy (figure 13). Work with this material and its abitlity to de-hydrate and re-hydrate has just started.

Test results from a Latent Heat Storage developed for a Solar Heating and Cooling System

Harald Mehling*, Stefan Hiebler, Christian Schweigler, Christian Keil, Martin Helm

Bavarian Center of Applied Energy Research (ZAE Bayem), Walther-MeiBner-Str.6, 85748 Garching,

Germany

* ph:++49 89 329442-22, fax: :++49 89 329442-12, e-mail: Mehling@muc. zae-bayern. de

Abstract

In solar thermal installations, full annual utilisation is desirable. This can be achieved with solar space heating during the cold season and solar cooling by means of sorption cooling in the warm season. When low temperature heating and cooling facilities like floor or wall heating systems or activated ceilings are applied for heating and cooling, a low-temperature heat storage using the latent heat of phase change materials (PCM) can be used to significantly improve the system in the cooling and also in the heating mode. After completion of tests on functional models, a full scale storage with about 2 tonnes of CaCl26H2O as PCM and capillary tubes as heat exchanger was built in fall 2006. The storage consists of two modules with a total volume of 1.5 m3 and has a design storage capacity of 120 kWh in the temperature range from 25 °C to 33 °C. At first, the performance of the storage was determined in standalone tests. Then, the storage was integrated into the new system for solar heating and cooling at the ZAE Bayern. This paper reports on the stand alone tests, and the tests performed after the storage was integrated into the system during summer operation in 2007 and winter operation 2007/2008.

Keywords: latent heat storage, solar cooling, absorption chiller, phase change

1. Introduction

In solar thermal installations with large capacity, full annual utilisation is desirable. During the cold season, solar heat serves for space heating. During the warm season, solar heat can be converted into useful cold by means of sorption cooling. A favourable situation is given when low temperature heating and cooling facilities like floor or wall heating systems, or activated ceilings, are applied for heating and cooling. In that case a low-temperature heat storage using the latent heat of phase change materials (PCM) can be used to significantly improve the system in the heating and also in the cooling mode. In the heating mode the heat storage is used to level the highly variable solar gain. In the cooling mode the storage is used to reject waste heat in combination with a dry cooling tower, instead of using a wet cooling tower. To allow the use of a single storage in the heating mode as well as in the cooling mode to reject waste heat, the heat has to be stored in a very narrow temperature range around 30 °C.

Results — Cost Analysis

The results of this analysis are best summarized in Fig.1 below. This figure shows the specific storage capacity [kWh/m3], as well as the specific storage cost [Euro/kWh stored heat], as a function of the ice packing factor (that is, the volume fraction of PCM in the storage). Zero percent IPF is thus equivalent to a conventional stratified water storage. This particular case is assuming a AT for storage of 25 °C.

As shown, when considering the capital (first) cost only, the cost of the PCM-storage is always higher than for a water storage (see IPF=0) although the difference is not very large. However, if the cost of “space requirement” is important, such as in a house, the PCM — solution quickly becomes cost

Fig. 1. A comparison of a PCM thermal energy storage (IPF>0) to a conventional hot water storage (IPF=0) with regards to specific storage capacity (diamonds) and cost (squares).

effective as compared to the hot water storage. Assuming a cost of space of 300 Euro/m3, the specific storage cost levels off at just below 48 Euro/kWh regardless of IPF such that the cost of a PCM storage is the same as that of a conventional stratified water storage. Then the advantage of the PCM storage is clear — approximately one third the space requirement as compared to a water storage.

Fig. 1. also shows that one important attribute affecting the specific storage capacity is of course the Ice Packing Factor so that designing a PCM storage with as high an IPF as possible is good for the technical competitiveness of this technology option for storage. As the IPF increases, the cost- effectiveness of the PCM-storage is also likely to be enhanced. This finding was taken into account when designing the storage prototype presented below.

Solar assisted district heating system with seasonal thermal energy. storage in Eggenstein-Leopoldshafen

F. Ochs1*, J. NuBbicker-Lux1, R. Marx1, H. Koch2, W. Heidemann1, H. MBller-Steinhagen1, 3

1 Institute of Thermodynamics and Thermal Engineering, Pfaffenwaldring 6, 70569 Stuttgart, Germany
2 Pfeil und Koch Ingenieurgesellschaft mbH PKi, Stuttgart, Germany
3 DLR Stuttgart, Institute of Technical Thermodynamics
* Corresponding Author, email: ochs@itw. uni-stuttgart. de

Abstract

The solar assisted district heating system with seasonal thermal energy storage in Eggenstein-Leopoldshafen (Germany) is the first system realized with existing renovated buildings. The system consists of 1600 m2 flat plate collectors and a 4500 m3 gravel-water thermal energy store (TES) for seasonal thermal storage. Experiences gained within the BMU-project “Further development of the pit heat store technology” contributed to the design of the seasonal TES. This paper focuses on the design and construction of the gravel — water store. The monitoring concept of the solar assisted district heating system with focus on the gravel-water TES is presented.

Alveolar brick cubicles

Two different cubicles were built:

1. Reference cubicle: The alveolar brick has an especial design which provides both thermal and acoustic insulation. No additional insulation was used in this cubicle.

2. PCM cubicle: Several CSM panels (Fig. 3) containing SP-25 A8 hydrate salt are located inside the cubicle, between the alveolar brick and the plaster plastering in order to increase the thermal inertia of the wall (in the southern and western walls and the roof).

Fig. 7 Alveolar brick.

Table 3 Physical properties of the alveolar brick.

Alveolar brick

Heat transmittance (W/m2K)

0.66

Thickness (mm)

290

Table 4 Physical properties of SP-25 A8.

Units

SP-25 A8

Melting point

°C

26

Congealing point

°C

25

Heat Storage Capacity From 15°C to 30°C

kJ/kg

180

Density (at 15°C)

kg/L

1.38

Specific Heat capacity

kJ/kgK

2.5

Heat conductivity

W/mK

0.6

Подпись: Table 4 Physical properties of SP-25 A8. Units SP-25 A8 Melting point °C 26 Congealing point °C 25 Heat Storage Capacity From 15°C to 30°C kJ/kg 180 Density (at 15°C) kg/L 1.38 Specific Heat capacity kJ/kgK 2.5 Heat conductivity W/mK 0.6

Fig. 7 presents the alveolar brick. The most important properties of the alveolar brick and SP-25 A8 are shown in Table 3 and Table 4.

Fig. 8 Demonstration cubicles built with alveolar brick.

Подпись: Fig. 8 Demonstration cubicles built with alveolar brick.
Fig. 8 shows the demonstration cubicles built with alveolar brick.

1.1. Instrumentation and registered data

To evaluate the insulating performance of each material the following data were registered for each cubicle.

• Wall temperature (east, west, north, south internal, south external, roof and floor).

• Internal ambient temperature (1.5 m) and humidity.

• Heat flux at the south wall (inlet and outlet).

• Electrical consumption of the heat pump.

• Solar radiation.

• External ambient temperature and humidity.

1.2. Experiments performed

The experimental set-up offers the possibility to perform two kinds of tests.

• Free-floating temperature, where no heating/cooling system is used. The temperature conditions in the cubicles are compared. The ones with PCM are expected to present a better behavior.

• Controlled temperature, where a heat pump is used to set the ambient temperature of the cubicle. The energy consumption of the cubicles is compared using different set points. The cubicles using PCM are expected to present lower consumptions.

Control Strategy

3.1. Passive and active thermal storage in the house

Simulations indicate that during a clear sunny or even a partially sunny day, no heating will be required [1]. The main challenge of the control strategy thus consists of gathering as much thermal energy as possible during sunny days, and storing it so that it can be used over a sequence of cloudy days (at least 2 days), thus minimising the use of the backup (i. e., the ground source). The many systems of this house make controlling it a challenging task. However, despite the inherent complexity of the problem, the key issue is the rational balance of the two thermal storage media for stockpiling solar thermal energy: the passive thermal storage in the building’s structure thermal capacity (mainly in the floor slab and the masonry wall) and the active thermal storage (TES tank).

The passive storage of the ANZEH is charged by the solar heat gains obtained through the windows (which are practically an essential component of the heating system) and by the radiant floor heating pipes. The passive storage is discharged when it gives heat to the indoor space. Naturally, this thermal energy is eventually released to the surroundings of the house. The TES reservoir (active storage) can be charged in four ways: (a) through the direct recovery of thermal energy from the BIPV/T air, via the HX; (b) with one or both HPs using the BIPV/T air; (c) with the HPs, but using the ground as the heat source; and (d) with excess energy from the solar collector loop. The TES is discharged mainly by its use as a source for the radiant floor heating system; it is also discharged by delivering thermal energy to the DHW tank and through natural heat losses to the surroundings.

The following considerations should be addressed in the design of the control strategy:

• Although temperature fluctuations are needed to take advantage of the thermal mass potential for storing thermal energy, comfortable indoor conditions must be maintained at all times.

• These temperature fluctuations occur at time scales of several hours, much longer than the time constants of the sensors and of the HVAC system.

• The COP of the HP(s), and thus the energy delivered to the TES, depends mainly on the temperatures of the BIPV/T air and the bottom of the TES tank. The COP also depends on the water flow rates on both sides of the HP(s), and the air flow rate through the HX.

• The thermal energy stored in the tank increases with its temperature. Since the COP of the HP(s) decreases as the temperature of the tank increases, the decision to charge the tank should be made based on the availability of thermal energy at the present time and in the future.