Category Archives: EuroSun2008-5

Improved new testing possibilities for air-collectors

image159 image160 image161

As explained in the abstract, Fraunhofer ISE has already operated a test facility for solar air collectors for some years. We have now started to extent and improve our testing possibilities. Our concept was to have a mobile testing facility. That means that either it may be used within the existing indoor test stand with the solar simulator. But additionally, we also made it possible to use the components of the air-collector testing loop at the outdoor testing facility with sun tracker. In both cases the testing facilities allow measurements with very high accuracy and reproducibility. Figure 1 shows the set-up of the air-collector testing loop. Exactly the same rules as described in EN12975 for the testing of water-collectors are used for the air-collector tests.

Fig. 1. Set-up of the air-collector testing loop for efficiency measurements

Information on the components of the loop are summarised here, and will be commented after the list:

• Two ventilator units, each with a flow rate performance between 0 — 500 m3/h, 400 Pa

• Two flow meters (MR) with a measurement range of 0 — 1000m3/h

• Two humidity sensors(FR) to measure the relative humidity of the air in the test loop

• four temperature sensor (TR) to measure the inlet — and outlet-temperature of the collectors, and temperature measurement points necessary to determine the density and the heat capacity of the air at the flow sensors

• water-to — air heat exchanger to condition the air in the test loop

• pressure difference sensor (PR)

• solar radiation (RR)

• Ambient air temperature (TR)

Compared to a water-collector test loop, we added some additional features due to our testing experience with air-collectors:

1. Not one, but two air ventilators are used. Although in principle solar air-collectors should be tight and do not have leakages in the absorber, many of them (especially at the beginning of a development process) are not air tight. This is disturbing the efficiency measurements. It is therefore very helpful, if two ventilators are installed in the test loop. Because they can be operated in such a way (one is pushing air, the other is sucking air) that with the proper adjustment as a result there is only a small difference between the absolute pressure of the air in the collector and the pressure of the ambient air. Thus, air leakage does only have a small influence on the efficiency measurement and development measurements can be carried out in a reproducible was. Of course, the air leakage rate is nevertheless important for the performance of a collector and should be measured and reported in separate measurements. And as mentioned already, the collector should be air tight.

2. Water is an incompressible fluid, but air is not. Therefore the influence of the absolute pressure of the fluid has to be taken into account. Figure 2 shows the dependence of air density on temperature and pressure. The variation within the typical measurement boundaries can be seen from Figure 2. In any case, the influence can easily be included in the measurement and evaluation procedures.

t

 

£

tfl

c

&

 

70 90 110

Temperatur in °C

 

-30

 

-10 10

 

30

 

50

 

130

 

150

 

170

 

190

 

image162

image163

image164

Figure 2: Dependence of air density on pressure and temperature.

Figure 3: Dependence of specific heat capacity of air on temperature and humidity (xw20 denotes an

absolute humidity of 20 g/m[16]).

Use PV to offset non-renewable energy use

Some of the housing renovation projects studied included a PV roof. Decisive is the price at which the local utility is required to buy back the solar electricity. For example in Switzerland as of Jan. 1, 2009 electricity providers must buy back solar power for 25 years for all approved pv installations built since Jan. 1, 2006. For systems <10 kW the buy-back rate for attached PV systems is €0.46 and for systems integrated into the roof or facade €0.56. [9].

An exemplary project where PV was part of a comprehensive renovation is the apartment building in Staufen. The 110 m2 PV installation on the south-facing roof (fig. 1) has a nominal output of 14.7 kWp. In 2006 it fed 14’300 kWh into the grid. The motivation of the owner, Guido Erni, was to provide retirement income. Also part of the renovation were insulation of the attic floor (140 mm), facade (200 mm) and basement ceiling (100 mm); a new ventilation system with 85% heat recovery and replacement of the oil furnace by a heat pump. The results: primary energy use for space and water heating was reduced 65% from 154 to 54 kWh/m2 [10].

Use passive solar design to reduce energy use and improve life quality

Replacing windows with highly insulated units can reduce heat losses to such an extent that solar gains cut heating costs (fig. 8). To minimize unnecessary window opening time and drafts, frame vents can be installed. [11]. Enlarging window openings in walls, when possible, amplifies these savings and admits more daylight. Daylight can also be led into interior spaces by a light pipe [12 + 13].

An example is the renovation of the row houses Kroeven in Roosendaal, the first large — scale passive house renovation project in Holland. Single pane windows were replaced by triple pane glazing in passive house frames.

In addition the walls were insulated with 200 mm XPS and the roof with 360 mm of

cellulose. A new ventilation system was added. A 90% savings in energy consumption resulted, with the annual primary energy for space and water heating being cut from 219 to 21 kWh/m2 [14].

Conclusions

Renovating existing housing can provide living space with superior comfort, very low energy consumption and a special charm. The examples presented here from Austria, Germany, Greece, Italy, the Netherlands and Switzerland demonstrate that it is possible to achieve energy savings up 90 percent, while preserving the special character of the projects. Solar energy is a viable, economic alternative to the costly, last increment of conservation measures in order to achieve the goal of drastically reducing dependency on non-renewable energy and production of CO2. In some of the projects photovoltaic panels were included in the renovation package. When the primary energy value of the solar electricity is deducted from the greatly reduced energy demand for space and water heating, these projects achieve a nearly zero-energy balance.

References

[1] IEA SHC: Renovation Examples, http://www. iea-shc. org/publications/task. aspx? Task=37

[2] Feist, W.: Passivhaus Kriterien, http://www. passivhausprojekte. de/kriterien. php

[3] GAG Ludwigshafen am Rhein Passivhaus im Mietwohnungsbestand: Hoheloogstrafie 1 und 3, WittelsbachstraBe 32, DE-67061 Ludwigshafen, www. gag-lu. de

[4] Calderaro, Valerio: Historic Building in Modena, IT, www. iea-shc. org

[5] Domenig-Meisinger, Ingrid: Passiv House Renovation, Makartstrasse, GIWOG Gemeinnutzige Industrie-Wohnungs-AG Linz http://www. hausderzukunft. at/results. html/id3951

[6] Fehr-Bigger, Hubert, Architekt, Dorfhaldenstrasse 30, CH-8880 Walenstadt Enz, D & Hastings, R.: One-Family House in Walenstadt CH, www. iea-shc. org

[7] Hastings, R. & Morck, O.: Solar Air Systems, Vol. 1 Built Examples, Vol 2 A Design Handbook, Earthscan, London, ISBN 1 873936 85 0 and 1 873936 86 9, www. earthscan. co. uk

[8] Grammer Solar GmbH: Twinsolar, Oskar-von-Miller-Str. 8, DE 92224 Amberg, www. grammer-solar-bau. de

[9] Stickelberger, David: Fakten zur Kostendeckenden Einspeisevergutung KEV fur Solarstrom, Swissolar Infoblat 16.Apr. 2008, www. swissoolar. ch

[10] Enz, D. & Hastings, R.: Apartment Building in Staufen, CH, http://www. iea-shc. org/publications/task. aspx? Task=37

[11] Passivent: Background Ventilation, 2 Brooklands Road, UK-Cheshire M33 3SS, www. passivent. com

[12] Velux: Sun Tunnel Natural Light, VELUX Company Ltd., Woodside Way, UK-Glenrothes Fife KY7 4ND, http://www. velux. co. uk/Products/SUN+TUNNELS/

[13] Glidevale Ltd.: Sunscoop Tublar Roof Lights, 2 Brooklands Road, Cheshire UK-M33 3SS www. glidevale. com

[14] Frank, E. and Bekx, M: Rowhouses, Kroeven in Roosendaal, Holland, Franke Architekten,

Postbus 151, 3360 AD Sliedrecht, Holland, info@frankearchitekten. nl

. Overview of technologies and status for solar heat

J.-C. Hadorn

Groupe Bemey — BASE Consultants SA, Geneva, Switzerland
ichadom@baseconsultants. com

Abstract

This paper presents the state of the art of the main storage solutions for storing solar heat. Keywords: heat storage, seasonal storage, water tank, duct storage, aquifer, solar

1. Storage of heat : overview of classical and advanced materials

Water is a good candidate for all kind of heat storage in the range -30 to 200 C. Table 1 compares basic materials and shows that water has a high volumetric heat capacity.

Thermal

conductivity

@20C

Density

@20C

Volumetric

heat

capacity

@20C

Thermal diffusivity @20C ‘

W/mK

Kg/m3

10+6J/m3

10-8 m2/s

Air

0.025

1.29

0.001

1938

Water

0.6

1000

4.180

14

Ice

2.1

917

2.017

104

Aluminium

237

2700

2.376

9975

Copper

390

8960

3.494

11161

Stainless Steel

16

7900

3.950

405

Concrete

1.28

2200

1.940

66

Marble

3

2700

2.376

126

Glass

0.93

2600

2.184

43

PVC

0.16

1300

1.950

8

PTFE

0.25

2200

2.200

11

Sand (dry)

0.35

1600

1.270

28

Sand (saturated)

2.7

2100

2.640

102

Wood

0.4

780

0.187

214

Cork

0.07

200

0.047

150

Foam glass

0.045

120

0.092

49

Mineral insulation materials

0.04

100

0.090

44

Table 1: Thermal properties of some materials for sensible heat storage

Heat storage candidates

image010

BASE CONSULTANTS

Figure 1: Energy density of some storage material: 3 ranges emerge as competitors to water: PCM, Sorption and Chemical storages.

New solutions have been recently investigated in Task 32 (Figure 1). There is a lot to of potential for medium temperature solutions but as we will see new materials are urgently needed.

Advanced Solar Housing Renovations

image001

Robert Hastings

Donau University

Department of Building & Environment
AT-3500 Krems
robert. hastings@aeu. ch

Abstract

I strongly favour renovating housing because older structures often have a charm and personality lacking in "modern" box architecture. A renovation for me is "advanced" if it fulfils contemporary expectations for appearance, functionality, space and light; and if purchased energy costs are drastically reduced. Often, however, renovations are only superficial. This is tragic because a sensible renovation will not be an issue again for many years. Therefore, clients should be convinced to set high standards for renovations. To achieve ambitious goals, many components and concepts drastically reduce heat losses. However, in existing buildings conservation opportunities are often limited by existing construction or historic preservation. In such circumstances, producing energy is an economical alternative to extreme conservation. The obvious means is harnessing the sun, be it by photovoltaics, solar thermal or passive solar design. When a renovation includes a sensible mix of conservation and solar measures, and drastically improves living quality, it can truly be said to be an "advanced solar housing renovation". Examples of comprehensive renovations and their performance from across middle Europe are presented here, based on documentation completed in a Subtask of IEA-SHC 37.

Keywords: housing, renovation, solar, international

Подпись:1. Introduction

Successful, innovative renovation projects are a good source of ideas, can convince a client to set ambitious goals and are helpful in setting realistic targets. Exemplary renovations from seven countries have been documented in an IEA-SHC project [1]. The combined strategies of conservation and solar use achieve up to 90 percent reduction in non-renewable energy demand. Primary energy use for space and water heating was reduced to between 40 and 70 kWh/m2a. The renovation strategies not only save energy, they also improved living quality, comfort and functionality, as well as increasing real estate values. As in new building design, careful detailing is the key to beauty, effectiveness and durability in renovation projects (fig. 1).

Storage using the building structure

To collect solar heat in a building windows are effective! In order to avoid high temperature swing, thermal mass must be however present inside the protected volume, i. e. inside the insulated envelope. Internal thermal mass can be used from 19 to 24C, barely more. Depth of penetration is however limited and all the thickness of a wall or slab cannot be used during a daily cycle. For concrete it has been shown that only the first 10 to 14 cm can be really used for a diurnal storage. In our example the necessary wall area becomes then 70 m2, free of carpets, paintings, furniture, wood or other thermal and radiation barriers. Any measure that can improve thermal mass in a building favours the comfort.

PCM in walls

To improve storage, several solutions of PCM (phase change materilal), embedded or not, into construction elements have been invented and tested since more than 30 years. Today 2 types of PCM

solutions for storing into of close to the structure of a building might see a future on the market: cool ceilings and PCM in microencapsulated balls inside a wall element.

PCM in microencapsulated polymers are now on the market. They can be added to plaster, gypsum or concrete to enhance the thermal capacity of a room, at a pre-defined temperature (22 or 24C). For renovation they provide a good alternative to new heavy walls.

Rock beds

Storage of air heated by a solar air system can be achieved in rock beds. The blocks of rock are between 1 to 10 cm diameter. Air is blown through the bed at low speed to heat up the rocks. Several solar houses have been built on this principle and works to satisfaction. It is not seasonal storage, and needs a air collecting and distributing system. Water solutions for storage are preferred because water collectors are more efficient than air collectors, and water can transfer much higher power rates than air.

Insulate and tighten the envelope. Goal, benefits and problems

Reducing heat losses is the basis for a rational renovation. When the facade or roof needs repairing, insulating at the same time decreases heating bills, improves comfort and eliminates a cause of mould. Renovating to the "Passivhaus" standard [2], however, often poses problems in renovation, i. e. anchoring thick insulation and detailing wall openings of window and door. Exterior insulation of historic buildings is often prohibited and interior insulation leaves thermal bridges.

Three examples are presented here: gutting the building to the structure (but still preserving much of its immense gray energy content), a gentle renovation of an historic building and a renovation including an innovative solar facade insulation system.

Water tanks for diurnal storage

The most cost effective way of storing solar heat is water. Water is a cheap and convenient material and tanks ranging from 50 to 1000 l are built by millions each year for the HVAC market. Modern solar tank combines several specific features that improve the overall efficiency of storing solar heat: Few thermal bridges, Enhanced insulation, even vacuum insulation in the future, Improved bottom insulation, Siphon introductions pipes to avoid natural convection losses, Reduced number of connections to avoid thermal bridges, Stratification enhancers, Internal devices to reduce speed of inlet water not to disturb stratification, Large heat exchangers or mantle heat exchanger.

2. Storage in PCM in tanks (Phase Change Material)

The idea of using PCM in a storage tank has been investigated in the 80s with paraffin. Although it works, the advantage is nowadays not strategic since the solar collector have been much improved and are less dependant of the collecting temperature in the range 50 to 80C than they were. Parafin has also a major drawback, its flammability. A few manufacturers propose PCM material for storing solar heat on the market (see table). From 20 to 80C, there is some choice.

Even for 0C, one company manufactures polymer balls that encapsulate water for ice storage. The same idea could be applied to a class 50 or 60C material that would end in a ball storage acting with the solar fluid fluid like a rock bed does with air.

PCM name

Type of product

Melting Temp. (C)

Heat of fusion (kJ/kg)

Manufacturer

RT20

Paraffin

22

172

Rubitherm GmbH

ClimSel C 24

n. a.

24

108

Climator

RT26

Paraffin

25

131

Rubitherm GmbH

STL27

Salt hydrate

27

213

Mitsubishi Chemical

AC27

Salt hydrate

27

207

Cristopia

RT27

Paraffin

28

179

Rubitherm GmbH

TH29

Salt hydrate

29

188

TEAP

STL47

Salt hydrate

47

221

Mitsubishi Chemical

ClimSel C 48

n. a.

48

227

Climator

STL52

Salt hydrate

52

201

Mitsubishi Chemical

RT54

Paraffin

55

179

Rubitherm GmbH

STL55

Salt hydrate

55

242

Mitsubishi Chemical

TH58

n. a.

58

226

TEAP

ClimSel C 58

n. a.

58

259

Climator

RT65

Paraffin

64

173

Rubitherm GmbH

ClimSel C 70

n. a.

70

194

Climator

n. a.: not available

Table 2: Some PCM available on the market for storage of solar heat (from IEA SHC 32, L. Cabeza)

New ideas are to combine PCM material with water in a hybrid storage so that the top part of the storage tank would stay at a maximum of 60C. Some first results both theoretically and in laboratory were found within IEA Task 32. PCM materials were found difficult to caracterize (supercooling and hysteresis effect). The advantage of PCM in tanks for short term storage was not demonstrated since the temperature range at which a combitank operates is quite large. The optimum position and operating temperature of the PCM and the material choice itself still needs investigation.

Strategy insulate the exterior

For non-historic buildings, exterior insulation is the standard solution, but to insulate to the PH Standard is advanced. The apartment building, Hoheloogstrasse in Ludwigshaven, is an example of an extreme renovation (figure 2). Wall, roof and the basement ceilings received 20, 24 and 12 cm of insulation. Windows were replaced with PH-quality units, thermal breaks were eliminating and mechanical ventilation with heat recovery added. As a result, the annual heating energy demand was reduced from 250 to 15 kWh/m2. After stripping the building to its structure, it was possible to integrate heat recovery ventilation and construct modern, spacious apartments. Finally, PV was integrated into the entire south-facing roof [3].

Seasonal storage requires large volumes

Seasonal storage is of course possible with sorption and even more with chemical storage (see figure 1). But there is no solution yet mainly due to very little research in the field. Since 30 years seasonal storage has been investigated to reach high solar fractions, up to 100% solar, the final dream.

How big should a tank be to reach 100% solar? Many studies and experience have been conducted to solve this question. There are a few examples that actually are in operation! In the 80s it was calculated that a seasonal water storage should be as big as half the house, limitating its interest. That was indeed true but what happened is the stong demand reduction for heating a house. Savings of energy prior to investing for a new production device!

For a recent passive house, the volume is much less. With 20 m3 of water it is possible to reach 100% solar in mid Europe, in average meteorological year! Since climate variations can be as high as 25%, an auxiliary heating system is still recommended. It is interesting to use wood as back up if the user is interested to reach 100% by renewable energy!

When trying to deliver as much as possible heat from solar to more than one house, say a group of 100 houses, there are several technologies that have been investigated and a few pilot plants built around the world. They can be ordered in three generic families:

o the water based

o the soil or earth based

o the aquifer based (basically a mixture of water and soil!).

Accessing huge volume of water requires a tank! the biggest tank we can build are in the order of 20 to 30’000 m3 at a cost that is above 150 euros per m3. Which is not high considering a 1 m3 storage tank for a combisystem can cost 1000 to 2000 euros! But 30’000 m3 is not enough in certain cases like a solar district heating system say for 500 dwellings in Denmark or Sweden. Soil has adequate thermal properties at a low cost!

We have seen that heat capacity of soil is in the order of 60 to 80% that of water and thermal conductivity is comparable to that of water:

Soil can store sensible heat like water but is a bad insulation material. An underground storage system must therefore be completely insulated by some insulating material to withstand 6 months of duty must be very big! Minimum volume not to insulate bottom and sides of an underground storage is 20’000 m3 as an order of magnitude. This means a minimum of 500 dwellings connected to the store! The true parameter is the time constant of a heat store. For a store to be really seasonal, its time constant should be over 365 days! The next table shows all seasonal storage technologies that have been worked during the past 30 years..

Large water tanks have been built with success for solar plants in Sweden, Denmark, Germany and Switzerland. The specific cost of a large tank is much reduced compared to that of a small storage tank for a one family house, but still with 150 to 250 euros/m3 for 10’000 m3 tanks it is still expensive.

Duct storage are a cheaper and reliable solution. The german project Neckarsulm is proving well that the technology of a shallow duct storage is operating as predicted by computer models at a cost divided by 4 to 5 compared to a tank storage.

The Neckarsulm storage has 528 boreholes 30 m deep, and a top insulation only thanks to its huge size of 140’000 m3 in the long run. The present volume of store is 63’200 m3 heated up by 6’337 m2 of collectors. The core of the store reached 62C in summer 2003. The store operates with great heat losses during its warming up time (5 years) but is expected to reach a 50% efficiency in 2005.

Duct storage used with a heat pump are common in several countries. The store operates at low temperature (0 to 30C), thus without any heat losses!

Aquifer storage was the most promising technology in the 80s because of its low cost to acces huge volumes of water.

China has a long tradition of cold water storage in aquifers. In Europe and the US, pilots plants to test the concept of storing solar heat without a heat pump at 60 to 90C into an aquifer at 30 to 200 m depth, were built in Denmark, France and Switzerland with limited success due to chemical problems (calcite precipitation) or due to buoyancy effects ruining the exergy of the store.

Подпись: Table 3: Seasonal storage technologies (Hadorn 1988)

Recent projects in Germany prove to work (almost) well if design is carefully based on the results of the site investigations. But still a strong natural groundwater flow negatively influences the results, and the storage without a heat pump would probably not reach a reasonable efficiency. Cold storage in aquifers has become a cost effective technology in some countries. Competition against electricity costs makes the alternative of free cooling with a COP of more than 20 attractive

Rule of thumbs for large seasonal storage are:

Minimum load: 500 MWh recommended

1.5 Подпись:Подпись:to 2.5 m2 solar collectors per MWh of load for 40 to 60% of load

1 to 2 m3 per m2 of solar collector Thermal insulation: min. 40 cm at 0.04 W/mK

Solar productivity: 200 to 300 kWh/m2 in mid Europe climate Storage cost : 150 to 250 euros/m3

1.5 to 3 m2 solar collectors per MWh of load for 40 to 60% of load

2 to 6 m3 of storage per m2 of solar collectors minimum 20’000 m3 if insulation only on top

15 to 50 W/m of borehole (double U-pipe, quartz bentonite filling) Storage cost: 30 to 60 euros/ m of borehole

Aquifer storage 2 to 6 m3 of water per m2 of collectors

Minimum 50’000 m3 of storage for a no heat pump system

Minimum depth: 10 to 20 m

Should preferably be used for cold storage

Storage cost: 5 to 20 euros/ m3 but very very dependant on local conditions A strong Regional groundwater flow (1 mi/month) can ruin the store

Regarding large seasonal storage, the present conclusions after many pilot plants are the following:

o Water technologies are reliable but more expensive. Water tightness is the main problem that can be overcome with stainless steel liners or special very dense concrete

o Pit storage have some potentials in terms of cost /performance ratio but are difficult to master

o Duct storage are the most simple. With heat pumps they work fine at low temperature range (0- 30C). The challenge of a no heat pump solar system is proved to be possible in the Neckarsulm project and in the Drake Landing project in Canada.

Aquifer storage is the cheapest technique for huge volumes, when it can be mastered. For cold storage, cost effectiveness is proved (payback time less than 5 to 7 years in Germany, Sweden, Netherlands on more than 200 installations). For warm storage (40 to 90C), the chemical composition of the aquifer and its geometry (natural convection problem) are very determinant and local conditions prevail so that a prognosis is always difficult to make. On site investigations and modelling are required making the process long and somehow costly for a result difficult to assess a priori.

Solar ponds (salted water) are also candidate for long term storage in desert.

High temperature storage (concrete or molten salts) will also be needed for thermal power plants at 400 to 500C to overcome clouds or cloudy hours. There are also preferred chemical reactions (ZnO) investigated since decades but with no commercial solutions yet.

3. Conclusions

Sorption and chemical storage are future candidates for solar heat storage. However recent work from IEA SHC Task 32 has shown that new materials are urgently needed. There are also other ways to indirectly store solar energy, i. e.using hydrogen as a storage material and a vector of distribution.

Water is still the best storage material for small scale solar installations.

There are ways to improve water tank storage efficiencies but we are reaching limits for diurnal storage. The progress will be more in the overall system and control strategy. The heat exchange also is a research topic.

PCM materials will have a come back in construction elements for cooling and less for heating. For combitank the competition with water is hard to win due to a large operating temperature range.

There is a need for some new investigations in sorption materials, where silicagel and zeolites have shown too limited possibilities. Chemical stores are also back in the R&D work but unfortunately to a low level.

For large seasonal storage, duct storages have an interesting future with or without heat pumps, and aquifer storage are best suited for cold storage and free summer cooling.

Seasonal storage for one family house is possible through an insulated water tank, but economics show systems with short term storage that can cover up to 50% of the load as more attractive at present. In the long run, chemical storage will find its way to the one family house but there is more R&D budget to allocate to this fundamental topic for a large deployment of solar in mid european climates.

References

[1] Hadorn J.-C. editor, (June 2005), Thermal energy storage for solar and low energy buildings — State of the Art, a IEA SHC Task 32 book, Printed by Servei de Publicacions Universidad Lleida, Spain, 170 pages ISBN 84-8409-877-X, available through Internet www. iea-shc. org Task32

[2] IEA SHC Task 26 (2004): Solar Heating Systems for Houses — A Design Handbook for Solar Combisystems, W. Weiss and al., James & James, 2004, 313 pages

[3] Task 32 reports are available at http://www. iea-shc. org/task32/publications/index. html

[4] Solar Engineering of Thermal Processes, John A. Duffie, William A. Beckmann 1991, 2nd edition, Wiley — Interscience

[5] De Winter (Ed.) Solar Collectors, Energy Storage, and Materials June 1991, MIT Press.. Historical! A must…

[6] Solar thermal energy storage, HP Garg, SC Mullick, AL Bhargava, D. Reidel Publishing Company, 1985

[7] Streicher Wolfgang, Sonnenenergienutzung, lecture book, Graz University of Technology, 2003

[8] Central Solar Heating Plants with Seasonal Storage : Status Report by Jan-Olof Dalenback, International Energy Agency, Book — January 1990

[9] Guide to seasonal storage, JC Hadorn, 1988, published in English by Public Works, Canada

[10] Thermal Storage Of Solar Energy. C. Den Ouden. Holland, The Hague, Martinus Nijoff Publishers, 1980

[11] Drake Landing Project, Canada http://www. dlsc. ca/