CONCERTO COMMUNITIES IN EU DEALING WITH OPTIMAL THERMAL
AND ELECTRICAL EFFICIENCY OF BUILDINGS AND DISTRICTS,
BASED ON MICROGRIDS
WP 2.3 - Del 2.3.3
Guidelines for the implementation of
large scale solar thermal energy systems and
long-term thermal energy storage into district heating systems
November 2015
Prepared by
Steinbeis Research Institut for
Solar and Sustainable Thermal Energy Systems
Meitnerstr. 8, 70563 Stuttgart, Germany
Tel.: +49 711/6732000-0, Fax: +49 711/6732000-99
[email protected], www.solites.de
DOCUMENT INFORMATION
Title:
Deliverable number 2.3.3
Guidelines for the implementation of large scale solar thermal energy systems and long-term thermal energy storage into district heating systems
Date:
November 2015
Task(s):
WP 2.3
Large-scale solar thermal integration and long-term heat storage
Authors:
Thomas Schmidt
SIG - Solites
[email protected]
Dirk Mangold
SIG - Solites
[email protected]
Thomas Pauschinger
SIG - Solites
[email protected]
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
INDEX
1. Introduction
1
2. Basic information
3
2.1. Types of large scale solar thermal system concepts
2.2. Economics
3. Large-scale solar thermal collector fields
3.1. Solar collector technology
3.1.1. Flat plate collectors
3.2. E vacuat ed-t ube-collectors
3.2.1. Solar collector paramet ers
4
10
12
12
12
13
14
3.3. Mounting options for solar collector fields
15
3.4. Solar collector field hydraulics
15
3.5. Security layout for large solar collector fields
17
4. Long-term thermal energy storage
4.1. Construction concepts for large-scale thermal energy storages
18
19
4.1.1. Tank thermal energy storage
21
4.1.2. Pit thermal energy storage
22
4.1.3. Borehole thermal energy storage
23
4.1.4. Aquifer Thermal Energy Storages
24
4.1.5. Cost of storages
26
4.1.6. Design guideline
27
5. Referenc es
29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
1. INTRODUCTION
Large-scale solar thermal systems, also referred to as solar (assisted) district heating systems (SDH),
are differentiated in systems with short-term or diurnal heat storage, designed to cover 10 to 20 % of
the yearly heat demand for space heating and domestic hot water preparation by solar thermal energy, and solar systems with seasonal heat storage with solar fractions of 50 % and higher. The so
called solar fraction is that part of the yearly energy demand that is covered by solar energy.
Block or district heating systems consist of a heating cent ral, a heat distribution net work and heat
transfer substations in the connected buildings. Large-scale and centralized heat production offers
high flexibility concerning the choice of the type of energy used. It allows the application of a seasonal
storage in an energy- and cost efficient way.
To gain solar thermal energy in SDH systems large solar collector areas are installed on ground or on
buildings that are preferably close to the heating central. The heat obt ained from the solar collectors is
transported via a solar network to the heating central and is directly distributed to the buildings. Surplus heat from the summer period can be fed into a seasonal thermal energy storage (S TES). In central Europe the sun provides more than two third of its yearly energy supply only during the summer
period (Fig. 1). Thus during the space heating period, when an ordinary residential house needs more
than 80 % of its yearly energy demand, the sun provides not sufficient energy for higher solar fractions. With the beginning of the space heating period, an S TES can deliver solar thermal energy that is
transported to the houses via the district heating net.
Fig. 1: Solar radiation and heat demand over a year in Central Europe
Page 1 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
Decisive for the optimum function of the solar system is a proper integration into the conventional
heating system and a careful design of the solar part as well as of all other components for heat supply: district heating network, heat transfer substations and building services.
Seasonal thermal energy storage offers a great potential for substituting fossil fuels using solar energy
for domestic hot water preparation and space heating. There are however further applications for
large-scale thermal energy storages as for example:
•
Increased use of biomass for electricity production in cogeneration
•
Increased use of geothermal energy
•
Increased use of waste heat from the industry
•
Increased use of waste energy from electricity production through combined heat and power
plants
In the latter case, heat storages can balanc e load variations and dissociate the electricity production
from the heat production through storage.
In a growing number of countries more and more electricity is produced from wind and solar. This
means more and more variation - both short and long term - in the electricity production and more difficult conditions for the traditional CHP units. The “smart district heating” concept (Fig. 2, source:
PlanEnergi, DK ) is developing to assist in solving the problems connected to these two issues. Smart
district heating is combining renewable energy technologies and thermal storage in such a way that
the district heating system is linked in a very flexible and constructive way with the liberal electricity
market. Main features of a smart district heating system are long term thermal energy storage, solar
collectors, heat pumps and combined heat and power units.
Dronninglund and Marstal
HP
CHP
35 – 40 000 m²
50 – 100 000 m3
Fig. 2:
STES
Buffer
storage
Load/ user
Example of a smart district heating system in Denmark (source: Planenergi, DK; HP: heat
pump, CHP: combined heat and power plant)
Page 2 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
2. BASIC INFORMATION
The possible contribution of a solar thermal system to a heat supply system is depending on the dimension of the main components of the solar thermal system, primarily the solar collector area and
the heat storage volume. The solar cont ribution is often referred to as solar fraction, which is basically
the yearly heat production from the solar thermal system referred to the total yearly heat demand of
the heating plant.
Fig. 3 shows the efficiency, profitability and solar fraction of a solar thermal system as a function of the
solar collector area. For a specific system (or a specific heat demand) the solar fraction increases with
an increase of the solar collector area. Because storage heat losses and number of periods where the
solar heat cannot be used due to fully charged storage capacities or low heat demand the efficiency of
a solar system is decreasing with increasing solar collector areas. This on the other hand leads to a
non-linear and restricted increase of the solar fraction and at the same time to a decrease of the profitability for larger collector areas.
Considering this, it is obvious that the main dimensioning of a solar thermal plant should be done in an
early phase of project development. Here the main decision for either an economically optimised solar
system with a limited solar fraction or for a system with a certain target s olar fraction has to be taken
with a clear picture of the consequences of this decision in terms of solar contribution and system
economy.
Fig. 3:
Development of solar collector field efficiency, profitability and solar fraction as a function of
solar collector area for a specific heat demand
Page 3 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
Besides the dimensioning of a solar thermal system a number of system boundary conditions have a
nameable influence on t he solar system efficiency. Besides the weather conditions at a given location
the major influencing parameter is the temperature conditions for the heat distribution t hat are defining
the operation temperatures of the solar thermal system. Fig. 4 shows the potential solar heat gain as a
function of system heat distribution temperatures (net temperatures in the figure) for different types of
solar collectors. As can be seen in Fig. 4 the area-related s olar gain can vary between 200 and
550 kWh/m²a. Especially for high distribution t emperatures the choice of a suitable solar collector type
is important for high solar gains. With a good solar collector also at high temperatures high solar gains
of 350 to 450 kWh/m²a can be reached.
600
Supply temp. ST (winter/summer)
Return temp. RT (winter/summer)
400
300
ST(110/90) RT(60/70)
ST(110/90) RT(50/60)
ST(100/80) RT(60/70)
ST(100/80) RT(50/60)
ST(90/70) RT(50/60)
ST(100/80) RT(40/50)
ST(70/70) RT(50/60)
ST(90/70) RT(40/50)
ST(100/80) RT(30/40)
100
ST(70/70) RT(40/50)
ST(90/70) RT(30/40)
200
ST(70/70) RT(30/40)
Area-related solar gain [kWh/(m² a)]
500
0
50
55
Flat plate collector
Fig. 4:
60
HT flat plate collector
65
70
Mean net temperature [°C]
Evacuated tube collector
75
80
85
Evacuated tube collector with CPC
Solar gain as a function of solar collector type and net temperatures (simulation res ults for
location Würzburg in Germany, HT: high-t emperature; CP C: compound parabolic concentrator)
2.1. Types of large scale solar thermal system concepts
In the last decades different system concepts for solar district heating (SDH) systems have been developed and realized in numerous projects. The main variables for distinguishing between the concepts are:
-
The way the solar thermal plant is connected to the net (central vs. decentral integration, see
Fig. 5)
-
The size of the solar thermal plant itself and the size of the district heating system in which the
solar thermal heat is fed-in. The latter range from small heat nets supplying a few buildings to
Page 4 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
systems supplying new construction areas or villages up to extended district heating systems
in large cities.
Fig. 5: Central and decentral int egration of a solar thermal plant into a district heating system
In consideration of the connection type, the size of the solar thermal system and also organizational
aspects seven types of solar district heating plants are distinguished, see Table 1. Their characteristics are described in the following.
Table 1: Definition of different types of solar district heating plants
Type
Heat de ma nd
Connec tion
Solar ther-
Solar fraction referred
in DH network
of solar
mal power
to total heat delivery
(GWh/a)
ther mal
(MW th)
to DH network (%)
plant (-)
1
SDH systems for block
heating in quarters
0.5 – 10
central
0.2 – 2
10 – 20 %
2 – 10
central
2 – 20
20 – 50 %
SDH systems with long
2
term heat storage and high
solar fraction for block
heating in quarters
Decentrally integrated so-
3
lar thermal plants for quar-
up to 100 % referred
20 – 5000
decentral
0.2 – 2
ters
4
SDH systems for small
cities and communities
to quarter (< 10 % of
total DH)
2 – 100
central
Page 5 of 29
0.5 – 50
10 – 20 %
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
Type
Heat de ma nd
Connec tion
Solar ther-
Solar fraction referred
in DH network
of solar
mal power
to total heat delivery
(GWh/a)
ther mal
plant (-)
(MW th)
to DH network (%)
2 – 100
central
0.5 – 50
10 – 50 %
20 – 5000
decentral
0.5 – 10
up to 10 %
20 – 5000
central
0.5 – 50
up to 20 %
SDH systems with combined electricity and heat
5
supply for small cities and
communities (‘Smart District Heating’)
Large-scale solar thermal
6
plants with decentral integration into large urban
district heating systems
Large-scale solar thermal
7
plants with central integration into large urban district heating systems
SDH systems for block heating in quarters (type 1)
In cases of renovation or new construction of urban quarters, local heating networks are a valid option
for heat supply. Due to the local limitation and depending on the building type and the building equipment such networks can be operated at low temperatures, which are in general favourable for integrating solar thermal plants. The solar fraction of such systems is up to 20 %. The main part of the heat
load is usually covered by a fossil- or biomass-fired boiler.
Due to low availability of space in urban areas, the solar collectors are often int egrated in the roofs of
buildings. Specialized suppliers provide different solutions for such roof-integrated collector plants. In
numerous projects, collecting the solar heat from the different collector fields with a separate network
and feeding it in the net through a buffer heat store located in the central heating plant turned out as
the most economically feasible solution. The buffer heat store enables the adaptation of the solar heat
supply to the actual load. Projects have also been realized, in which the local block heating system
works as subsystem of an extended high-t emperature district heating system. In this case, the latter
supplies the necessary auxiliary heat to the local block heating system.
When realising a loc al district heating system of this type, the early cooperation between the designers
of the solar thermal plant, the operators, the building company and the architects is of essential importance. Thereby a thorough tuning of heating net work, solar thermal plant and heat demand has to be
performed. Local heating systems are usually operated by contractors or municipal utilities. Therefore
the municipality administration has an important role in creating favourable framework conditions for
such projects, in particular regarding legal aspects and permission procedures.
Page 6 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
SDH systems with long term heat storage and high solar fraction for block heating in quarters
(type 2)
The system type for quarters and urban areas (type 1) can be extended with a long term seasonal
heat store. By this the solar contribution to the total heat demand of the quart er can reach up to 50 %,
by storing the solar heat from the summer period until the heating period in winter. Since 1996, eleven
of these large-scale solar thermal plants with long term seas onal heat storage were built in Germany
and the technology was demonstrated within a national research programme.
The central element of such a system is the long term seasonal heat store. It accumulates a large
share of the solar heat produced during summer and stores it until the heating period in aut umn and
winter. Only with such large seasonal heat stores with volumes reaching today from 1 000 to
200 000 m³, solar fractions of 50% and more of the total heat demand can be reached. Seasonal heat
storage is only technically and economically feasible with a size of 1 000 m³ or more. Therefore also
for the district heating system a minimum size is required. Fig. 6 shows the tank storage of 5 700 m³
for a solar district heating system in Munich during construction and in the final stage.
Fig. 6:
Tank thermal energy storage with a volume of 5 700 m³ for the solar district heating system
Munich-Ackermannbogen during construction and in the final stage (source Solit es)
Systems are usually realized and operated by municipal utilities or larger enterprises of the district
heating sector. The success of such projects depends on the close collaboration of all parties and a
good consistency between system planning and development or renovation of the quart er. This is particularly important for projects with long development durations and several construction phases.
Decentrally integrated solar thermal plants for quarters (type 3)
Urban quarters that are connected to a larger district heating system offer the possibility of a decentral
integration of a solar thermal plant. For example housing companies have the option to install large
solar thermal plants on their building blocks. The solar heat production can even exceed the total heat
demand of the building or the quarter and all of the produced solar heat is fed into the district heating
system. Any heat demand of the building is on the other hand supplied through a standard substation
Page 7 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
by the district heating system. In this way the district heating systems work as storage for the solar
thermal plant. The latter remains simple and can be realized at low costs.
In this model the solar thermal plants are in most cases owned by the housing associations. The
plants are operat ed either by the housing companies or by heat supply enterprises. E valuations carried out by [Dalenbäck, 2013] have shown that professional operation by the enterprises lead to a better maintenance and thus performance of the solar thermal plants. Contracts between the housing
companies and the district heating net company regulate the feed-in tariff for the solar heat. The purchased heat is billed according to standard contracts.
SDH systems for small cities and communities (type 4)
In Germany, Austria, Denmark and Sweden, district heating systems are frequently used to supply
heat to small cities and communities in rural areas. A special case are so called “bio energy villages”,
which cover their local electricity and heat demand through local resources and renewable energies.
So far mainly biogas plants were used for this purpose. An alternative concept is the combination of a
large scale solar thermal plant and a biomass heating plant. The solar thermal plant covers the energy
demand during summer in order to avoid the inconvenient partial load operation of the biomass plant
leading to low efficiency and increased emissions.
In rural areas the collector fields of solar thermal plants are often mount ed on the ground close to the
heating central. This leads to very low specific system costs. For the dimensioning of the plant, the
maximum summer load of the district heating system is taken as a reference. It is composed of the
summer heat load (mostly domestic hot water preparation) and the heat losses of the district heating
network. Usually the solar thermal plant covers about 15 – 20% of the total heat load.
Projects of this type are often initiated by local citizens. The plant is often realized and operated by
citizen cooperatives or by local suppliers. The involvement and concrete participation of citizens are
essential success factors when realizing such projects. Citizen participation inc reas es the acceptance
and the willingness to connect as client to the district heating system. The ambition is not profit maximization but to achieve a long‐term favourable price using renewable energies. The necessary professionalism during planning, construction, operation and customer service is a key challenge. A further
success factor is the cooperation with a competent designer for district heating systems and renewable energies. The low heat density in rural areas often turns out to be challenging for the profitable
realization of such systems.
SDH systems with combined electricity and heat suppl y for small cities and communities
(‘Smart District Heating’) (type 5)
An extension of type 4 are medium size district heating systems for small cities and communities that
include large-scale solar thermal plants combined with further technologies for electricity and heat
generation as well as large heat stores. The technology of such holistic local energy supply systems
was demonstrat ed by pilot plants in Denmark and is referred to as „Smart District Heating“ [Soerensen, 2011].
Page 8 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
The central element of this concept is a large multi-functional heat store. It is used for operation optimization of the electricity and heat generation systems and contributes to the flexibility of the overall
system, especially when it comes to highly fluctuating electricity prices. Such innovative systems have
been realized in the Danish communities Marstal, Braedstrup and Dronninglund. The capacities of the
solar thermal plants are in the 50 MW th range and solar contributions bet ween 15 and 50 % are
achieved.
Large-scale solar thermal plants with decentral integration into large urban district heating
systems (type 6)
Large urban district heating systems are usually operated with heat from combined heat and power
plants, heating plants or industrial waste heat. Fuels are often natural gas, coal, waste or biomass.
The decentral integration of large-scale solar thermal plants is one possibility to increase the share of
renewable energy sources in such district heating systems. Plants of this type have so far been realized in Austria.
The decentral integration of a solar thermal plant is possible at any arbitrary place in the district heating system. However, the specific conditions have to be thoroughly checked for each potential connecting point for feed-in. In particular the net temperatures, the net pressure and the hy draulic conditions are relevant aspects for the feasibility. There must be sufficient load and flow-rat e at the feed-in
point for absorbing the produced solar heat. This way, the solar thermal plant can be integrated directly without buffer heat store and controlled in order to reach a set temperature (e.g. the district heating supply temperature) for feed-in. However, the feed-in pumps have to be dimensioned with sufficient capacity for overcoming the pressure difference bet ween the supply and return pipe of the district
heating net. This pressure differenc e reaches up to 9 bar in realized projects. Due to the technical requirements as described above, the decent ral integration of solar thermal plants is only economically
and technically feasible for plants of a relevant capacity (e.g. minimum 150 kW th).
Some of the realized plants in Austria are operated by a cont ractor. The technical requirements and
economic al conditions for the feed-in of solar heat are regulated in a contract between the district
heating net work operator and the contractor.
Large-scale solar thermal plants with central integration into large urban district heating systems (type 7)
In many countries the integration of wind turbines and photovoltaic plants lead to a highly fluctuating
electricity production and thus to significant changes in the electricity markets. These effects also alter
the operating conditions for the heat production units of district heating systems: While the runtime of
CHP plants decreases due to electricity market changes, the share of fossil-fired heating plants increases. This opens the opport unity to substitute the boiler heat of heating plants with solar heat. For
this purpose large scale solar thermal plants can be built and integrat ed directly on the sites of central
heating or CHP plants. The premises around large heating plants often offer good possibilities for
building larger solar thermal plants. First plants of this type are presently in the planning phase.
Page 9 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
2.2. Economics
If conditions are favourable today solar heat generation costs of below 50 Euro/MWh (net and without
subsidies) are being reached. This is possible in countries on middle latitudes, with a significant share
of district heating and for ground mount ed solar thermal systems with a nominal capacity of more than
1 MW th providing a solar fraction of less than 10 %. In some countries subsidies for renewable energies can be assumed on top. In Denmark large scale solar thermal plants with capacities up to
50 MW th are being realized. The operat ors of these plants state heat generation cost of around
30 Euro/MWh (net and without subsidies).
However, several factors can significantly influence the economics of a solar district heating plant. The
following are particularly important:
-
The share of capital costs in the heat generation costs is high in the case of solar thermal energy. Therefore, the internal interest rate used for the calculation has a strong impact on the
heat generation costs.
-
Costs for the land as well as the realization of distributed or roof-integrated collector areas can
increase the total cost.
-
Systems with higher solar fractions are more complex and large heat stores are needed. The
specific system gains also decrease accordingly for thes e systems leading to higher heat
generation costs.
Fig. 7 presents results of an estimation of system cost and heat generation cost for the seven types of
SDH systems as described in section 2.1. The coloured sections represent cost ranges from favourable to not so favourable conditions regarding economics for the solar thermal system.
Page 10 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
Fig. 7: cost estimation for solar district heating systems (net and without subsidies)
Page 11 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
3. LARGE-SCALE SOLAR THERMAL COLLECTOR FIELDS
This section introduces basic technical information regarding solar thermal collector fields.
3.1. Solar collector technology
For the thermal use of solar energy solar thermal collectors trans fer the solar irradiation into thermal
energy. In the following the basics of the solar collector technology is summarized.
For large solar collector fields the use of special large module collectors is recommendable. By the
use of large modules the number of connections bet ween collectors is reduced, which on the one
hand reduces the amount of piping in the field and by this also the thermal losses and on the other
hand reduces the amount of potential bad spots in the field. At last also the effort for the field installation is reduced nameable.
In Europe in most of the cases flat plate solar collectors with harp absorbers have been used for large
installations. However, recently also large fields formed from evacuated tube collectors have been realised.
3.1.1. Flat plate collectors
Solar flat plate collectors (FPC) convert solar irradiation into thermal energy in a very basic way, see
Fig. 8. They are able to mak e use of direct and diffuse solar irradiation.
Fig. 8: Vertical section of a flat plate collector
Page 12 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
Solar absorbers normally are made from copper or aluminium. They are furnished with special selective coatings to enhance the process of energy transfer from the solar irradiation to the heat transfer
fluid inside the absorber pipes that are connected to the back side of the absorber sheets. To reduc e
heat losses the back and the sides of the framing are heat insulated. A high transparency glass cover
reduces heat losses to the front side.
3.2. Evacuated-tube-collectors
E vacuat ed-t ube-collectors (ETC) are, like flat plate collectors, equipped with selective-coat ed abs orbers. To reduce thermal losses the space around the absorber is evacuated. To keep the vacuum for a
long period so called “getters” are installed inside the pipes that bind existing air molecules chemically.
The getter material mostly is barium that has a silver colour when there is a good vacuum and turns
into white when there is a leakage in the tube.
Fig. 9: Vertical section of evacuated-tube-collectors
Depending on the type of E TC there is either a metal-glass (Fig. 9 left) or a glass-glass connection
(Fig. 9 right) to separate the evacuated space from the surroundings. In the latter two glass pipes form
a thermos jug and the absorber pipes are in the non-evacuated cent re.
As the production of the vacuum is complex E TC collectors are normally more expensive compared to
FPC collectors. However, because of the higher efficiency the overall economy of this collector type
can be better especially for high operation temperatures.
Page 13 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
3.2.1. Solar collector parameters
To be able to compare different solar collector products often efficiency curves are used. These
curves show the efficiency as a function of the temperature difference between the solar collector
mean temperature Tm and the ambient temperature Tamb referred to the total (global) solar irradiation
on the collector surface Eglob.
The efficiency curve of a solar collector is defined by three coefficients that are determined in stanst
dardised test procedures: the zero-loss coefficient ɳ0 (also called optical efficiency), the 1 order heat
nd
loss coefficient a1 and the 2 order heat loss coefficient a2. With these paramet ers the solar collector
efficiency at specific operation conditions can be calculated with the following formula:
𝜂 = 𝜂0 − 𝑎1
(𝑇𝑚 − 𝑇𝑎𝑚𝑏 )
𝐸𝑔𝑙𝑜𝑏
− 𝑎2
(𝑇𝑚 − 𝑇𝑎𝑚𝑏 )2
𝐸𝑔𝑙𝑜𝑏
Efficiency parameters for a wide range of solar collectors can be found in a database for solar collectors that have been tested according to EN12975 hosted at www.solarkeymark.dk.
Fig. 10 exemplarily shows efficiency curves of different solar collector types. For higher temperature
differenc es between the solar collector and the ambient the efficiency decreases as the thermal losses
increase. If the solar irradiation Eglob increas es the operation point moves to the left on the x-axes and
the efficiency increases.
Fig. 10: Exemplary efficiency curves of different solar collector types
Page 14 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
3.3. Mounting options for solar collector fields
In large installations solar collectors normally are either mounted on ground or on roofs of buildings.
Also substructures like e.g. car ports, noise protection barriers etc. are used.
If land is available at low cost this is often the most economic possibility for the realisation of a large
solar collector area. In cent ral Europe and in urban areas this option is often not available as land is
either not available at all or too expensive. In these cases solar collectors can be installed on roofs of
buildings. Here it is possible to place the solar collectors on top of an existing roof covering as e.g.
roof tiles or to replace the roof covering by the solar collectors. The latter enables a good archit ectural
integration of the solar collectors and allows for cost savings as e.g. roof tiles can be saved.
In general a collector installation on buildings is a little more expensive as the int erfaces to the buildings have to be properly solved to allow a long-term operation wit hout problems. Also the piping works
for the connection of the solar collector field to a solar net work cause some more effort compared to
an installation on ground.
3.4. Solar collector field hydraulics
The piping layout of the solar collectors and the connecting pipes has to guarantee a uniform distribution of the fluid flow through the entire solar collector field. This is important for a high field efficiency
and to avoid stagnation in parts of the field with lower flow rates compared to ot her parts. To allow for
a uniform flow distribution both the collector internal piping as well as the connection of the solar collectors has to be considered.
To reach a uniform flow distribution inside the collector the pressure drop through the single absorber
channels has to be high compared to the distributor and collector pipes inside the collector. The same
is required for the parallel and serial connection of the collector rows: the pressure drop through a single row has to be high compared to the connection pipes between the rows.
In large solar collector fields consisting of flat plate collectors with harp absorbers often many collectors are connected in a row to allow for a high temperature increase, see Fig. 11. This is also referred
to as a large hydraulic lengt h. Single collector rows are then connected in parallel to form large collector fields.
Fig. 11: Solar collector field row wit h harp abs orbers (Source: [Knabl et.al, 2014])
In installations with flat plate collectors wit h meander abs orbers collectors often four connection points
are used and all abs orber channels in a collector row are connected in parallel, see Fig. 12.
Page 15 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
Fig. 12: Solar collector field row wit h meander abs orbers (Source: [Knabl et.al, 2014])
The connection piping inside a collector field should be kept as short as possible by hydraulic dimensioning of pipe diameter to necessary flow. Fig. 13 shows some general possibilities for these connections. Case (a) should be the preferred way with a proper hydraulic calculation and an adaption of the
pipe diameters according to the flow rates at different locations of the field. In case (b) the so-called
Tichelmann-system is used. The idea is to have the same hy draulic length for each hy draulic path.
This leads to an equal flow distribution within the field. However, the overall piping length is longer
compared to case (a) leading to higher thermal losses and higher cost for the installation. In case (c)
the flow through the single collector rows can be adjusted by balancing valves. This solution as well
leads to higher investment cost and long-term maintenance effort compared to case (a). Case (d)
shows a very unfavourable constellation in terms of hydraulic design that should be avoided whenever
possible.
Fig. 13: Possibilities for the connection of collector rows (Source: [Knabl et.al, 2014] )
Page 16 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
3.5. Security layout for large solar collector fields
A possible layout for the security system of large solar collector fields is presented in Fig. 14. In most
cases it is sufficient to install one central security valve in the central room for the installations. In case
of stagnation caused by a malfunction or a low heat demand the security valve opens the hydraulic
circuit and prevents damages caused by overpressure. In this configuration the security valve is located on the supply line in order to prevent central installations as heat exchangers and pumps from
being damaged by steam entering the room with the installations. On the return line this protection is
ensured by the non-return valve illustrated in front of the pump, see Fig. 14.
The fluid leaving the solar circuit through the security valve is collected in a storage tank and can be
refilled to the circuit when the temperat ures are reduc ed. For security reasons this should first take
place in the following night when temperatures in the entire circuit are low and there is no risk of pressure hammers due to spontaneous evaporation in hot parts of the field.
In some countries an automated re-filling is not allowed after a blow-off through the security valve. If
so an additional overflow valve with an opening pressure below the one of the security valve can reduce the effort for the maint enance personal.
Solar collector f ield
Three-way-valve
deaeration
Securityvalve
Expansion
vessel
Central
heating plant
Security
vessel
Vessel f or
solar f luid
storage
Pressure
maintenance
Fig. 14: Security layout for large solar collector fields
Page 17 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
In the configuration displayed in Fig. 14 it is not possible to have closing valves bet ween the single
collectors and the security valve. If for maintenance reasons a possibility for separation of single collector rows is desired it can b realised by the solution also shown in Fig. 14: a two-way-valve on the
return line of the collector row and a three-way-valve opening to the ambient when the connection to
the security valve is closed.
For pressure maintenance different systems can be used. One possibility is to use diaphragm expansion vessels that can absorb the entire expansion volume of the circuit. This is the standard solution in
small scale applications. For large solar systems this solution can require very large volumes. If so
alternatively an automated pressure maintenance system can be used that is removing some fluid
from the circuit when the pressure increases and is re-filling fluid when the pressure is decreasing.
If diaphragm expansion vessels are used an additional security vessel should be installed in front of
the diaphragm expansion vessel to avoid damages of the diaphragm caused by high temperatures in
case of stagnation.
4. LONG-TERM THERMAL ENERGY STORAGE
When heat from solar collector fields is integrated into a district heating network oft en thermal energy
storage is necessary. Main reason is that the storage of thermal energy enables to cope with the deviating solar heat during the course of one day, several days or even of a year. So the surplus heat
supply during high solar irradiation periods can be stored for heat demand periods with low solar irradiation e. g. during the night or winter time. This increases the solar contribution to the system. At the
same time the thermal energy storage helps to balanc e the demand of varying heat capacity rates.
In other applications the storage of thermal energy decouples the supply of electricity from the supply
of heat. This is of importance when e.g. CHP-plants are integrated into district heating net works.
Three main applications of thermal energy storage are distinguished:
a) Buffer storage for short term energy storage
b) Large scale thermal energy storage (1 000 – 50 000 m³) for long term / seasonal thermal energy
storage
c) Large scale thermal energy storage for multiple usage (e. g. solar heat and waste heat)
Application a) is state of the art and is not discussed here. This section mainly deals with large scale
thermal energy storages of applications b) and c) (see Fig. 15).
Page 18 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
District heat station
Thermal energy storage
Fig. 15. Seasonal thermal energy storage within a district heating network
4.1. Construction concepts for large-scale thermal energy storages
Four main types of large-scale or seasonal thermal energy storages are used worldwide. The four
storage concepts shown in Fig. 16 include tank and pit thermal energy storage (TTES and P TES) with
and without liners, borehole thermal energy storages (B TES) and aquifer thermal energy storages
(ATES ).
Fig. 16: Construction concepts for large-scale or seasonal thermal energy storages
Page 19 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
New advanced storage techniques are phase change mat erials (P CM), thermo chemic al storages and
sorption storages. These techniques are however not yet ready for the use in large seasonal thermal
energy storage applications. For more details see [Schmidt, T. et.al, 2008].
Table 2 shows a comparison of the storage concepts presented in Fig. 16 regarding heat capacity and
geological requirements. Because of the lower specific heat capacities of a gravel-water mixture and
different underground materials storage volumes have to be significantly larger compared to water to
be able to store the same amount of heat at the same temperature differenc e.
Table 2: Comparison of storage concepts regarding heat capacity and geological requirements
TTES
PTES
BTES
ATES
storage medium
water
water*
gravel-water*
soil / rock
sand-wat er
60 - 80
30 - 50
15 - 30
30 - 40
1.3 - 2 m³
3 - 5 m³
2 - 3 m³
- drillable ground
- groundwater favourable
- high heat capacity
- high thermal conductivity
- low hydraulic conductiv-10
ity (kf < 10 m/s)
- natural ground-water
flow < 1 m/a
- 30 - 100 m deep
- natural aquifer
layer with high hy draulic conductivity
-5
(k f > 10 m/s)
- confining layers on
top and below
- no or low natural
groundwater flow
- suitable water
chemistry at high
temperatures
- aquifer thickness of
20 - 50 m
heat capacity in k Wh/m³
60 - 80
storage volume for 1 m³ water equivalent
1 m³
1 m³
geological requirements
- stable ground conditions
- preferably no groundwater
- 5 – 15 m deep
- stable ground conditions
- preferably no groundwater
- 5 – 15 m deep
*: Water is more favourable from the t hermody namic point of view. Gravel-water is often used if the
storage surface is to be designed for further usage (e.g. for streets, parking lots etc).
Seasonal thermal energy storages are still in the phas e of development and technological research.
However, each of the technologies is successfully demonstrated and operated in a number of plants.
The aim is to reach market readiness by 2020. Today’s research focuses on large multi-functional
heat storage systems that are additionally used for CHP optimization, power-to-heat technologies or
storage of waste heat. The combination of these applications significantly improves the overall economical feasibility and flexibility of such systems.
Page 20 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
4.1.1. Tank thermal energy storage
For the construction of ground buried thermal energy storages there are no standard procedures regarding wall construction, design of charging devic es, etc. available today.
Due to the size and geomet ry and also due to the requirements in terms of leak age detection and lifetime most techniques and materials have their origin in landfill construction. However, with res pect to
high operation temperature mat erials and techniques cannot be simply transferred.
Dimensions of pilot and research tank thermal energy storages and pit thermal energy storages that
have been realized over the last 25 years for solar assisted district heating systems, range from several 100 m³ up to 200 000 m³ [Schmidt, T. et.al, 2008; Schmidt, T. et.al., 2011].
Tank thermal energy storages have a structure made of concrete, of
steel or of fibre reinforced plastic (sandwic h elements). Concrete tanks
are built utilizing in-situ concrete or prefabricat ed concrete elements. An
additional liner (polymer, stainless steel) is normally mounted on the
inside surface of the tank. The insulation is fitted outside the tank.
Above ground tanks (see Fig. 17) are state of the art. Because of the
high investment cost they are in general only used as buffer tanks with
volumes up to 200 m³. Yet some above ground seasonal steel storage
tanks were built in Sweden in the 1980s [Schmidt, T. et.al, 2008].
In Crailsheim, Germany a 100 m³ buffer storage was built using prefabricated concrete elements and a stainless steel liner. A further 480 m³
Fig. 17:Above ground tank
in-situ concrete storage serves as a buffer for a 39 000 m³ BTES. Both
tanks can be operat ed at temperat ures up to 108°C as they are operated with a pressure level of three
bars [Bauer, D. et.al, 2007].
Additional TTES facts:
-
Multifunctional application area (short / long term storage)
-
Special case retrofitted TTES in Hamburg (DE): use as seasonal thermal energy storage for
solar heat and for optimization of a connected CHP-heating network [Schmidt, T. et.al, 2010]
-
Charging equipment has to avoid mixture of the thermal stratification
-
Waterproof liner made from stainless steel panels
-
Un-pressurized operation temperat ure up to 95°C
-
Wall construction has to consider combined heat and mass trans fer (steam)
Page 21 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
4.1.2. Pit thermal energy storage
Pit thermal energy storages (P TES ) are constructed without static constructions, by means of mount ing insulation and a liner in a pit. The design of the lid depends on the storage medium and geometry,
whereas in the case of gravel- or soil / sand-water thermal energy storages the lid may be constructed
identical to the walls. The construction of the lid of a water-filled PTES requires major effort and is the
most expensive part of the thermal energy storage.
By definition, pit thermal energy storages are entirely buried. In large P TES the soil dug from the
ground is used to create banks which mak e the storage somewhat higher than the ground level. The
lid can be only equipped with a membrane for rain and UV protection.
In Denmark a number of large water-filled P TES were realised, e.g. in Marstal (75 000 m³), Dronninglund (60 000 m³), Gram (125 000 m³) and Vojens (200 000 m³).
Table 3: Hot water vs. gravel water pit thermal energy storage [Schmidt, T. et.al, 2008]
Hot water
Gravel / sand / soil water
pit thermal energy storage
pit thermal energy storage
+ thermal capacity
+ operation characteristic
+ low static requirements
+ thermal stratification
+ simple cover
+ maintenance/repair
- thermal capacity
- sophisticated and expensive cover
- charging system
- cover load
- additional buffer storage where necessary
- costs for landfill of excavated soil
- maintenanc e / repair
- gravel costs
Additional PTES facts:
-
Gravel fraction of 60 to 70 % (if gravel is used)
-
Soil / sand instead of gravel can be used alternatively
-
Thermal insulation of cover, optionally of side walls and the bottom is necessary (depending
on storage volume)
-
Charging and discharging process by direct water exchange or indirect by plastic pipes in
gravel lay er
-
Max. storage temperatures 80 - 90°C, depending on temperature stability of liner material
-
Wall construction has to account for combined heat and mass transfer (steam)
-
Less vertical thermal stratification with gravel-wat er compared to pure water as storage medium
Page 22 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
4.1.3. Borehole thermal energy storage
Underground thermal energy storage systems can be divided into two groups [Sanner, B. et.al, 2001]:
-
Systems where a technic al fluid (water in most cases) is pumped through heat exchangers in
the ground, also called "closed" systems (BTES)
-
Systems where groundwater is pumped out of t he ground and injected into the ground by the
use of wells, also known as "open" systems (ATES)
An advantage of closed systems is the independency from aquifers and water chemistry, an advantage of open systems is the generally higher heat trans fer capacity of a well compared to a borehole.
This makes ATES usually the cheapest alternative, if the subsurface is hydrogeologically and hydrochemically suitable.
In a B TES the underground is used as storage material. There is no exactly separat ed storage volume. Suitable geological formations for this kind of storage are rock or water-saturated soils without
natural groundwater flow. Heat is charged or discharged by vertical borehole heat exchangers (BHE)
which are installed into boreholes with a depth of typically 30 to 100 m below ground surface. BHEs
can be single- or double-U-pipes or concentric pipes mostly made of synthetic materials (see Fig. 18).
BTES do not have a vertical but a horizontal temperature stratification from the centre to the borders.
This is because the heat trans fer is driven by heat conduction and not by convection. At the boundaries there is a temperature decrease as a result of the heat losses to the surroundings. The horizontal
stratification in the ground is supported by connecting the supply pipes in the centre of the storage and
the return pipes at the boundaries. A certain number of B HEs are hydraulically connected in series to
a row and certain rows are connected in parallel. During charging, the flow direction is from the centre
to the boundaries of the storage to obtain high temperatures in the centre and lower ones at the
boundaries of the storage. During discharging the flow direction is reversed.
At the top surfac e of the storage an ins ulation layer reduc es heat losses to the ambient. Side walls
and bottom are normally not insulated because of inaccessibility.
Compared to A TES systems BTES systems are easier to realise and to operate. They need less
maintenance and have a high durability. Because of the closed loop system BTES systems usually
require also more simple procedures for authority approvals, unless storage temperatures of more
than approx. 50°C are foreseen. Table 4 shows typical general values for B TES systems.
Table 4: Typical values of BTES system for heat storage application
Borehole diamet er
Borehole depth
100 - 150 mm
30 - 100 m
Distance bet ween boreholes
2-4m
Thermal ground conductivity
2 - 4 W/(m·K)
Flow rate in U-pipes
0.5 - 1.0 m/s
A verage capacity per m borehole
length
Min. / max. inlet temperature
20 - 30 W/m
Typical cost of B TES storage per
m borehole length
50 - 80 €/m
Page 23 of 29
-5 / > +90°C
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
ground
surface
double U- pipe
150 mm
heat
insulation
25 mm
borehole wall
pipe connection
in sand underlay
covering
layer
grouting
(e.g. bentonite-sandcement-suspension)
return
single U- pipe
borehole
supply
concentric pipe
grouting
borehole
heat exchanger
Fig. 18:
borehole depth (30 - 100 m)
injection tube for
grouting material
Common types and vertical section of borehole heat exchangers. (Sourc e: ITW, University of
Stuttgart)
Additional BTES facts:
-
Modular design: additional boreholes can be easily connected and the storage can be expanded
-
Because of low capacity rate for charging and discharging often a buffer storage is integrated
into the system
-
Permission from water authorities normally necessary for heat storage application
4.1.4. Aquifer thermal energy storage
Aquifers are below-ground widely distributed and water filled permeable sand, gravel, sandstone or
limestone layers with high hydraulic conductivity. If there are impervious layers above and below and
no or only low natural groundwater flow, they can be used for thermal energy storage. In this case, two
wells (or groups of wells) are drilled into the aquifer layer and serve for extraction or injection of
groundwater. During charging periods cold groundwater is extracted from the cold well, heated up by
the heat source and injected into the warm well. In discharging-periods the flow direction is reversed:
warm water is extracted from the warm well, cooled down by the heat sink and injected into the cold
well. Because of the different flow directions both wells are equipped wit h pumps, production- and injection pipes.
Because the storage volume of an A TES cannot be thermally insulated against the surroundings heat
storage at high temperatures (above 50 °C) is normally only efficient for large storage volumes (more
than approximately 20 000 m³) with a favourable surface to volume ratio. For low temperature or cooling applications also smaller storages can be feasible.
Page 24 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
Properties and conditions that have to be considered
are:
-
Stratigraphy (sequence of layers)
-
Grain size distribution (mainly prime porosity
aquifers)
-
Structures and fracture distribution (mainly fractured aquifers)
-
Aquifer depth and geometry, hydraulic boundaries included
-
Storage coefficient (hydraulic storage capacity)
-
Leakage factor (vertical hydraulic influence)
-
Degree of consolidation (hardness)
-
Thermal gradient (temperature increase with
depth)
-
Static head (ground water level)
-
Natural ground water flow and direction of flow
-
Water chemistry
Fig. 19: Layout of a well for charging and
discharging. (Source: Geothermie Neubrandenburg GmbH, DE)
Additional ATES facts:
-
Aquifers near to the surface are often used for drinking water extraction
-
At high charging t emperat ures water treatment can be necessary (chemical and biologic al
processes can lead to deposition, corrosion and degradation in the system)
-
Permission from water authorities normally necessary for heat storage application
Page 25 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
4.1.5. Cost of storages
Construction cost of the four storage concepts vary significantly. However, there is not one optimum
storage concept for all applications and not every storage concept can be built everywhere. Fig. 20
shows a typical cost allocation for one example of eac h of the four storage concepts.
5,700 m³ TTES in Munich, 2007
4,500 m³ PTES in Eggenstein, 2007
exploitation of
storage volume
13%
charging device others
3%
3%
charging
device
13%
liner
26%
ground
work
12%
ground
work
16%
liner
35%
953 000 €
insulation
13%
433 000 €
exploitation of
storage volume
38%
insulation
30%
37,500 m³ BTES in Crailsheim, 2008
20,000 m³ ATES in Rostock, 2000
ground work
3%
ground work
2%
others
6%
others
13%
charging
device
18%
171 000 €
520 000 €
insulation
24%
Fig. 20:
charging
device
39%
exploitation
of storage
volume
52%
exploitation
of storage
volume
45%
Exemplary alloc ation of construction cost for different storage concepts (cost figures in Euros
(€) without planning and VA T)
Fig. 21 presents the cost data of built pilot and demonstration plants. The listed storages are high
temperature heat storages (working temperatures up to 95 °C) and are mostly integrated into solar
district heating plants with seasonal storage.
Fig. 21 shows a cost decrease with increasing storage volumes. Appropriate sizes for seasonal heat
storage are located between 2 000 and 20 000 m³ water equivalent. Within this range the investment
costs vary between 40 and 250 Euro/m³. Generally, TTES are the most expensive ones. On the other
hand, they have some advantages concerning the thermodynamical behaviour and they can be built
almost everywhere. The lowest costs can be reac hed with A TES and B TES. However, they often need
Page 26 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
additional equipment for operation like buffer storages or wat er treatment and they have the highest
requirements on the local ground conditions.
500
Investment cost per m³ water equivalent [€/m³]
Ilmenau
450
Crailsheim
Tank (TTES)
Pit (PTES)
Rottweil
Boreholes (BTES)
Steinfurt
400
Aquifer (ATES)
Others
350
300
250
Stuttgart
Hanover
Hamburg
200
Eggenstein
150
Munich
Chemnitz
Friedrichshafen
100
Attenkirchen (Hybrid)
Neckarsulm-1
Marstal-1 (DK)
Neckarsulm-2
Brædstrup (DK)
50
Crailsheim
Marstal-2 (DK)
Rostock
0
100
1,000
10,000
100,000
Storage volume in water equivalent [m³]
Fig. 21:
Specific storage costs of demonstration plants (cost figures without VAT, storages without
country code are loc ated in Germany).
The economy of a storage system depends not only on the storage costs but also on the thermal performance of the storage and the connected system. Therefore each system has to be examined separately. To determine the economy of storage, the investment, maintenance and operational costs of
the storage have to be related to its thermal performance.
4.1.6. Design guideline
For the choic e of a suitable storage concept for a specific plant all relevant boundary conditions have
to be taken into account: local geological situation, system integration, required size of the storage,
temperature levels, power rates, no. of storage cycles per year, legal restrictions etc. Finally, decisions
should be based on an economic optimisation of the different possibilities.
For all concepts a geological investigation has to be made in the pre-design phase. The highest demands with regard to this are made by A TES and B TES. The legal requirements have to be checked
in the pre-design phase as well. In most countries the usage of the ground for heat storage has to be
approved by local water aut horities to make sure that no interests regarding drinking water are af-
Page 27 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
fected. This can also become necessary if the ground surrounding a storage tank is heated up by heat
losses.
After construction the storages have start -up times between two to five years, depending on the storage conc ept, to reach normal operating conditions. Within this time, the surrounding ground is heat ed
up and the heat losses of the storage are higher than during long-term operation.
A crucial point in all storage applications are return temperat ures from the heat distribution systems. In
systems without a heat pump the return temperature defines the lowest temperature level in a system
– and by this the lowest usable temperature level for discharging the storage. In many installations
measured return temperatures are much higher than design values. This results directly in a strongly
reduced useable heat capacity and a lower performance of the connected heat storage.
Page 28 of 29
A CONCERTO Project
Guidelines for the implementation of large scale
solar thermal energy systems and long-term thermal energy storage into district heating systems
5. REFERENCES
Bauer D., Heidemann W., Müller-Steinhagen H., (2007), Der Erdsonden-Wärmespeicher in Crailsheim,Proc. OTTI, 17. Symposium Thermische Solarenergie, Kloster Banz, Bad Staffelstein, Germany
st
Dalenbäck, J-O. (2013), Decentralized SDH systems – Swedish experiences, Proceedings of the 1
International Solar District Heating Conference in Malmoe, Sweden. A vailable from: www.solar-districtheating.eu [accessed 2013].
Knabl, S. Fink, C. Ohnewein, P. Mauthner, F. Hausner, R. (2014): Guidelines for requirements for collector loop installation including precautions for safety and expansion, AEE – Institute for Sustainable
Technologies (AEE INTE C), in: IEA -SHC Tech Sheet 45.A.2, IEA-S HC Task 45, International Energy
Agency, France
Mangold D., Benner M., Schmidt T., (2001), Langzeit -Wärmespeicher und solare Nahwärme, Bine
Informationsdienst, Profiinfo, 1/01, Germany
Sanner, B. (2001) A different approach to shallow geothermal energy - Underground Thermal Energy
Storage (UTES), Int ernational Summer School on Direct Application of Geothermal Energy. Skopje,
Bad Urach, Germany
Schmidt, T. Mangold, D. Ochs, F. (2008): Seasonal Thermal Energy Storage, in: High solar fraction
heating and cooling systems with combination of innovative components and methods – State o fthe
Art of Similar Applications, EU Project High-Combi, Workpackage WP 2.2, Deliverable D6,
www.highcombi. eu
Schmidt T., Mangold D., (2010), Conversion of Germanys first seasonal solar thermal energy storage
into an innovative multifunctional storage, EuroSun, International Conference on Solar Heating, Cooling and Buildings, Graz, Austria
Schmidt T., Mangold D., Sørensen P.A., From N. (2011), Large-scale heat storage, IRES 2011 6t h
International Renewable Energy Storage Conference, Eurosolar, Berlin, Germany
Soerensen, P-A. and Nielsen, J-E (2011), Smart District Heating, Proceedings of Intersolar Conference 2011, PlanE nergi, Denmark
Solites (2015): www.saisonalspeicher.de (soon also available in English:
www.seasonalheatstores.com, Solites, Stuttgart
Page 29 of 29