Solar Heat Networks

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Solar Heat Networks Poster

Solar Heat Networks (SHN), sometimes called Solar District Heating (SDH) systems, are a type of low carbon heat network[1][2] where a proportion of the consumer's heat is sourced from a centralised large-scale solar heat farm. Essentially, a solar heat network uses a large number of solar panels to heat water, which is then stored and piped to homes, factories, offices and other buildings, to heat them up. Typically, solar heat farms are ground-mounted and range in capacity from 0.5 to 100 megawatt (MW) peak. Solar heat networks can be classified by their solar fraction (f)[3], which is the proportion of the annual heat load supplied by solar energy:

  • Solar fraction less than approximately 30 %: the solar heat farm is sized to meet the maximum summer peak load and would typically include an above ground thermal storage tank (accumulator) that operates to balance the daily variations in heat demand.
  • Solar fraction greater than approximately 30 % (typically 40 to 60 %): the solar heat farm is sized to meet more than the minimum summer peak load and would typically include an underground pit or borehole seasonal thermal store.
  • Solar fraction greater than 90 %: very high solar fractions are currently economically challenging[4], however technically possible. For example, the Drake Landing Solar Community system in Canada achieved a solar fraction of 97% in its fifth year of operation.[5][6]

The solar heat farm for the solar heat network can also be decentralised, with several distributed relatively small solar heat farms feeding into a single centralised plant. This type of setup is typically used in cites where access to land is limited. A good example of this type of system is operating in Lehen (City of Salzburg, Austria) where a distributed 2,000 m² solar heat farm (installed on the roofs of the buildings) supplies 30 % of the heat required by the network[7].

Solar heat farms currently represent about 1 % of global solar heat installed capacity. This is anticipated to rise to at least 15 %, however, as solar heat farm technology expands beyond Denmark, which is by far the leader in this technology worldwide.[8]

The main argument for the use of solar heat networks rather than solar heat systems for individual buildings is:

"To reach a high solar fraction for a given town, a large number of roof mounted solar collector systems will in general be required, as the size of each system is limited by the roof area available. Given a certain area required to reach a specific solar fraction, a large ground-mounted system will often have a much lower total cost due to economy of scale."[9]



Northview Junior High School (USA) solar energy system, designed and installed by Honeywell in 1974[10]

The first large-scale solar heat networks started to be deployed in the USA and Europe around the 1970s.

In 1973, the Research Applied to National Needs (RANN) organisation in the USA fast-tracked the Solar Energy School Heating Augmentation Experiment project with the aim of demonstrating the use of solar heat at large-scales. The Northview Junior High School (Osseo, Minnesota) solar energy system (see image on left) was designed as a solar heat network and consisted of a 465 m2 ground-mounted solar heat array (flat plate collectors) and a 13.6 m3 hot water tank (the system was designed and installed by Honeywell).[10]

In Europe, the first solar heat networks were installed in Sweden from around 1979[11], where “in 1979 and in 1980 solar heating plants connected to newly-built residential areas at Ingelstad, outside the city of Växjö, and at Lambohov, outside the city of Linköping, were taken into operation.” [12] By 2007, there were around 119 solar heat networks installed across Europe.[13]

Solar heating plan in Saltum, Denmark (1988)[14]

The first solar heat network in Denmark was put into operation in the spring/summer 1985 at Vester Nebel. It consisted of 296 m2 of flat plate collectors, which were installed on the field in front of the district heating substation (straw and oil) and supplied approximately 100 houses with solar heat.[15][16]

The second system was installed in 1988 in Saltum, a village of about 600 inhabitants. This system, installed by Saltum Varmevaerk A.m.b.A, consisted of 1,100 m2 of ground-mounted flat plate collectors (5 % solar fraction).[17] The solar heat farm was installed on a slope at the boarder of the village and mounted on concrete blocks placed on top of the soil (see image on right).[18]

The concept of sizing a solar heat farm to provide 100 % of the summer heat load of the heat network (approximately 20 % solar fraction) appears to have originated from the development of the Marstal Fjernvarme A.m.b.a. (Marstal District Heating Ltd.) solar heat farm in Ærø, Denmark[19]. In the document ‘Summary technical description of the SUNSTORE 4 plant in Marstal’ it states that the system was designed (in 1994) so that the “solar fraction should be 100 % in the summer months, the demand to the supply temperature was 72 –70° C” and “to comply with this demand, it was considered to build the system with adjustable flow to reach a higher supply temperature, even when the solar radiation was low. This method of controlling a solar collector plant was however not very known, most plants runs with constantly high flow and a variable supply temperature”[20][21][22][23].

Illustration of a typical solar heat network, similar in setup to what is used at Marstal [24]

In 2018 Arcon-Sunmark Large-Scale Solar Systems Integration Co[25] built a “22,000 m2 solar collector field will from now on cover more than 90% of the heat demand in Langkazi. The system also has a 15,000 m3 storage pit in order to store the energy so it can be used when demanded. As part of the project a 25,000 meter long piping infrastructure has been constructed in the town in order to connect 82,000 m2 households to the solar district heating system.”[25]


The Figure below, from the 2019 IEA SHC Solar Heat Worldwide solar heat market report, shows that the current, worldwide, installed collector area of large-scale solar heat systems (>350 kWth; 500 m) is around 1.8 million m2 (339 systems in total).[8]

Large-scale systems for solar district heating and large residential, commercial and public buildings worldwide – annual achievements and cumulated area in operation in 2018[8]

"Worldwide [2016 figures], Denmark is a good example for a mature and commercial solar district heating market but other markets are catching up, especially China. In several other countries smaller niche markets exist, such as in Austria where 28 systems >500 m² are installed to feed into district heating networks, smaller micro grids in urban quarters or local biomass heating networks and to supply large residential, commercial and public buildings. Other countries to note are Germany with 28 large-scale systems (some of these with seasonal storage), Sweden (24 systems), France (17 systems), Poland (14 systems), Greece (13 systems) and Switzerland (10 systems)."[26] Although Germany is currently considered a niche market, relative to Denmark, experts anticipate accelerated growth in the coming years[27]:

“By 2050, the Federal Government would like to massively increase the contribution of solar thermal energy to district heating. With a share of 15 percent, this corresponds to 12 terawatt-hours per year. This requires an installed capacity of around 21 gigawatts, i.e. a collector area of around 30 million square meters. We therefore need an additional 1 million square meters per year. That means that the roll-out has to increase by a factor of 50 compared to this year!” (Dirk Mangold, head of the Steinbeis Research Institute Solites)[28]

Collector area and capacity of large-scale systems by country in 2018[8]
Meteonorm map of yearly sum of global horizontal irradiation (1991–2010) for Europe[29] (Source: Meteonorm)

The 2019 Solar Heat Worldwide report[8] shows that more than 1.3 million m2 of the large-scale solar heat systems are used for solar heat networks in Denmark. Given that Denmark does not have exceptionally high levels of solar radiation compared to the rest of Northern Europe (much of Denmark receives about 1000 kWh/m2, which is equivalent to about 2.7 kWh/m2 per day)[29], why did the solar heat network market in Denmark develop so rapidly over the last 10 years? The answer to this is complex,[30][31][32][33] however, Denmark does have 2 important features which contributed to this growth:

  1. existing heat networks;
  2. relatively high natural gas price[34][35][36].

Eurostat natural gas prices for household consumers, second half 2018 (EUR per kWh)[37] have created a comprehensive solar heat farm database, with the top solar heat networks, in terms of array area, shown in the table below.

Top Solar Heat Networks in Terms of Area[38]
City Operation Start Owner Country Area (m2) Capacity (kWth)
Silkeborg 2016 Silkeborg Forsyning Denmark 156694 110000
Vojens 2012 Vojens Fjernvarme Denmark 70000 49000
Gram 2009 Gram Fjernvarme Denmark 44836 31385
Dronninglund 2014 Dronninglund Fjernvarme Denmark 37573 26300
Zhongba 2019 City of Zhongba China 34650 20000
Marstal 1996 Marstal Fjernvarme Denmark 33300 23300
Gråsten 2012 Gråsten Fjernvarme Denmark 30206 21144
Ringkøbing 2010 Ringkøbing Fjernvarmeværk Denmark 30000 21000
Brønderslev 2016 Brønderslev Forsyning Denmark 26929 19000
Toftlund 2013 Toftlund Fjernvarme Denmark 26000 18200
Aalestrup 2016 Aalestrup-Nørager Energi Denmark 24129 16900

Market Potential

Solar heat potential in the UK for 20 % solar fraction and price limit of solar heat in €/MWh[9]

PlanEnergi, as part of the IEA Solar Heating and Cooling TCP ‘Solar Heat and Energy Economics in Urban Environments’ Task, estimated the solar heat potential in Europe, assuming 20 % solar fraction and solar heat cost limits of between 25 and 60 €/MWh.[9]

The Figure on the right shows the potential for solar heat production in the United Kingdom for the full range of cost limits. The analysis shows that at a cost of 35 €/MWh, around 1,800 GWh/yr of solar heat potential could be unlocked.

The Figure below shows the potential solar heat production for a 30 €/MWh cost limit for countries in Europe. Poland has a significant potential for solar heat, but the authors of the report suggest that this is currently limited by legislative restrictions which inhibit the use of land for anything other than agriculture. The United Kingdom has the second largest potential in Europe for solar heat production.

The main conclusions were:

  • “The analysis indicates that a roll-out of large-scale SDH is possible and economically feasible in most countries – there seem to be plenty of space; and,
  • Solar thermal and seasonal storages in intelligent combinations with other production options can improve feasibility of fully decarbonised DH systems”.[9]
Potential solar heat production targeting 20 % solar fraction with a cost limit of 30 €/MWh[9]

A report by the Asian Development Bank, published in July 2019, concluded that “the market potential [in China] is huge. A conservative estimate suggests that 150–200 projects could be implemented in the coming years”.[39]


Levelised Cost of Heat (LCOH)

In the economic evaluation of heat networks, Levelised Cost of Heat (LCOH) can be used to compare costs of different sources of heat generation on a consistent basis. The LOCH[40], measured in £/MWh or p/kWh, is the average annual minimum price at which solar heat network operator can sell the heat to the consumer in order to break even over the lifetime of the project[41]. It can also be called the Levelised Cost of Energy (LCOE), Levelised Energy Cost (LEC) or the Levelised Cost of Generation (LCOG):

Levelised cost of generation is the discounted lifetime cost of ownership of using a generation asset converted into an equivalent unit cost of generation in £/MWh or p/kWh. This is sometimes called a life cycle cost, which emphasises the cradle to grave aspect of the definition.[42]

One of the major advantages of supplying solar heat through a heat network is that the Levelised Cost of Heat (LOCH) is significantly lower due to economies of scale. The table below shows the LOCE (in Euros) for three solar water heating systems using data collected in 12 countries in 2016: Australia, Austria, Brazil, Canada, China, Denmark, France, Germany, India, Israel, South Africa and Turkey. It shows that the larger the collector array area, the lower the LOCE.

Levelised Cost of Heat (LOCH) for different solar heat applications[8]
System Type Typical Collector

Array Area (m2)



Solar heat networks 10,000 4
Solar heat system for multi-family homes 50 8
Solar heat system for multi-family homes 4 - 6 12

In 2017, the IEA Solar Heating and Cooling (SHC) TCP investigated the LOCH for a range of solar heat system types for Denmark, to evaluate how the economies of scale can enable the delivery of low-cost clean heat (see figure below). The Figure shows that for solar heat systems installed on individual buildings (either Single-Family Houses (SFH) or Multi-Family Houses (MFH)), that the lowest LOCH that can be achieved is around 12.1€-ct/kWh, whilst the LCOH for solar heat network systems (>10,000 m²) goes down to 3.6 €-ct/kWh (including the cost for a diurnal storage). This is a potential 70 % reduction in cost to the consumer. The results clearly show why solar heat networks have been successful in Denmark:

"The low LCOH in combination with a tax on natural gas makes large-scale solar thermal a commercial business case for district heating (consumer) co-operatives all over Denmark"[43]

Specific investment costs (left Y-axis in €/m2) and Levelised Cost of Heat (right Y-axis in €-ct/kWh) for different solar thermal applications in Denmark (2017) (orange: small-scale domestic systems, green: large-scale commercial applications) [26]

Example Cost of a Solar Heat Farm

An example of the typical costs of a solar heat farm, case study for the solar heat farm at the Solarbioenergy village Mengsberg in Germany.[44][45]


Heat Networks Investment Project (HNIP)

Design Guidance

Typical System Specification

A typical system in Denmark would be for a small town of 4,000 inhabitants, in the range of 5,000 - 15,000 m2, with an annual solar collector yield of 400 kWh/m2, investment of €1.3-3.0 million, 20 % solar fraction, located within 200 m from the main network, diurnal storage of around 0.2 m3 per square meter of collector, financed by a long-term, low interest loan (20-25 years, 2-3 % interest rate).[9]

How much land area do you need for a solar heat farm?[46]

Example diagram of solar heat combined with a gas boiler
Illustration showing solar fraction with short term heat store and seasonal heat store

Design Guides

The Danish consultancy, PlanEnergi, in association with the Danish District Heating Association, have produced an overview of the main lessons from installing solar heat networks in Denmark. The guide, Solar District Heating Inspiration and Experiences from Denmark, “brings together some of the most important experiences from the inspiring development of solar district heating systems that have been seen in Denmark in the past years”.[4] The guide outlines the main stages of the development of a solar heat network project:

  • Preparation and planning
  • Establishing the system
  • Commissioning
  • Operation and maintenance

The guide also brings together some lessons on developing solar heat networks at: Løgumkloster, Jægerspris, Ejstrupholm, Marstal, Gram, and Dronninglund.

The IEA Solar Heating and Cooling TCP has also produced a 'Design Handbook: Installation, Commissioning and Operation of Large Scale Solar Thermal Plants'[47] which provides a broad overview of the design and installation of large-scale solar heat systems.

Installation Instruction Videos

Aalborg CSP A/S, a "Danish developer and supplier of solar district heating solutions"[48], have produced a series of instructional videos showing the installation of solar heat farms:

Detailed Guidance

Links to detailed guidance on various aspects of the design, feasibility, modelling and performance of solar heat networks, amongst other topics, can be found in the table below.

Funding Models
Public participation models for solar district heating Austria Solar Download
A funding programme of the Climate and Energy Fund for the Heat Transition Austria Solar Download
General Guidance Author / Source Download
Nomenclature Download
Solar district heating: Inspiration and experiences from Denmark PlanEnergi Download
Success factors in solar district heating Download
Solar district heating guidelines: Collection of fact sheets Download
Categories of solar district heating systems Download
Introduction to solar district heating: From idea to operating system Download
Supervision of construction and commissioning Download
Solar heat combined with other fuels Download
Preliminary investigations Download
Solar district heating in urban planning Download
Calculation tools and methods Download
Implementation of solar heat networks in cites Download
Collector Field
Solar collectors Download
Where to place the solar collectors Download
Temperature variations Download
Correction of collector efficiency parameters depending on variations in collector type, fluid type, collector flow rate and collector tilt DTU Download
Requirements & guidelines for collector loop installation AEE-INTEC Download
Simulation of large collector fields DTU Download
Thermal and hydraulic investigation of large-scale solar collector field Chemnitz University of Technology Download
Heat exchangers Download
Control strategies Download
Pipes, pumps and pressure ratings Download
Safety components Download
Thermal Storage
Design Guide: Stored Hot Water Solutions in Heat Networks 2019 Hot Water Association Download
Construction concepts, costs and design guidelines for large-scale or seasonal thermal energy storages Download
Seasonal thermal energy storage: Report on state of the art and necessary further R+D SOLITES Download
Seasonal bore hole thermal energy storage: Guidelines for materials & construction NRCAN Download
Seasonal water pit heat storage: Guidelines for design & construction PlanEnergi Download
Underground thermal energy storage (UTES): State-of-the-art, example cases and lessons learned HEATSTORE Download
Long term storage and solar district heating PlanEnergi Download
Large storage systems for heat networks FLEXYNETS Download
Design and construction of large-scale heat storage for district heating in Denmark PlanEnergi Download
SUNSTORE 3: Pit heat store with heat pump PlanEnergi Download
SUNSTORE 4: Final Project Report (large scale (district) heating system with long term heat storage) EU Seventh Framework Programme Download
Investigating heat storage technologies and large heat pumps for the bridge heating system (Danish) PlanEnergi Download
Boreholes in Brædstrup PlanEnergi Download
Categorization and applications of large solar heating and cooling systems SOLID Download
Contracts and Financing
Ownership and financing Download
Permissions, tendering, contracts and guarantees Download
Heat networks: ensuring sustained investment and protecting consumers (General) BEIS Download
Heat networks market study (General) Competition and Markets Authority Download
Solar Heating from A-Z: Large-Scale Turnkey Solutions Arcon Sunmark Download
Financing Heat Networks in the UK (Guidebook) Grant Thornton UK LLP Download
Performance Guarantees
Performance guarantee: Guarantee power outputof large collector fields PlanEnergi Download
Performance guarantee: Giving and checking guarantee for annual outputof collector fields PlanEnergi Download
ESCO Models
ESCo models: General SOLID Download
ESCo models: Best practice example: Lisbon SOLID Download
ESCo models: Best practice example: Graz SOLID Download
ESCo models: Energy performance contracts SOLID Download
In-situ Performance
Dynamic Collector Array Test (D-CAT) - Quality assessment of large-scale solar thermal plants under real operating conditions Climate and Energy Fund Download
Purpose of monitoring systems: Information about the selection of data, recording of data and accuracy of monitoring equipment Download
Drake Landing Solar Community: 10 years of Operation DLSC Download
Ecosystem Services
The Natural Capital Value of Solar Solar Trade Association Download
The Effects of Solar Farms on Local Biodoversity: A Comparative Study Solar Trade Association Download
Overview of Opportunities for Co-Location of Solar Energy Technologies and Vegetation NREL Download
Native Vegetation Performance under a Solar PV Array at the National Wind Technology Center NREL Download
Biodiversity Guidance for Solar Developments BRE National Solar Centre Download
Potential ecological impacts of ground-mounted photovoltaic solar panels BSG Ecology Download
Evidence review of the impact of solar farms on birds, bats and general ecology 2016 (NEER012) Natural England Download
Solarparks - Gewinne fürdie Biodiversität (German) bne Download
Solarthermieanlage als Biotop (Solar use of areas should bring ecological added value) (German) Bundesministeriums für Wirtschaft und Energie Download

Case Studies

Location Author Array Size Link
Silkeborg, Denmark Arcon-Sunmark 156,694m2 Link
Vojens, Denmark Arcon-Sunmark 70,000m2 Link
Dronninglund, Denmark Arcon-Sunmark 37,573m2 Link
Trustrup-Lyngby, Denmark Arcon-Sunmark 7,245m2 Link
Oulun Seudun Sähkö, Finland Savosolar 356 m2 Link
Etelä-Savon Energia, Finland Savosolar 118 m2 Link
Myrskylä municipality, Finland Savosolar 60 m2 Link
Grenaa Varmeværk, Denmark Savosolar 20,673 m2 Link
Jelling Varmeværk, Denmar Savosolar 20,125 m2 Link
FORS A/S, Denmark Savosolar 9,200 m2 Link
Ystad Energi AB, Sweden Savosolar 534 m2 Link
Løgumkloster Fjernvarme, Denmark Savosolar 15,300 m2 Link
Lolland Forsyning, Denmark Savosolar 4,700 m2 Link



Dronninglund Fjernvarme (Dronninglund District Heating) by PlanEnergi and Niras


Hadsten Varmeværk


Drake Landing Solar Community

SWLB | SolarHeatGrid

Further Resources


The International Energy Agency (IEA) Solar Heating and Cooling (SHC) Technology Collaboration Programme (TCP) has run three Tasks on solar heat networks:

  • Towards the Integration of Large Systems SHC into District Heating and Cooling. This is currently active and led by Sabine Putz from SOLID. The task "aims to identify technical and economic requirements for a commercial market introduction of solar district heating and cooling (DHC) in a broad range of countries".
  • Large Scale Solar Heating and Cooling Systems. This task operated from January 2011 to December 2014 and was led by Jan Erik Nielsen from PlanEnergi in Denmark. "The main objective of this task is to assist in the development of a strong and sustainable market of large solar heating and cooling systems by focusing on cost effectiveness, high performance and reliability of systems. The work's main focus will be on the system level and how to match a system configuration to the local needs and conditions".
  • Central Solar Heating Plants with Seasonal Storage. This task operated from June 1979 to June 1988 and it showed that: "Through the use of central solar heating plants with large, seasonal storage systems, solar energy can be collected during the summer and stored until needed in the heating season when solar radiation is often insufficient. As a result of the Task 7 collaboration, significant advances were made in this technology”.

Solar District Heating Europe is a website which gathers the results of three European research projects:

  • SDHtake-off – Solar district heating in Europe (2009 – 2012)
  • SDHplus – New business opportunities for solar district and cooling (2012 – 2015)
  • SDHp2m – Solar District Heating from policy to market (2016 – 2018)

The website contains a solar heat network plant database and a searchable report database.

SolarHeatData.EU was stated in 2008 with the “aim of gathering both general descriptions of the large scale solar heating systems in Denmark and showing the solar heat output from these systems”. The site shows the location of each plant, information about the plant (year of construction, number of collectors, estimated annual heat production) and current generation data. Information on the monitoring methods and main features of the website can be found here.

International Solar District Heating Conference

The International Solar District Heating Conference focuses on “sharing market development and policy making experience, recent projects as well as latest trends in technology and system concepts”. [49]

  • 1st International Solar District Heating Conference (2013) Malmö, Sweden
  • 2nd International Solar District Heating Conference (2014) Hamburg, Germany
  • 3rd International Solar District Heating Conference (2015) Toulouse, France
  • 4th International Solar District Heating Conference (2016) Billund, Denmark
  • 5th International Solar District Heating Conference (2018) Graz, Austria

Proceedings and presentations for the conference can be download here.

Solar Thermal World is a news website focusing on solar heat. News articles related to solar heat networks can be found here.

General Heat Network Resources

Heat and the City is a research team studying sustainable heating for low energy buildings and cities. One of their projects is the UK District Energy Vanguards Network which is a "UK-wide knowledge exchange network whose core consists of local authorities and housing associations actively developing or operating district energy systems".


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