ArticlePDF Available

Is It Possible to Obtain More Energy from Solar DH Field? Interpretation of Solar DH System Data

Authors:

Abstract and Figures

Europe has a course to zero emissions by 2050, with a strong emphasis on energy sector. Due to climatic conditions in Latvia, district heating (DH) plays an important role in the energy sector. One of the solutions to achieve the set goals in DH is to introduce emission-free technology. Therefore, the popularity of installation of large-scale solar collector plants continues to increase in DH in Europe. The first large-scale solar collector field in the Baltic States was installed in 2019. Solar collector active area is 21 672 m ² with heat storage water tank 8000 m ³ . The article shows the first operation results of this system and evaluates influencing factors. The results of the analysis show that system productivity is mainly demanded by solar radiation, and the strongest correlation between these parameters were established in May. The highest correlation between ambient air temperature and produced thermal energy is reached when ambient air temperature is between 7 °C to 15 °C and production process has not been externally regulated. The temperature difference between flow and return temperatures of the heat carrier affect solar collector performance minimally and strong correlation was not observed.
Content may be subject to copyright.
Environmental and Climate Technologies
2021, vol. 25, no. 1, pp. 12841292
https://doi.org/10.2478/rtuect-2021-0097
https://content.sciendo.com
1284
©2021 Roberts Kaķis, Ilze Poļikarpova, Ieva Pakere, Dagnija Blumberga.
This is an open access article licensed under the Creative Commons Attribution License (http://creativecommons. org/
license s/by/4.0).
Is It Possible to Obtain More Energy from Solar DH
Field? Interpretation of Solar DH System Data
Roberts KAĶIS1 *, Ilze POĻIKARPOVA2, Ieva PAKERE3, Dagnija BLUMBERGA4
1–4 Riga Technical University, Institute of Energy Systems and Environment, Azenes iela 12/1, Riga,
LV-1050, Latvia
Abstract Europe has a course to zero emissions by 2050, with a strong emphasis on energy
sector. Due to climatic conditions in Latvia, district heating (DH) plays an important role in
the energy sector. One of the solutions to achieve the set goals in DH is to introduce
emission-free technology. Therefore, the popularity of installation of large-scale solar
collector plants continues to increase in DH in Europe. The first large-scale solar collector
field in the Baltic States was installed in 2019. Solar collector active area is 21 672 m2 with
heat storage water tank 8000 m3. The article shows the first operation results of this system
and evaluates influencing factors. The results of the analysis show that system productivity is
mainly demanded by solar radiation, and the strongest correlation between these parameters
were established in May. The highest correlation between ambient air temperature and
produced thermal energy is reached when ambient air temperature is between 7 °C to 15 °C
and production process has not been externally regulated. The temperature difference
between flow and return temperatures of the heat carrier affect solar collector performance
minimally and strong correlation was not observed.
KeywordsDistrict heating; large scale solar collector field; regression analyses
1. INTRODUCTION
A Green New Deal target is zero emission by 2050, as well as not to exceed global warming
under 1.5 °C. One of the main steps in this resolution is to decarbonise the energy sector. The
Green New Deal will promote decarbonisation of the economy in the energy sector and ensure
longer investment periods [1].
In recent years, use of sustainable heat sources in district heating (DH) has been growing,
heating network losses are reduced and digitalization is taking place to achieve the targets. A
variety of sustainable energy sources are used in DH in the EUsolar energy, heat pumps,
waste heat [2]. The system is moving to the 4th generation by implementing low potential heat
sources, lowering network temperature, integrating smart grid technology, different storage
technologies and promoting interaction with prosumers. When the operation of DH systems
becomes more complex, it is important to plan long-term development of heat production,
transmission and energy efficiency measures at the consumers side [3], [4].
Solar energy is a high potential for use in district heating [5][7]. Solar collectors are
emission-free technology and an appropriate solution to reduce CO2 in the DH system.
Implementation of solar collector systems is increasing, because it uses unlimited solar energy
and have low maintenance costs [8]. Denmark is the world leader in the use of large-scale
solar collectors in DH, where approximately 160 000 m2 of solar collector active area have
* Corresponding author.
E-mail address: roberts.kakis@rtu.lv
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1285
been installed [9]. In Denmark, 70 % of all large-scale solar collector are installed by
Arcon-Sunmark. The high solar collectors’ efficiency and long warranty decrease the payback
time [10].
Climatic conditions are one of the main influencing factors in the operation of solar
collectors [11], [12]. Yearly, the global radiation in Denmark is approximately 1000
1150 kWh/m2, but total radiation on collector surfaces is around 11001200 kWh/m2. The
total radiation to the collector area is affected by the installation angle, which in Denmark is
in average 30 to 40 degrees, depending on the goal to get the maximum capacity in the
summer or the maximum produced energy [13]. In Latvia, yearly global radiation is
approximately 10001200 kWh/m2 [14].
Fig 1. Global irradiation in Latvia [14].
As can be seen in Fig. 1, Latvia is divided into zones in the north-west of Latvia radiation
is higher. However, radiation levels in Denmark and Latvia can be considered equivalent.
The main aim of the article is to evaluate the performance of the first large scale solar
system in Baltic States after one-year operation period by determining the potential solar yield
under different impact factors.
2. METHODOLOGY
Several solar collector systems influencing factors and their importance have been
evaluated in this research. Influencing indicators also show how effectively the overall system
works.
2.1. Case Study
In this paper the analysed case study is large scale solar collector system installed in Latvia,
Salaspils. The total active area of collectors is 21 672 m2 with integrated water storage tank
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1286
of 8000 m3. The system is operating since September 2019, and it is the first field of
large-scale solar collectors for district heating in the Baltic States.
In total, 1720 Arcon-Sunmark A/S HT-Heat Boost 35/10 solar collectors have been
installed, size of each collector is 2.0 × 6.3 meters. The collector has high absorber efficiency
~83 %. The solar system is operating with a temperature regime 45/63 ºC, but in the thermal
accumulation system the temperature can be raised to 85 ºC, if necessary, which allows to
increase the total amount of stored heat.
TABLE 1. OVERVIEW OF MAIN SOLAR SYSTEM PARAM ETERS
Parameter Value
Total active area 21 672 m2
Number of installed solar collectors 1720
Operational temperature regime of solar field 45/63 °C
Volume of thermal storage tank 8000 m3
Absorber efficiency 83 %
Gross efficiency 77 %
Heat loss coefficient, a1 2.27 W/km2
Heat loss coefficient, a2 0.0181 W/km2
The operating modes are also important for the solar yield evaluation. Operation is
influenced by two factors: the intensity of solar radiation and the set temperature. When the
solar irradiance is below 250 W/m2, the solar field is preheated, but the heat production starts
when the set temperature is reached. Production is divided into two stages low temperature
and high temperature. Low temperature production starts when the solar irradiance is from
250 W/m2 to 650 W/m2.When the solar irradiation is above 650 W/m2, high temperature
production starts. The overheating protection of solar field during high solar irradiation
period is done in two different ways. The first option is to cool it through the collector field
at night. The second option is to reduce efficiency by raising the inlet temperature in the
collector field.
The performance of solar collectors is determined according to the produced solar heat
according to Eq. (1) [15]:
2
g c 0 1ma 2ma puo
η ()()PA GaTT aTT fff

=⋅⋅⋅−⋅− ⋅

, (1)
where
Pg Guaranteed performance (thermal power output), W;
Ac Collector area corresponding to the collector efficiency parameters, m2;
η0 Optical efficiency;
a1, a2 Heat loss coefficients W/(K·m²);
G Solar irradiance on collector plane W/m²;
Ta Ambient air temperature °C;
Tm Mean temperature of solar collector fluid, °C;
fp Safety factor, considering the pipe heat losses in the collector field and transmission
lines;
fu Safety factor, considering measurement uncertainty;
fo Safety factor for other parameters.
In the particular study the estimated pipe losses are 3 %, therefore the used fp is 0.97. The
value of fu is assumed to be 0.95 for the total measurement uncertainty estimated to be 5 %.
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1287
The used safety factor for other parameters fu is assumed as 0.95, considering non-ideal flow
distribution and unforeseen heat losses.
Mean temperature of solar collector fluid is calculated according to Eq. (2) [15].
c,in c,out
m
()
2
TT
T+
=
, (2)
where
Tc,in Hot side temperature (equal to collector outlet temperature), °C;
Tc,out Cold side temperature (equal to collector inlet temperature), °C.
From the above equations it can be seen that the solar field performance is affected by the
following factors:
Collector area;
Collector optical efficiency;
System losses;
Ambient air temperature;
DH return temperature inlet temperature;
Collector outlet temperature.
The next step is to understand which factors in a particular system can be affected by the
system operator and which cannot be changed. As this is the first project in the Baltic States
the solar radiation is also analysed in order to identify how the intensity of solar radiation
affects the overall system performance. Therefore, the solar collector yield is evaluated
depending on the solar intensity, DH heat carrier return temperature and the heat carrier flow
rate.
2.2. Solar System Monitoring System and Input Data
The monitoring system is designed to ensure security and to be able to calculate the
efficiency of solar collector field. Data reading points corresponds to solar district heating
guideline is shown in Fig. 2.
Fig. 2. Technical scheme of the measurement points [15].
The particular solar collector field in Salaspils is equipped with four solar radiation meters
installed in opposite quadrants of the collector field. Solar radiation meters are connected to
circulation pumps, which increases or decreases the flow depending on the average
measurement of the two radiation meters. The cloudy weather is the main reasons why two
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1288
measurements are compared. Radiation meters are located far enough away from each other
and, while a shadow from a cloud may have been fallen on the one of the radiation meters, at
the same time other radiation meters receive huge amount of solar radiation. Circulation
pumps and flow is also connected and regulated from temperature meters, which displays the
temperature in the solar collector system.
The daysaverage radiation measurement is calculated using every measurement above
300 W/m2, because, as experience shows, at this value it is possible to start thermal energy
production and obtain useful temperature, all measurements which are under this value are
not taken into account. During the night production stops. Radiation measurements are
recorded automatically and shall be performed as minimum once every two minutes or even
more frequently. Up to 25 000 measurements are made at the 24-hour period.
The produced thermal energy from solar collectors is also measured and recorded
automatically, in order to evaluate the solar energy yield.
3. RESULTS
In the first year since solar collectors are installed in Salaspils, the annual share of thermal
energy produced by solar collectors is about 20 % of the total produced amount of heat. The
amount produced using solar collectors was 11 088 MWh, whilst the total produced amount
of thermal energy in Salaspils heating plant were about 58 GWh. As it is shown in Fig. 3, the
highest share of solar field production is observed in June, July and August, when the solar
energy share reached 46–49 % comparing to the total production. Although there are two
wood chip boilers installed in Salaspils DH plant, it was concluded, that the best solution to
cover the peaks demands in summer period is by using the natural gas boilers. Natural gas
boilers can be started immediately without the time-consuming boiler starting process, which
would be the case if biomass boilers were used. Natural gas boilers in combination with solar
collectors is used only in the summer months. Biomass boilers continue to operate in April
and June, as they are not sopped after the heating season. Therefore, total annual share of
thermal energy produced with natural gas boilers was only 10 %.
Fig. 3. Share of thermal energy produced by solar collectors and total produced thermal energy in 2019.
During the high solar radiation periods, when the sun reaches its highest intensity for
several days, the installed 8 000 m3 storage tank cannot accumulate all of the produced solar
energy, and the overheating protection of solar field starts. However, there are also moments
0
1000
2000
3000
4000
5000
6000
April May June July August
Total Produced with solar collectors
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1289
of low solar irradiation and cloudy weather for several days, in which all of stored heat is
consumed and production by solar collectors is insufficient. In these moments, natural gas
boilers are used for heat production, the operation of which is more efficient than biomass
boilers when switched on for a shorter time.
Another operational parameter that can be regulated by the operator is the solar field set
point. Set point is a manually adjusted temperature mark at which the solar collector field
circulation pumps start their operation and move the heat carrier towards the heat exchanger,
where heat is removed. In summer months the ambient air temperature and solar radiation is
much higher than in spring. This results in higher solar thermal energy production, but the
demand of thermal energy at the consumer’s side is lower compared to May and April. In
such cases, the set point should be increased, thus reducing the amount of energy produced
and adjust it with the heat consumption. The opposite situation is observed in spring, when
demand for thermal energy is high enough and all solar energy is either consumed, or stored.
Consequently, the set point is adjusted lower, sometimes even under the network flow
temperature to reduce the load from the biomass boilers and decrease the fuel consumption.
The obtained monitoring data are analysed by using regression analysis method by
determining the correlation between different parameters. This method reflects the
relationship between two factors, which means that when one value changes, the
corresponding one also changes. The maximum value of correlation coefficients is one.
The data presented have been collected throughout the sunny season, which in Latvia’s case
is from April until August. Fig. 3 shows that solar radiation is the main impacting factor of
produced solar thermal energy.
Fig. 4. Produced solar energy dependency of solar radiation.
Of all the parameters studied, which may affect the performance of solar collectors directly,
solar radiation has the highest correlation coefficient with average value reaching 0.6112.
Similar results have been observed in the studies carried out in large-scale solar DH systems
in Denmark [12]. The largest data distribution has been observed in July, which is due to the
regulation of already mentioned set point. At high productivity over several days and
insufficient heat demand, the set point was raised reducing produced thermal energy.
R² = 0.521
R² = 0.8249
R² = 0.5189
R² = 0.5799
0
10
20
30
40
50
60
70
80
90
100
300 400 500 600 700 800 900
Produced thermal energy, MWh
Solar radiation, W/m2
April May July August
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1290
However, the highest correlation is observed in May because solar collectors are placed in
such a position that they produce heat energy with the highest efficiency and productivity in
May. This analysis proves it. If this correlation between solar radiation and solar collector
productivity is the strongest in May, the most appropriate conditions have therefore been
created to make production the most efficient at this time.
Fig. 4 presents a correlation between ambient air and produced thermal energy. The
correlation between these parameters is lower in June, July and August, but higher in April
and May. Average correlation between ambient air temperature and produced thermal energy
is 0.2837, which means that the correlation is low and other conditions impact solar collector
performance more.
Fig. 5. Solar collector efficiency dependency of ambient air temperature.
Another analysed impacting parameters that were examined in this study were flow and
return temperature of heat carrier. Study showed that correlation between these parameters
and produced thermal energy with solar collectors is very low. The data was very distributed
and knowing that the set point value was changed regularly, several times a day, the analysis
of these parameters is very complicated. When the temperature difference changes several
times a day, it is difficult to register and divide data on how much heat energy was produced
with each specific temperature difference between supply and return temperatures.
Fig. 5 shows the comparison of the performance about numerous solar heating plants in
Denmark and the particular solar DH plant in Salaspils. All the heating plants included in this
comparison are using the same solar collector technologies from Arcon-Sunmark A/S,
HTHEATstore 35/10. The data was calculated as average solar performance and solar
radiation results during the period from 2012 to 2016 presenter in [13]. It is clear that the
Salaspils solar heating plant has achieved high solar yield. The specific solar performance
(511 kWh/m2) is high compared to other solar DH systems with similar average solar
radiation.
R² = 0.3123
R² = 0.4989
R² = 0.2032
R² = 0.1972
R² = 0.2071
0
20
40
60
80
100
120
0 5 10 15 20 25 30
Produced thernmal energy, MWh
Ambient air temperature, °C
April May June July August
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1291
Fig. 6. Comparison of solar heating plant performance demanding of solar radiation in Denmark [13] and solar heating
plant installed in Salaspils (2020).
The difference between the results could arise, because the solar heating plant in Salaspils
is actively working only the first year, when the equipment and technologies are the most
effective, although the efficiency of solar collectors does not fall significantly during the
following years. The monitoring of system performance will be continued and more driving
factors will be identified in the future.
Overall, the solar performance of solar collector field installed in Salaspils is high, however,
it would be possible to produce even more thermal energy, if the plant had more storage for
the energy, or if the demand were higher. But the problem is that the weather cannot be
controlled and the demand shrinks at times, when the production (with solar collectors) is the
most productive warm and sunny days. On the further research authors has planned to
analyse the efficiency of solar collectors by excluding the influence of solar radiation, to
better identify the effects of other factors, which dependency on the efficiency are
significantly lower compared to solar radiation. One of the main tasks will be more detailed
analysis of the set point by dividing several components temperature limits and flow control,
on the primary (solar collector) and secondary (rest of the system) sides of the system.
4. CONCLUSION
The article presents the operational data analysis of a first large-scale solar collector field
in Latvia, connected to district heating network, and demanding of several factors.
The results of the analysis show that the system’s productivity is mainly demanded by solar
radiation and the strongest correlation between these parameters is identified in May. Solar
collector performance is strongly affected by adjusted set point which sometimes has to be
settled on a seemingly unbeneficial value, but it is done with the long-term aim to keep the
solar collector system safe. Correlation between solar radiation and collector productivity is
linear.
200
250
300
350
400
450
500
550
600
800 850 900 950 1000 1050 1100 1150 1200 1250
Solar performance, kWh/m
2
Solar radiation, kWh/m2
Solar heating plants in Denmark Solar heating plant in Salaspils
Environmental and Climate Technologies
____________________________________________________________________________ 2021 / 25
1292
Solar collectors installed in Salaspils are producing with the highest productivity at a time
when it is most beneficial:
Solar radiation is high and it would be possible to produce a large quantity of heat;
Placement creates suitable conditions for efficient production;
The temperature of ambient air is within such limits as the demand for heat has not yet
fallen so low that the volume produced could not be realised.
The highest correlation between ambient air temperature and produced thermal energy is
reached, when ambient air temperature is between 7 to 15 °C and production process has not
been externally regulated. The temperature difference between flow and return temperatures
of the heat carrier affect’s solar collector performance minimally and strong correlation was
not observed.
ACKNOWLEDGEMENT
The research is funded by the Ministry of Economics of the Republic of Latvia, project “Assessment of Latvia’s renewable
energy supply-demand economic potential and policy recommendations”, project No. VPP-EM2018/AER_1_0001.
This research/publication was supported by Riga Technical University's Doctoral Grant programme.
REFERENCES
[1] Mastinia R., Kallisa G., Hickelc J. A Green New Deal without growth? Ecological Economics 2021:179:106832.
https://doi.org/10.1016/j.ecolecon.2020.106832
[2] Sayegh M. A., et al. Trends of European research and development in district heating technologies. Renewable and
Sustainable Energy Reviews 2017:68(2):11831192. https://doi.org/10.1016/j.rser.2016.02.023
[3] Li Y., Rezgui Y., Zhu H. District heating and cooling optimization and enhancement Towards integration of
renewables, storage and smart grid. Renewable and Sustainable Energy Reviews 2017:72:281294.
https://doi.org/10.1016/j.rser.2017.01.061
[4] Lund H., et al. 4th Generation District Heating (4GDH) Integrating smart thermal grids into future sustainable energy
systems. Energy 2014:68:1–11. https://doi.org/10.1016/j.energy.2014.02.089
[5] Pelda J., Stelter F., Holler S. Potential of integrating industrial waste heat and solar thermal energy into district heating
networks in German. Energy 2020:203:117812. https://doi.org/10.1016/j.energy.2020.117812
[6] Rezaie B., Rosen M. A. District heating and cooling: Review of technology and potential enhancements. Applied
Energy 2012:93:210. https://doi.org/10.1016/j.apenergy.2011.04.020
[7] Lake A., Rezaie B., Beyerlein S. Review of district heating and cooling systems for a sustainable future. Renewable
and Sustanable Energy Reviews 2017:67:417425. https://doi.org/10.1016/j.rser.2016.09.061
[8] Carpaneto E., Lazzeroni P., Repetto M. Optimal integration of solar energy in a district heating network. Renewable
Energy 2015:75:714721. https://doi.org/10.1016/j.renene.2014.10.055
[9] Weiss W., Spörk-Dür M. Solar Heat Worldwide 2018 version (Global Market Development and Trends in 2017
Detailed Market Figures 2016). Graz: Steinhuber Infodesign, 2018.
[10] Tian Z., et al. Large-scale solar district heating plants in Danish smart thermal grid: Developments and recent trends.
Energy Conversion and Management 2019:189:6780. https://doi.org/10.1016/j.enconman.2019.03.071
[11] Adsten M., Perers B., Wackelgard E. The influence of climate and location on collector performance. Renewable
Energy 2002:25(4):499509. https://doi.org/10.1016/S0960-1481(01)00091-X
[12] Noussan M., et al. Data Analysis of the Energy Performance of Large Scale Solar Collectors for District Heating.
Energy Procedia 2017:134:6168. https://doi.org/10.1016/j.egypro.2017.09.619
[13] Furbo S., et al. Yearly thermal performances of solar heating plants in Denmark Measured and calculated. Solar
energy 2018:159:186196. https://doi.org/10.1016/j.solener.2017.10.067
[14] The European Commission’s science and knowledge service [Online]. [Accessed 21.01.2021]. Available:
https://ec.europa.eu/jrc/en/pvgis
[15] SDH. Solar district heating guidelines. Graz: Steinhuber Infodesign, 2012.
... At the ____________________________________________________________________________ 2023 / 27 967 same time, solar energy is versatile with negligibly small carbon emissions. The main demerit of solar energy is its weather-dependent nature, mainly the variation in solar irradiation [17]. The energy output of solar energy system varies throughout the seasons. ...
Article
Full-text available
With the prevalent energy crisis and climate changes, decarbonising energy sector has become the need of the hour. An environmentally friendly way is the utilisation of solar energy which mainly involves the deployment of photovoltaic (PV) and/or solar thermal technology. Unlike electricity generation, the application of photovoltaics for the district heating & cooling (DHC) is relatively new. Also, this energy route is yet to be fully explored. This paper aims to provide an overview of the photovoltaic applications in the context of DHC sector. At first, the utilisation of solar energy in the DHC sector is briefly described and then the review of the available literature is carried out. It was understood that PV integration in the district heating and/or district cooling system can take place in different topologies such as PV technology, energy storage, and system configuration (centralized/distributed). On one side, this technology options support design flexibility based on local scenarios (i.e., climatic conditions, building types, energy cost). On the other side, selecting the best configuration remains a challenging task for design and planning engineers. The research database on the studied topic needs to be enhanced, with a focus on PV’s role in district cooling (DC). It is deduced that right technical and economic boundary conditions in the chosen region is important for the accelerated photovoltaic integration. Also, lower environmental impact throughout the whole life cycle of solar PV integrated DHC system is reported. PV assisted DC systems have the potential to revolutionize cooling sector, especially in the places where daytime electricity costs are high. Based on the SWOT analysis, it is concluded that there is an enormous opportunity for PV integration in the DHC sector with the upgradations in DH networks, developing DC networks and rising adoption of HPs. This overview is expected to be beneficial to researchers, policymakers and other stakeholders of district energy sector.
... In this paper, the authors analyze the operation of a multi-source DH system in Latvia. The analysed case study is the first large-scale solar collector field for DH in the Baltic States installed in Latvia, Salaspils [43,44]. The total active area of collectors is 21 672 m 2 with an integrated water storage tank of 8000 m 3 . ...
... In addition, natural gas boilers are used for peak demand coverage (see Figure 3). A thermal storage tank of 8000 m 3 is used to accumulate solar energy [66]. However, the capacity would not be suitable to store additional heat amount from waste heat sources. ...
Article
Full-text available
To recover thermal energy from different sources, its quality and possibilities for utilisation are essential. The wide range of engineering solutions includes a direct connection to the district heating (DH) system and the integration of low-quality heat using heat pumps to increase the temperature level of recoverable heat. Therefore, this article compares waste heat valorisation strategies for integration into existing DH networks, low-temperature DH, and ultra-low heat supply systems using the multi-criteria assessment method. In addition, a local scale assessment was performed to identify the waste heat role in existing RES-based DH systems. The results show that the highest waste heat valorisation rate could be reached when integrated into low-temperature DH systems due to high waste heat potential and suitable temperature conditions. However, a local scale assessment shows a significant impact on the already implemented solar technologies, as waste heat could cover around 70% of the summer heat load.
... Innovative DH companies have taken steps to improve the solar energy fraction in their plants. It is reported that the solar energy fraction in the Salaspils DH plant reached 46-49% of the total heat generation during the summer seasons [41]. Further, an increase in interest in solar energy utilization is anticipated in the coming years. ...
Article
Full-text available
The decarbonization of the district heating (DH) sector is receiving attention worldwide. Solar energy and heat pump technologies are widely considered in existing and new DH networks. There is a need to understand the influence of solar energy on district heating experimentally. However, only a few university laboratories are focused on district heating aspects. Further, the concept of such laboratories is not adequately disseminated in the scientific literature. The main objective of this paper is to develop a conceptual design of a solar energy laboratory with a focus on district heating systems. The proposed concept forms part of the preliminary study carried out by a research group at the Tallinn University of Technology. First, a brief literature review on solar energy laboratory development is provided. Then, the conceptual design of such a laboratory is presented, along with a case study. Regardless of project size, the main components of a district heating-based solar energy laboratory are solar collectors, thermal energy storage (TES) tanks, and a control system. The proposed laboratory is expected to serve multiple roles, such as a practical laboratory to provide interdisciplinary courses for students, a research and experimental platform for researchers, and a cradle to achieve the campus green initiative. It is roughly estimated that the thermal energy output from the proposed laboratory would meet around 25% of the heat demand of the institutional building during the summer season (May, June, July, and August). It is expected that the present study will be a reference material for the development of innovative energy laboratories in educational institutions.
... The performance of solar collectors and desirability of their use under different climatic conditions has also been evaluated by Haine and Blumberga [12], Pop et al. [13] and Krawczyk et al. [14], while Xiao et al. [15] and Kakis et al. [16] studied the influence of ambient meteorological parameters (such as solar radiation intensity or temperature) and system operating parameters on the solar performance of flat plate solar collectors. ...
Article
Full-text available
Solar collectors are devices that enable the use of solar radiation, e.g., for hot water preparation or space heating. They are playing an increasingly important role in Europe and around the world, mainly due to the easy availability of the sun, as an energy source. The advisability of their use depends on a number of factors, of which climatic conditions are an extremely important one. This paper presents the results of energy simulations of a solar collector-based domestic hot water system for the capitals of five selected Central and Eastern European Countries (CEEC): Riga, Warsaw, Prague, Bratislava, and Zagreb. Using TRNSYS software, a theoretical model of the system was developed and dynamic simulations were carried out for the entire year. The amount of useful energy generated by the flat-plate collectors, their efficiency, as well as the auxiliary energy requirements and the amount of energy needed to meet the load were estimated and compared. The extent to which changing the area of solar collector affects the operation and efficiency of the system for different locations was also analysed. The results showed that in terms of efficiency, the use of solar collectors is most favourable in placed southernmost Croatia and in Slovakia, where it was also achieved the lowest annual auxiliary energy demand. The least favourable location turned out to be Riga. It is also worth noting that regardless of location, the area of solar collector has a significant impact on the efficiency of the entire system.
... Non-fuel heat sources are almost not used in the Baltic district heating systems. The Salaspils district heating system in Latvia [28,29] is the only district heating network in the Baltic States with a significant share of non-fuel solar heat in heat generation. Taking into account all the above conditions, it is necessary to evaluate the potential contribution of large-scale HP implementation to the reduction of energy-related CO 2 emissions in the region. ...
Article
The current state of power-to-heat integration into the heat supply in the Baltics is examined. In the socio-economic analysis, three scenarios for prospective district heating electrification development in the Baltic countries until 2050 were investigated and compared: Baseline scenario, Grid Tariff scenario, and Investment Support scenario. Large-scale HPs were analysed as key future technologies. Furthermore, the results are focused on excess heat and renewable thermal energy sources used for heat supply, as well as expanding the representation of DH areas. In 2050, large-scale HPs will generate more than half of the heat for the Baltic states in the Baseline scenario, while biomass plants will generate one-third. One of the dominating fossil fuels in heat supply (natural gas) consumption should gradually decrease from 7.9 TWh in 2020 to 1.4 TWh in the same period. Large HPs generated the lowest quantity of heat in the Grid Tariff scenario. The current network tariffs in each of the Baltic countries, it may be inferred, are an impediment to the introduction of HPs. In 2050, only up to 1/4 of thermal energy will be produced by large-scale HPs. Investments in large-scale HPs are half-subsidised in the Investment Support scenario, which greatly affects the introduction of HPs, and, according to this scenario, in 2050, up to 68% of heat will be produced via HPs in the Baltic states. Detailed results are presented for Estonia, Latvia, and Lithuania.
... Non-fuel heat sources are almost not used in the Baltic district heating systems. The Salaspils district heating system in Latvia [28,29] is the only district heating network in the Baltic States with a significant share of non-fuel solar heat in heat generation. Taking into account all the above conditions, it is necessary to evaluate the potential contribution of large-scale HP implementation to the reduction of energy-related CO 2 emissions in the region. ...
Article
Full-text available
These researches concern the application of renewable energy sources in district heating systems. In Ukraine, district heating systems cover approximately 50% of the demand for thermal energy in the residential and communal sector. District heating systems 2G are most often used, which are characterized by high temperatures of the heat coolant, the lack of accounting for heat energy consumption during transportation of the heat coolant, and the use of fossil fuels. In the countries of the European Union, the introduction of district heating systems is considered one of the key directions for the transition to a decarbonized, environmentally safe and efficient energy system. The development of district heating systems technologies makes it possible to lower the temperature of the heat coolant in heat networks and increase the use of renewable energy sources. Ukraine will eventually become a full-fledged member of the European Union, and this determines the need to find ways to bring Ukraine's heat supply systems to the 4G level, in particular to low-temperature district heating systems with the most efficient use of renewable energy sources and waste heat. This article examines climatic, physical-geographical and social features, regulatory, technical and financial-economic opportunities and barriers to the implementation of low-temperature district heating systems in Ukraine. As a result of analytical studies, it was established that there are prerequisites for the introduction of low-temperature heat supply systems in Ukraine, however, a number of technical, regulatory, social and financial and economic measures need to be implemented to bring district heating systems up to 4G indicators. These studies allow establishing measures that require further research for the possibility of introducing low-temperature district heating systems in particular and environmental safety of heat supply systems in general.
Article
Full-text available
The German Federal Government identifies the integration of industrial waste heat and solar thermal energy into district heating systems as two measures to decarbonize the heating and cooling market in The Climate Action Plan 2050. This work determines the theoretical potential of industrial waste heat and solar thermal power within the cities' boundaries and in relation to the cities' district heating systems. A prognosis for the year 2030 and 2050 will be given. Poor information about industrial waste heat is bypassed by taking industrial emission from the dehst and by calculating the overall installed energy by stoichiometry. 10%, 20%, 30% of the so calculated primary energy input is assumed to be meaningful integrable waste heat. The potential of solar thermal power is estimated by the solar fraction that is given with 1%, 5% and 15%. The results show a high, currently unused potential of industrial waste heat sources and solar thermal power for the integration into district heating. In some cities, these energy sources can supply the heat demand of the city's district heating system completely.
Article
Full-text available
District Heating systems are an interesting opportunity for the increase of renewable energy share in the heating and cooling sector. The possibility of a centralized heat production allows the integration of multiple sources, including RES such as biomass, heat pumps and solar energy. This paper provides an operation analysis of the energy performance of large scale solar collectors supplying heat to DH systems in Denmark. Thanks to the availability of hourly data it has been possible to track the evolution of the collectors' performance throughout the year, and compare it with the available radiation. The results show the good reliability of such systems, which are generally able to convert 40% to 60% of the available radiation, with annual production yields higher than 400 kWh/m 2 y. The conversion efficiency shows some seasonal variations, being the winter months the less favorable, probably because of a lower direct radiation. The DH systems considered in the study show a similar performance but with some differences: other parameters such as slope, azimuth and operating temperatures could be the causes of these variations.
Article
Full-text available
The implementation of European Directive 2012/27 calls for the presence of a renewable share inside efficient district heating and cooling. Solar thermal energy can be a viable contribution to this aim but particular attention must be put into its integration inside the district heating systems. In fact, the variable and non-controllable nature of renewable heating must be handled by fulfilling users demand and coordinating its output with other controllable sources. Thermal energy storage is often necessary for exploiting the renewable sources at their best. An optimisation procedure has been developed to find the dispatching strategy for the different power sources present in the network. The optimisation procedure can be used at the planning level to find out the best sizing proportions of solar and conventional sources and for defining the optimal capacity of storage. After a brief description of the optimisation procedure and of its simulation modules, one test case is presented and results about advantages due to solar heating are discussed.
Article
The IPCC warns that in order to keep global warming under 1.5°, global emissions must be cut to zero by 2050. Policymakers and scholars debate how best to decarbonise the energy system, and what socio-economic changes might be necessary. Here we review the strengths, weaknesses, and synergies of two prominent climate change mitigation narratives: the Green New Deal and degrowth. Green New Deal advocates propose a plan to coordinate and finance a large-scale overhaul of the energy system. Some see economic growth as crucial to financing this transition, and claim that the Green New Deal will further stimulate growth. By contrast, proponents of degrowth maintain that growth makes it more difficult to accomplish emissions reductions, and argue for reducing the scale of energy use to enable a rapid energy transition. The two narratives converge on the importance of public investments for financing the energy transition, industrial policies to lead the decarbonisation of the economy, socializing the energy sector to allow longer investment horizons, and expanding the welfare state to increase social protection. We conclude that despite important tensions, there is room for synthesizing Green New Deal and degrowth-minded approaches into a ‘Green New Deal without growth’.
Article
Large solar collector fields are very popular in district heating system in Denmark, even though the solar radiation source is not favourable at high latitudes compared to many other regions. Business models for large solar heating plants in Denmark has attracted much attention worldwide. Denmark is not only the biggest country in both total installed capacities and numbers of large solar district heating plants, but also is the first and only country with commercial market-driven solar district heating plants. By the end of 2017, more than 1.3 million m2 solar district heating plants are in operation in Denmark. Furthermore, more than 70 % of the large solar district heating plants worldwide are constructed in Denmark. Based on the case of Denmark, this study reviews the development of large solar district heating plants in Denmark since 2006. Success factors for Danish experiences was summarized and discussed. Novel design concepts of large solar district heating plants are also addressed to clarify the future development trend. Potential integration of large solar district heating plants with other renewable energy technologies are discussed. This paper can provide references to potential countries that want to exploit the market for solar district heating plants. Policy-makers can evaluate the advantages and disadvantages of solar district heating systems in the national energy planning level based on the know-how and experiences from Denmark.
Article
The thermal performance of solar collector fields depends mainly on the mean solar collector fluid temperature of the collector field and on the solar radiation. For Danish solar collector fields for district heating the measured yearly thermal performances per collector area varied in the period 2012–2016 between 313 kWh/m² and 577 kWh/m², with averages between 411 kWh/m² and 463 kWh/m². The percentage difference between the highest and lowest measured yearly thermal performance is about 84%. Calculated yearly thermal performances of typically designed large solar collector fields at six different locations in Denmark with measured weather data for the years 2002–2010 vary between 405 kWh/m² collector and 566 kWh/m² collector, if a mean solar collector fluid temperature of 60 °C is assumed. This corresponds to a percentage difference between the highest and lowest calculated yearly thermal performance of about 40%. This variation is caused by different weather conditions from year to year and from location to location. Approximately half of the variations of yearly thermal performances can be related to variable weather conditions.
Article
District heating and cooling (DHC) systems are attracting increased interest for their low carbon potential. However, most DHC systems are not operating at the expected performance level. Optimization and Enhancement of DHC networks to reduce (a) fossil fuel consumption, CO 2 emission, and heat losses across the network, while (b) increasing return on investment, forming key challenges faced by decision makers in the fast developing energy landscape. While the academic literature is abundant of research based on field experiments, simulations, optimization strategies and algorithms etc., there is a lack of a comprehensive review that addresses the multi-­‐faceted dimensions of the optimization and enhancement of DHC systems with a view to promote integration of smart grids, energy storage and increased share of renewable energy. The paper focuses on four areas: energy generation, energy distribution, heat substations, and terminal users, identifying state-­‐of-­‐the-­‐art methods and solutions, while paving the way for future research.
Article
Abstract The present study explores the implementation of district heating and cooling systems across a broad set of case studies reported in the literature. Topics addressed include their history, system identification, energy sources, design considerations, environmental impact, economic feasibility, performance analysis and the role of energy policy. The history of district heating and cooling systems reveals how available technology has influenced the configuration of district energy systems as more efficient and cleaner methods of providing heating and cooling have arisen. This leads to system identification based on primary energy sources, including the deployment of more and more renewable energy streams. Advantages and disadvantages of each energy source are examined in detail. Policies created by government and international entities will have a major impact on the future of research and development in district energy systems. Incentives may become necessary for creating favorable conditions for the efficient construction and utilization of district heating and cooling. Outcomes of these policies influence design considerations underlying any district energy system and their contribution in sustainability. Studies on greenhouse gas emissions along with the economic impacts of district energy construction are part of the design process and optimization of district energy systems should include economic and environmental considerations and not solely thermal efficiency. District heating and cooling systems are often integrated with components such as absorption chillers, cogeneration and thermal energy storage. Performance analysis using exergy and energy analysis have revealed several sources of irreversibility in district heating systems with these elements. If understood properly, these can greatly enhance system operation. Awareness and accommodation of the many factors discussed in this paper can improve the soundness of any district heating or cooling installation.
Article
There is a considerable diversity of district heating (DH) technologies, components and interaction in EU countries. The trends and developments of DH are investigated in this paper. Research of four areas related to DH systems and their interaction with: fossil fuels, renewable energy (RE) sources, energy efficiency of the systems and the impact on the environment and the human health are described in the following content. The key conclusion obtained from this review is that the DH development requires more flexible energy systems with building automations, more significant contribution of RE sources, more dynamic prosumers׳ participation, and integration with mix fuel energy systems, as part of smart energy sustainable systems in smart cities. These are the main issues that Europe has to address in order to establish sustainable DH systems across its countries.
Article
This paper defines the concept of 4th Generation District Heating (4GDH) including the relations to District Cooling and the concepts of smart energy and smart thermal grids. The motive is to identify the future challenges of reaching a future renewable non-fossil heat supply as part of the implementation of overall sustainable energy systems. The basic assumption is that district heating and cooling has an important role to play in future sustainable energy systems – including 100 percent renewable energy systems – but the present generation of district heating and cooling technologies will have to be developed further into a new generation in order to play such a role. Unlike the first three generations, the development of 4GDH involves meeting the challenge of more energy efficient buildings as well as being an integrated part of the operation of smart energy systems, i.e. integrated smart electricity, gas and thermal grids.