Effectiveness of using Organic Rankine cycle engine in small- scale district heating systems
Author: Sarah Ebhuomhan
Abstract
Co-production of electricity and heat using biomass is important in providing sustainable energy for the future. District heating systems, which are well established in Sweden, provide a great opportunity to co-produce electricity simultaneously. With small-scale district heating comprising majority of the district heating systems in Sweden, there is great potential for electricity production. The overall efficiency of the primary energy use of the cogeneration system of electricity and heat from district heating systems shows its sustainability advantage and energy benefits. The cost effectiveness of such systems gives an idea as to how profitable, market-competitive, and feasible the implementation of such systems might be. This thesis analyses the effectiveness of co-producing electricity in small-scale district heating systems, using Organic Rankine cycle (ORC) engines from an overall energy system viewpoint. A small-scale district heating system in Ingelstad Sweden with an annual heat capacity of 6.0 GWh, was used as a case study. The additional fuel use and the amount of electricity that could be produced if an ORC turbine was integrated into the system as well as its production cost were estimated. Results show that a net electricity production of 219 MWh could be yielded annually at a production cost of 102.0
€/MWh, but an additional primary energy of 239.9 MWh could be consumed.
Consequently, the system yielded about double the quantity of electricity when compared to the case of producing in standalone power plants. Considering the situation where the co-produced electricity is used to replace that from different fossil-based standalone power plants, a carbon abatement cost of 90.4 - 158.4 €/ton CO2 is accounted. Sensitivity analysis done on this integrated system showed that increased heat demand would serve to improve the energy efficiency of the system, if applied, while lower discount rates and elongation of the equipment lifetime would serve to improve the cost-effectiveness of the system. The positive results from the study reveal that, the integration of ORC engines into small-scale district heating systems is promising from an overall energy system viewpoint.
Table of contents
1. Introduction ... 4
1.1. Background ... 4
1.2. Purpose & goals ... 5
1.3. Limitations ... 6
2. Theoretical Background ... 7
2.1. District heating production and uses ... 7
2.1.1. Small scale district heating in Sweden ... 8
2.1.2 Supply side ... 10
2.1.3 Demand side ... 10
2.2. Co-production of electricity in district heating system ... 11
2.2.1. Options for the co-production of heat and electricity ... 12
2.2.2. Small-scale electricity co-production and ORC turbines ... 13
2.3. Effect of coproduction in district heating system from an overall energy perspective ... 15
2.3.1. Energy demand and supply ... 15
2.3.2 District heat and electricity price in Sweden ... 18
3 Methodology ... 23
3.1. Electricity to be co-produced from the system ... 23
3.2. Primary energy use and avoided production ... 24
3.3. Cost of electricity production ... 25
4. Case Study... 27
4.1. District heating system in Ingelstad ... 27
4.2. Small scale Organic Rankine Cycle Engine ... 29
4.3. Assumptions and Calculations ... 30
4.3.1. Total Fuel Consumption and Fuel cost Calculation ... 30
4.3.2. Avoided production and emissions ... 32
4.3.3. Carbon abatement costs ... 32
5. Results ... 34
5.1. Connections of ORC system in the small-scale district heating system ... 34
5.2. Operation and production of electricity ... 34
5.3. Effects on primary energy and cost ... 35
5.3.1 Comparison to Standalone power plant... 35
5.3.2. Cost and effects ... 36
5.4. Sensitivity analysis ... 37
5.4.1. Changes in discount rate ... 37
5.4.2. Extended equipment lifetime/with changes in discount rates ... 38
5.4.3. Marginal power production technologies and Carbon abatement costs 39 6. Discussion and Conclusions ... 41
References ... 43
1. Introduction
1.1. Background
As an energy dependent world, and with an ever-increasing population, there is an ever increasing need to develop and provide energy in the most sustainable ways.
Satisfying electric and heat demand are critical for survival and sustainable ways to generate both simultaneously are always of interest and great advantage to meeting energy needs.
For heat energy supply, district heating has commonly been used to meet the heat demand of regions in some cities and countries of the world. One of the regions of the world where district heating has long been established is Scandinavia. Domestic heating in major towns and cities in Scandinavian countries like Sweden, Denmark, and Finland, is provided for by district heating (Gebremedhin, 2014). In a district heating system (DHS), hot water is generated centrally as the major product, which is then distributed to customers connected to the network through pipelines for their various heating needs. One of the sustainability advantage of district heating system is that biomass and other sources of heat like industrial waste heat, can be used as primary resources for the system.
Electricity is one of the most versatile forms of energy and according to the international energy agency (IEA, 2019), demand for electricity is set to further increase, due to growing demand for digital connected devices, increased electrification of transport and heat, air conditioning etc. With this increase in demand, comes an increase in CO2 emissions, related to the primary energy used to satisfy the demand. However, in line with sustainability, this increase in CO2
emissions is undesirable. In other to satisfy this growing demand for electricity, and at the same time reduce CO2 emissions, it is essential to improve the overall conversion effciciency such as to co-produce heat and electricity (European Commision, 2004), especially from heat sources where the primary energy source is renewable, like in district heating systems (Sven Werner, 2017).
In Sweden, district heating constitutes about 55% of the market share for all heat supply to buildings, as at 2014 (Swedish Energy Agency, 2015). This district heating system consists of small-scale and large-scale systems, of which small-scale district heating systems consists about 75% of the over 500 district heating systems in Sweden (Energiföretagen, 2019) (Sven Werner, 2017). The usage of district heating in colder regions is larger than in warmer regions (Winther, 2018). Since heat loads
are directly proportional to outdoor temperatures, a strong seasonal variation is created by this weather dependence (Sven Werner, 2017). The demand for heat is at its peak during winter, and in the summer, there is production of residual heat, due to low heating demand (Göteborg Energi, 2012). The district heat system has its distribution networks in which a design temperature difference of about 50℃ is used to perform the distribution of hot water in the networks. During recent years, annual average distribution temperatures have been 86℃ in the supply pipes and 47℃ in the return pipes (Sven Werner, 2017). This temperature range at which district heat is supplied and returned to the system, presents an opportunity for the heat to be utilized to co-produce electricity.
High efficiency co-generation of heat and electricity has several benefits. It is particularly advantageous in reducing primary energy use and contributes to the improvement of security of energy supply (European Commision, 2004). However, regarding co-producing electricity from district heating systems, the scale of the district heating production (large scale or small-scale) is a critical factor in determining the cost-effectiveness of the system. For small-scale district heating systems, co-generation of heat and electricity is generally not cost-effective, if typical combined heat and power technologies are used (Truong & Gustavsson, 2014).
However, given the number of small-scale systems in the Swedish district heating system of approximately 75% (Energiföretagen, 2019), the cogeneration of electricity in such systems is a potential sustainability opportunity. Therefore, it is necessary to investigate other technologies like the Organic Rankine cycle (ORC), which have been considered as a potentially cost-effective means (Brandin et al., 2011) (Nyström et al., 2011) of co-producing electricity from small-scale district heating systems.
The technology of the ORC is based on heating up organic working fluid using low temperature heat source. This is possible due to the use of organic working fluid, such as ammonia which has a lower boiling point than water that enables it to be evaporated at a low temperature level. The working fluid, after being evaporated is expanded in a turbine which drives an electricity generating generator to produce electricity (Salman, 2017). As a heat to power conversion technology, the Organic Rankine cycle have been proven to be effective (Markides, 2015) (White & Sayma, 2019).
1.2. Purpose & goals
The purpose of this study is to investigate the effects of installing ORC engines in a small-scale district heating system currently based on heat-only boilers. The amount of electricity that can be generated by this incorporation will be calculated, and the effects of this electricity generation on the primary fuel (biomass) demand will be
investigated as well. The cost efficiency of the system will be calculated, and the sustainability advantages resulting from the system, evaluated. To meet the goal of the study, the following questions will be examined.
• How is the heat supply and return temperatures of small-scale district heating systems, and how does it affect electricity generation?
• What capacity of ORC engine can be used in an integrated system?
• How much electricity can be produced using the desired ORC engine, taking into account the variation of heat demand?
• How does the electricity generation affect the fuel to be fed into the boiler?
• How does weather and temperature variation affect the performance of the system?
• How will the electricity generated be sold, and what are the cost benefits that could result from this?
• What is the cost-effectiveness of the integration option?
1.3. Limitations
The efficiencies of the boilers in the small-scale district heating facility were not considered in this work. The changed operation condition of boiler may influence the conversion efficiency of boilers. However, these changes were not considered in this work. It would be worthy to note that the weather variations for other years might affect the result differently, if used. In this work, the performance of the equipment was assumed to be the same throughout the project lifetime. Possible changes in equipment performance were not taken into consideration.
2. Theoretical Background
In the following sections of the theory, the basic idea of district heating and the Swedish district heating system are examined. Other concepts discussed include the co-production of electricity and heat in the district heating system, the working principle of the Organic Rankine cycle engine, and an overall energy perspective regarding the effects of district heating.
2.1. District heating production and uses
For district heating, the fundamental idea is to satisfy local customer demands for heating by utilizing local fuel or heat resources that would otherwise be wasted. A heat distribution network of pipes is used as a local market place to distribute this heat to the customers (Frederiksen & Werner, 2013). The European district heating systems have seen the introduction of some renewable heat from geothermal wells, solar collectors, and biomass fuels into the system in recent decades. Hereby, the focus for district heating systems is to use a combination of recycling and renewable heat, providing a substitution of primary energy supply to meet various societal heat demands (Sven Werner, 2017).
The above fundamental idea about district heating is being fulfilled in the Swedish district heating system. Several different fuels are being used to produce district heating, and since the 1980s, there has been a major transition towards renewable fuels, as Figure 2.1 demonstrates. Biomass accounted for 63% of the input energy for district heating production in 2015, while waste heat accounted for about 8% of the input energy (Swedish Energy Agency, 2021c). High security of supply, low carbon dioxide emissions, and efficient use of available heat sources characterise the current situation in Sweden.
Figure 2.1: Input energy used for district heat production, 1980-2019 (Energiföretagen, 2020).
District heat is bought by customers from the heat distribution systems and utilised in industrial premises and processes, single-family residential houses and used for ground heating to remove snow. Of all dwellings in multi-family residential houses, 93% were connected to district heating by 2014. These include small scale district heating systems in small towns and villages (Sven Werner, 2017). According to the Bioenergi Svebio Bioheat Roadmap of 2021 (Svebio, 2021), there are 556 district heating systems (Small-scale and large-scale) in Sweden as at 2021. These district heating systems are displayed on a map of Sweden in Figure 2.2. Small-scale district heating systems dominate the Swedish district heating system (Energiföretagen, 2021a), and are thus of great importance in analysing the district heating sector of Sweden.
2.1.1. Small scale district heating in Sweden
District heating for larger communities, using biomass as primary fuel is well established in Sweden. The capacities of these type of boilers are typically in the range 20 to 150MWth. However, the installation of district heating for smaller communities and use of biomass fuel have been made economically interesting, due to increased taxes on fuel oil and developments of the district network technology.
These types of small-scale boilers usually have capacities in the range of 0.1 to 2MWth (Lundgren, 2002). In several district heat markets, small-scale district heat
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0
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systems (SS-DHS) using heat-only boilers are dominant. Of the 556 district heating systems in Sweden, about three-fourths have been reported to be small-scale district heating system (Energiföretagen, 2021a), denoted in Figure 2.2 by the red dots.
These are district heating systems with annual district heat production of less than 100GWhheat/ year. There is space for at least 10GW new installed thermal power in small scale biomass fired district heating network, according to Swedish statistics and official reports. This could mean over two thousand boilers in Sweden (Lundgren, 2002).
Figure 2.2: District heating systems in Sweden, 2021 (Svebio, 2021).
An overview of a district heating system is shown in Figure 2.3. It shows that a district heating system consists of two parts, which are both connected in a closed-loop system. The two sides are the supply side and the demand/consumer side.
2.1.2 Supply side
On the supply side, which is the hot side of the district heating circuit, cold return water from the consumer side is reheated using a heat exchanger. An exhaust gas or steam extraction from a boiler or industrial processes is commonly used to exchange the heat. In the heat exchanger, due to the sudden temperature drop, the exhaust gas or steam is either condensed into liquid or cooled down. Depending on the desired temperature, the outgoing temperatures of the heat exchanger supplying the district heat could be in the range of 60-105℃. Depending on the amount of pumps present on the supply side, the mass flows can vary greatly (Gadd & Werner, 2014) (Lund et al., 2014)
2.1.3 Demand side
The demand side is where the heat is absorbed by the consumers, who are the customers of the energy company. Before every facility or building connected to the district heating system, individual heat exchangers are placed, to heat up the internal water circuit (Gadd & Werner, 2014). The heat exchanger thus transfers heat from the district heating water to the water in the building, which is then used for space heating or general domestic use. The usage of this heat in the district heating water leads to it being cooled down, and is thus sent back to the district heating plant to be heated again (Karlsson & Ottosson, 2018). By using valves to adjust the intake amount of water, the flow rate of water is easily controlled. For Swedish district heating systems, the average return temperature is about 47.2℃ (Gadd & Werner, 2014).
Figure 2.3: District heating system(Winther, 2018).
2.2. Co-production of electricity in district heating system
The co-production of different energy carriers involves the general idea of an integrated energy system that provides different energy products in a decentralized way simultaneously (Rubio-Maya et al., 2011), in this case electricity and heat. This integration and co-generation of energy provide several benefits. These include:
primary energy, environmental and cost benefits (Serra et al., 2009) in terms of reduction of network losses, a reliable energy supply, a reduction of greenhouse gases and economic saving (Rubio-Maya et al., 2011). This energy co-generation and integration is applied in various systems, an example of which is the integration of combined heat and power (CHP) units in district heating plants. Another example is the production of steam and electricity in industry, and this integration is expected to be further expanded (Gustavsson & Truong, 2011).
The use of district heat in Sweden has increased over the years and the current supply exceeds 10% of the total primary energy use and also exceeds the final energy use by approximately 14% (Swedish Energy Agency, 2021b). The average ratio of district heat systems in Sweden converting electricity to heat is however, low, about 0.13 (Energiföretagen, 2021a). This shows great potential for Swedish district heating systems to integrate the co-production of heat and electricity into their systems (S Werner, 2007). In a district heat production system, it is common to have different production units. For peak-load demand, mostly fossil fuel-based boilers with low investment per unit of produced energy are used. Due to the high investment costs for CHP units, they are generally used for base-load production, as they require high utilization (Gustavsson & Truong, 2011). Depending on the scale of the community served and the local climatic conditions, district heating systems vary in size (Truong & Gustavsson, 2014), from large and medium scale to small- scale systems. It has been reported by some authors (Gustavsson & Truong, 2011) (Truong & Gustavsson, 2013) that for medium to large scale district heating systems, the co-generation of heat and electricity could be an economic choice, especially if the value of the co-generated electricity in the CHP system is
equivalent to the cost of producing the electricity in standalone power plants, with corresponding technology. Small-scale district heating systems have however been reported to be limited in the choice of technologies that can be used for co-
generation of electricity and heat to achieve minimum cost option, due to their low heat demand (Truong & Gustavsson, 2014), usually less than 100GWhheat/year (Energiföretagen, 2021a). In Sweden, where small-scale district heating systems are dominant, to effectively co-generate electricity and heat in district heating systems, due to the demonstrated benefits, the technologies that could result in cost-
effectiveness of electricity and heat co-generation for small-scale district heating system have to be given due attention, as this will determine how effective the application of cogeneration of Swedish DHSs will be.
2.2.1. Options for the co-production of heat and electricity
Regarding CHP technology options, there are generally two different types of CHP systems, which are the topping cycle and bottoming cycle. In the topping cycle, the fuel supplied to the system is used to generate electricity primarily. The heat that is left after the electricity generation is then used for its desired applications. Examples of this type of CHP includes gas turbines and reciprocating engines or fuel cells. In the bottoming cycle, however, the fuel supplied is primarily used to generate heat.
The heat generated is used for its desired application, before being used to generate electricity. For the bottoming cycle type of CHP, steam turbines are the most used.
Organic Rankine cycle turbines are also an example of a bottoming cycle, useful for when the heat energy is at a relatively low temperature (Breeze, 2017a) like in the case of small-scale heat-only boilers. For the above-mentioned technologies, the CHP technologies which are more suitable for heat-to-power conversion (bottoming cycle CHP) in relation to biomass as the primary energy source are going to be further discussed, and these technologies are mainly the steam turbines and the Organic Rankine cycle turbines.
A steam turbine works with steam or pressurized water, as the name implies. The working principle of a steam turbine is to use pressurized water, converted to steam to drive a turbine that generates electrical power. The water is pressurized by heating it with a burning fuel (Breeze, 2017a), in this case the combusted biomass. This CHP technology thus, requires high temperature heat, as steam to produce electricity and is more suitable for systems, where this high-quality heat requirement is met. Small- scale district heating systems, with lower heat demand, usually do not meet this high heat requirement, and thus require an alternative bottoming cycle CHP type. In addition to heat temperature requirement, profitability/economic viability of the CHP option to be used in any system is a vital factor to consider. To access the economic viability of a CHP plant, the capital cost of installing the facility, the cost of fuel it will use and the cost of operating and maintaining the plant over its lifetime has to be estimated. This is then usually compared to the equivalent costs associated with a stand-alone power unit, or the cost of buying power from the grid and the cost of providing heat from a different local energy source (Breeze, 2017b). In these estimations, the profitability of the CHP plant will depend on the technology used, size, energy prices, location and the operational strategy (Thorin et al., 2015).
For the co-generation of electricity and heat using a district heating network, the heat demand in the network determines the operating load. The power production in the combined heat and power (CHP) plant, is completely dependent on the heat demand.
A low heat demand would result in a low power production. With the variation of heat demand during the year, the full load operating time of the production unit can
be limited. In Sweden, CHP production is widely used to cover part of the building heat demand by district heating. However, most of the CHP plants are steam-turbine based with biomass as fuel feedstock (Thorin et al., 2015). These are typically not economically viable for small-scale district heating systems (Truong & Gustavsson, 2014). Organic Rankine turbines, which can make use of low-grade heat present an alternative CHP option for these small-scale systems and will be further discussed in that light.
2.2.2. Small-scale electricity co-production and ORC turbines
Organic Rankine cycle (ORC) originated from and shares the same principle as the Steam Rankine cycle, but using different working fluids. While plenty organic fluids are suitable for ORC, water is the only working fluid for Steam Rankine cycles (SRC).
The primary and decisive indicator for the selection between ORC and SRC is the heat source (Tian & Shu, 2017). For heat sources at moderate or low temperatures, Organic Rankine cycles (ORC) using organic working fluid instead of water, present the best conversion efficiency from heat to power. The organic working fluid follows the source better, due to its smaller latent heat and lower boiling point (Wei et al., 2011). Organic fluids vary in terms of their boiling points, and as such for an ORC system, a suitable organic fluid that matches the temperature of the heat source is selected (Johansson & Söderström, 2014).
As shown in Figure 2.4, the ORC system consists of four major components, which are the pump, evaporator, expander, and condenser. Organic fluid is pumped to the evaporator and there it undergoes heating and vaporization by the heat source. The vaporized fluid then expands and produces mechanical energy output through the shaft of the expander. The mechanical energy produced, is converted to electrical power by a generator, which is usually coupled with the expander shaft. Exhaust products from the expander get converted back to liquid, by passing through the condenser, where extra heat is removed by secondary cooling fluid (Chowdhury et al., 2015).
Figure 2.4: A typical Organic Rankine cycle (Chowdhury et al., 2015) .
The temperature difference between the heat source and the heat sink, the heat of vaporization of the working medium and the phase of the heat source,determine the conversion efficiency of the system. A key factor in the thermodynamic performance of the ORC process is the working medium and further important factors that affect the conversion efficiency are the temperature and flow of both the heat source and cooling water (Johansson & Söderström, 2014). The evaporator of the ORC can operate under the subcritical or supercritical conditions, depending on the working pressure. In subcritical conditions, the operating pressure of the evaporator is below the critical pressure of the working fluid, while the operating pressure of the evaporator is above the critical pressure of the working fluid in supercritical conditions. The work output and efficiency of the ORC system are affected by the operating pressure of the evaporator. For an ORC operating at subcritical conditions, the efficiency is usually low, due to the high exergy destruction and loss resulting from the cycle being run at a lower pressure ratio (Schuster et al., 2010).
The energy analysis of the system is based on the first law of thermodynamics. The energy efficiency is expressed in (1) below as:
𝜂𝑒𝑛= 𝑊𝑔𝑄−𝑊𝑝
𝑖𝑛 (1) where
- 𝜂𝑒𝑛 is the energy efficiency,
- Wg is the power output of the generator (kW), - Wp is the power output of the pump (kW) and - Qin is the energy input of heat (kW)
The specific net power output is expressed in (2) as:
ω= 𝑊𝑡− 𝑊𝑔
𝑚 (2) where
- ω is the net power output,
- Wp is the power output of the pump (kW), - m is the mass flow rate (kg/s).
While the energy analysis above estimates the heat and energy losses, the optimal conversion of energy is not analysed (Wei et al., 2011).
For a district heating system using biomass as the primary fuel, a typical system is made up of an Organic rankine cycle module and a biomass feed boiler connected through a thermal oil or water loop. Biomass is combusted in a process like that used in conventional steam boilers. The thermal oil or water is used as a heat transfer medium, and this provides several advantages. These include large inertia and insensitivity to load changes, low pressure in the boiler, simple and safe operation, and control. Electricity production occurs as the heat carried by the thermal medium gets transferred to the Organic rankine cycle module and gets converted to electricity (Tchanche et al., 2011).
2.3. Effect of coproduction in district heating system from an overall energy perspective
2.3.1. Energy demand and supply
In regions with cold climate, the primary energy use for space heating of buildings, is significant. In 2010, heating of buildings including tap water heating, constituted about 23% of the total primary energy use in the EU-27 countries (Connolly et al., 2014). Heat is normally produced with heat only boilers in small-scale district heat production systems (Energiföretagen, 2021b), while CHP units are used to produce a major proportion of the district heat in large scale district heat systems (DHS). The amount of cogenerated electricity in CHP units, depend on the district heat demand,
and will therefore be influenced by any change in heat demand in the district heated buildings (Truong & Gustavsson, 2016). The idea of incorporating an ORC engine to produce electricity simultaneously as heat is being produced in small-scale district heat systems, functions as a CHP system for the small-scale DHS as well, so this applies to this type of system as well. If heat demand is at a reduced temperature, the primary energy required to produce the heat as well as the electricity will be reduced, and this can improve the efficiency of the system in coproducing heat and electricity on a small scale. This depends on the heat demand of the buildings in the system network.
In Sweden, multi-apartment buildings dominate the district heat use, and about 84%
of the floor area of apartment buildings get their heat from district heating (Wahlström, 2012). In 2011, these buildings consumed about 50% of the total district heat produced in the country (Swedish Energy Agency, 2021b). Of these multi- apartment buildings, more than half were constructed between 1950 and 1975.
Energistatistik revealed in 2014 that, the average space heating and hot water use in district heated multi-apartment buildings was about 135-145 KWh/m2 for buildings that were constructed between 1960 and 1980. It was likewise reported that, for buildings constructed after 2010, the average space heating and hot water use was about 88KWh/m2 (Statens energimyndighet, 2015a). This show that for newer buildings, which have been constructed more efficiently than older ones, the district heating demand is reduced, and this affects the primary energy as well, leading to an increase in primary energy efficiency, even as electricity is coproduced with the heat.
As the fuel (primary energy) to be used in this work is biomass, a study of biomass use in Sweden is presented. In the district heating sector in Sweden, the biomass used is predominantly wood fuels, most of which are domestically produced and supplied by forest owners and forest industries, including their subsidiary companies. Sweden is a country with large forest resources and a considerably sized forest industry. With the activities of the forest industry, large quantities of by-products and forest residues are generated. These were previously left unused, but with the introduction of biomass in the district heating systems, a market was created for these by-products and residues (Ericsson & Werner, 2016).
The Swedish district heating systems first saw the introduction of biomass use as primary fuel around 1980, and for the next ten years, biomass use remained fairly small. Most Swedish buildings had initially used fuel oil to cover their heat demands in the 1960s, while district heating accounted for only about 3% of the heat market (Statens energimyndighet, 2015b). In the early 1990s, however, massive biomass expansion began and continued until around 2010. Biomass use has been fairly stable since then. The use of biomass in district heating production, amounted to 99 PJ (Peta- Joules) in 2014 (SCB, 2015a).
The major wood fuels, which are the dominant type of biomass used in the Swedish district heating sector are wood chips and sawdust, which in 2013, accounted for 70%
of the biomass used for district heating production. The other major wood fuels consist of wood pellets and briquettes, accounting for 16% of the biomass used for district heating production. Other forms of biomass used in this sector consists of bio- oil (accounting for 5%), tall-oil, which is a by-product of pulp production and accounts for 2% of biomass in use, wood waste of various types (accounting for 3%) and unspecified biomass fuels, accounting for 4% of biomass in use (SCB, 2015a).
The introduction and wide use of biomass in the district heating systems have led to a 90% decrease in carbon dioxide emissions from the level it was in the 1970s to 2014. In the early 1970s when fuels of fossil origin were used in the district heating systems, the annual average specific carbon dioxide emissions was about 90 g/MJ of heat delivered, while in 2014, this value was about 9 g/MJ (Ericsson & Werner, 2016)
Figure 2.5: Real biomass prices in the price level for biomass fuels delivered to the district heating sector in Sweden, 1993-2017 (Swedish Energy Agency, 2021a).
The real prices of different biomass fuels from 1993 to 2017 is shown in Figure 2.5.
The prices were high in the 1980s, as biomass use was introduced in the system, but due to a combination of higher supply and productivity in forestry, there was a gradual decline of prices until 1995. A fairly stable trend was observed until 2002. This decrease in prices have been interpreted by some analysts as an example of technology learning in biomass production and use. However, due to higher demand, the biomass prices started to increase after 2002, reaching a peak in 2011.A slow decline in the biomass prices is observed from then till 2014, at which the price for
forestry wood chips is 180 SEK/MWh. The Figure shows that forestry wood chips are generally cheaper than densified wood fuels (pellets and briquettes), while demolition wood and other recycled wood are cheapest. This is because they require more environmental permits, more advanced boilers for their processing and more advanced cleaning equipment (Ericsson & Werner, 2016). According to the Swedish Energy Agency, (Energiföretagen, 2020) (Statistics Sweden, 2020), in 2019, the average price paid for processed wood fuels (pellets and briquettes) by electricity and district heating companies was 305 SEK /MWh, while 169 SEK /MWh was paid for wood chips and wood fuels of other types (bark and shavings). This price values for wood chips and processed wood fuels shows a close relation to the price as at 2014.
2.3.2 District heat and electricity price in Sweden
a. District heat price
The district heating price in Sweden is set based on local conditions, and so varies between different locations. The factor with the greatest impact on pricing is fuel prices. Networks, typically smaller ones characterised with houses at large distances from each other generally have higher prices, while larger networks usually have lower prices. These could also be networks that simultaneously produce heat and electricity (Energiföretagen, 2020). The average district heating prices in Sweden is shown in Figure 2.6. These prices correspond to different house types and can be seen to have generally been on the increase from 2005 to 2020, as the figure shows.
As large cities have the advantage of having a higher customer density, compared to smaller cities, higher costs are usually found in smaller municipalities. For a detached house that uses 15,000 kWh/year of heat, the district heating cost is about 13,700 SEK/year (VAT inclusive). Most networks are in the range of SEK 12,000 – 16,000 as the annual cost for heat. Over the past five years, prices have been relatively stable, with an increment of about 1-2% (Energimarknadsbyrå, 2019). The price model most commonly used is an energy price, expressed in SEK/kWh, which is same throughout the year without a seasonal breakdown. The energy price is a combination of a fixed cost for single-family houses or a power component for various apartments in an apartment building. There are however different pricing models used by district heating companies, which includes season prices model, power price model, flow price model and the fixed price model (Energimarknadsbyrå, 2019), all examined below.
For the season prices model, the cost of district heat is based on the season. Relatively inexpensive heat production methods like waste incineration are often used in the summer, while more expensive facilities are put into operation during winter, due to
increased heat demand. This has led to many district heating companies using a seasonal division of energy price, depending on the season of use. This division is usually done in two or three seasons which are summer, winter and spring/autumn.
The price during summer and winter can gave large differences giving that the price could be 67 öre/kWh from December to February (winter months), 22 öre/kWh from June to September (summer months) and 33 öre/kWh from April to May and same for October and November (Energimarknadsbyrå, 2019). An annual energy cost is obtained by multiplying the energy consumption of the different months by the corresponding monthly price.
For the power price model, the cost of district heat is based on the heating power needs of the property and is either charged as a fixed annual fee or charged as a variable part based on the individual consumption of the customer. The power price is normally based on consumption in kWh over a period, usually one year, with the effect divided by a category number, usually the estimated useful life in hours. The average value of some of the highest measured hourly values in kWh/h (probably in winter period) can also serve as a basis for the power price. Power prices have greater impact on the heat costs for customers who have large variations in their heat demand (Energimarknadsbyrå, 2019).
The flow price model bases the cost of district heat on the number of cubic meters of district heating water that flows through the heat exchanger of a customer’s house.
The flow price model aims to motivate customers to have heat exchangers with high efficiency, so that the temperature of the return water back to the network is as low as possible (Energimarknadsbyrå, 2019). This is because, the more efficient the heat transfer at the customers’, the more low-value waste heat being utilized by the supplier. The lower the temperature of the return water, the more efficiency is increased, as reheating can then start at a lower temperature. The lower the flow in the system, the lower the heat losses and cost of pumping. Some companies however have a seasonal division for the flow price, whereby the customer only pays for the flow price depending on systems, like during winter only (Energimarknadsbyrå, 2019).
Figure 2.6: Average district heating prices in Sweden for different house types, 2005-2020 (Energiföretagen, 2020).
For the fixed price model, part of the district heating price can be fixed, say for a detached house such that the price is the same, regardless of consumption.
Consumption, therefore, does not affect this part of the district heating cost. The fixed part of the price for apartment buildings, can be divided into intervals, depending on the annual heat consumption of the customer (Energimarknadsbyrå, 2019).
b. Electricity price
In Sweden, electricity network is divided into four zones based on geography (Svenska kraftnät, 2021), and correspondingly there are four price areas SE1, SE2, SE3 and SE4 in the Swedish electricity market. Figure 2.7 shows the electricity price areas in Sweden. To control the transmission of electricity between regions and promote the construction of power generation and transmission capacity in and to areas with electricity deficits, the electricity price areas were implemented. The northern parts of the country, SE1 and SE2 are on average, characterized by an excess of electricity production. This is due to the relatively low overall power consumption and available hydropower. Electricity consumption often exceeds production in the Southern parts, SE3 and SE4. This in turn results in these parts having relatively higher electricity prices (Nordpool, 2021).
Figure 2.7: Electricity price areas in Sweden (SCB, 2015b).
In Sweden, there is an electricity price base called the spot price for electricity. This usually follow the spot price for electricity on the Nordic electricity exchange, Nordpool. The spot price is the price at which electricity suppliers buy the electricity, and is a thus a base electricity price, governed by the demand and supply of electricity.
To get the actual electricity prices for consumers, in addition to the spot price, taxes, the electricity supplier and network owner’s fees and VAT are added (elen, 2021).
Figure 2.8 shows the spot price of electricity for the different electricity price areas in Sweden, from January to December 2020. From the figure, the fluctuation of electricity spot prices is visible, with the electricity area SE4 having average higher electricity prices than the other price areas. The average cost of producing electricity in Sweden is 109.87 €/MWh (Nordpool, 2021), as shown in Figure 2.9 as at May, 2021.
Figure 2.8: Electricity spot prices (SEK/MWh) for the price areas in 2020 (Nordpool, 2021)
Figure 2.9: Cost and flow of electricity production in Sweden, March 1st 2021 (Nordpool, 2021).
0 50 100 150 200 250 300 350 400 450
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
SE1 SE2 SE3 SE4
3 Methodology
To achieve the aim of this work, firstly, a groundwork of literature study, carried out to understand both the district heating system and the ORC system, as well as to identify the factors that affect the cost and energy efficiency of heat and electricity in the Swedish energy system was conducted. Data showing the supply and return temperatures from the district heating facility considered in this work, was then used to calculate for the electricity that can be co-produced from the system, as well as the suitable capacity of ORC engine required to deliver optimum electricity production.
The primary energy required to meet this electricity production was also calculated, using the total efficiency of the system. Data from literature, as well as the studied district heating facility (Ingelstad, VEAB) and the ORC equipment supplier (Againity) were used to make these calculations. The cost of the selected ORC engine was evaluated, and this was used to calculate for the levelized cost of the electricity (LCOE) produced. A comparison was made between the primary energy use of the ORC incorporated system and a standalone system, producing the same amount of electricity. This was to determine the effectiveness of primary energy use of the system. Also, sensitivity analysis was carried out, and appropriate recommendations to improve the energy efficiencies and cost-effectiveness of such ORC-district heating CHP systems were made.
3.1. Electricity to be co-produced from the system
The amount of electricity to be produced from the system, as described in the theory is dependent on the supply and return temperatures of the facility, regarding the heat production. It was thus necessary to understand the temperature profile of the facility, and this was analysed for a one-year period, as shown in Figure 4.3. The required data was sourced from Växjö Energi, VEAB for the Ingelstad small-scale district heat facility.
The Againity ORC system has been reported to yield up to 20% electrical efficiency.
Given the total efficiency of the system to be the summation of the heat and electrical efficiencies (Sartor et al., 2014), we have that:
Total efficiency = 𝐻𝑒𝑎𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 + 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑒𝑛𝑒𝑟𝑔𝑦 , (1)
Of which,
𝜂𝑡𝑜𝑡𝑎𝑙 = 𝜂𝑡ℎ+ 𝜂𝑒𝑙 (2) 𝜂𝑡ℎ = 𝑃𝑡ℎ
𝑃𝑝𝑟𝑖𝑚
𝑎𝑛𝑑 𝜂
𝑒𝑙 = 𝑃𝑒𝑙 𝑃𝑝𝑟𝑖𝑚
where
- 𝑃𝑡ℎ is the thermal power
- 𝑃𝑒𝑙 is the electrical power generated by the plant,
- 𝑃𝑝𝑟𝑖𝑚 represents the primary energy used to generate both the electrical and thermal power, 𝑃𝑒𝑙 and 𝑃𝑡ℎ (Sartor et al., 2014).
The electricity to be produced from the system was calculated from Eqtn (1). For this system, a moderate electrical efficiency of 10% was used. The suitable capacity of the ORC engine to be used was determined to be 50kW, from an estimation made by Againity, based on the supply and return temperatures for the existing facility in Ingelstad, as well as the ambient temperature for the period of one year (Feb 2020- Feb 2021). The maximum operating temperature of the boilers was assumed to be 120
℃.
3.2. Primary energy use and avoided production
The energy (fuel) consumed by the system was calculated as well from (1) using a total efficiency value of 99.6%, as reported by Againity. The calculation of the primary energy gave insight to the extra amount of fuel that needed to be fed in to the boilers, in other to optimally get the desired production of electricity by the ORC engine, and as well deliver heat to the district heating consumers at the required temperature. According to the analysis made by Vallios et. al (Vallios et al., 2009)for the fuel consumption of a typical district heating boiler, the annual fuel consumption is expressed as:
𝑚𝑓,𝑎 = 𝑄𝑡𝑜𝑡𝑎𝑙
𝑛𝑡∗𝑁𝐻𝑉 (3)
where
- 𝑚𝑓,𝑎 is the annual fuel consumption (tons/yr) - 𝑄𝑡𝑜𝑡𝑎𝑙 is the annual thermal energy load (MWh)
- 𝑛𝑡 is the overall efficiency of both the boilers and the district heating network - NHV is the net heating value per selected biomass type and mass unit
(kWh/kg).
From this analysis, the mean biomass flowrate per hour is then expressed as:
𝑚𝑓,ℎ = 𝑚𝑓,𝑎
𝑇𝑎 (kg/h) (4)
Where Ta is the annual operating hours of the system (sum of operating hours for each temperature period).
The energy savings calculated for this system, was by comparing this extra primary energy demand, compared to a standalone power plant producing the same amount of electricity.
3.3. Cost of electricity production
The lifetime of the ORC engine was reported by Againity to be 20 years, therefore a lifetime of 20years was used for the base case cost calculations. The production cost of electricity, to be determined will be expressed in SEK/MWh. The parameters which this cost depend on include: the capital cost of the project, fuel cost and operation and maintenance cost. For a given plant, the capital cost per unit of electricity depends on the investment costs, interest rate and return on equity and the equivalent utilization time or load factor of the plant. By adding the capital cost, fuel cost and operation and maintenance cost, the levelized cost of electricity (LCOE) is calculated (Kehlhofer et al., 2009). The estimation of the investment cost was made by Againity, based on the supply, and return temperatures for the facility, as well as the corresponding ambient temperature at each period. The discount rate was assumed to be 4% (Hansson et al., 2016), for the base case, based on the average discount rate used in Sweden, as at the period of performing this work (2021).
The cost of electricity produced was calculated from eqtn (3):
LCOE = Sum of cost over lifetime
Sum of electricity produced over lifetime
=
∑ 𝐼𝑡+ 𝑀𝑡+ 𝐹𝑡
(1+𝑖)𝑡 𝑛𝑡=0
∑ 𝐸𝑡
(1+𝑖)𝑡 𝑛𝑡=0
……….…………… (3)
Where,
𝐼𝑡– Investment expenditures in the year t,
𝑀𝑡– Operation and maintenance expenditures in the year t, Ft – Fuel expenditures in the year t,
i – Discount rate
Et – Electricity generated in the year t.
Among different types of plants, present value is generally the basis for economic comparisons. At different times, the various costs for the plant are incurred. These costs are however corrected to a single reference time for financial calculation purposes. Generally, this is the date on which commercial operation starts (Kehlhofer et al., 2009). The purpose of the discount rate is to calculate how these costs change with time, over the lifetime of the plant (during the periods of operation).
4. Case Study
4.1. District heating system in Ingelstad
The Växjö Energi (VEAB) facility in Ingelstad is investigated in this work as a small- scale district heating system. The facility is located in Ingelstad, Växjö Sweden, which falls under the electricity price area, SE4. An aerial view of the facility is shown in Figure 4.1. This small-scale facility is made up of three biofuel boilers, which are a 1MW wood fuel boiler, a 0.7MW pellet boiler and a 3.5MW bio-oil boiler. In general, approximately 9.5GWh of heat is produced per year, and about 150 customers get their heat from the plant. As a result, the biofuel used in this small- scale system replaces about 1000m3 of fossil-fuel oil.
During February 1st, 2020 - February 28th, 2021, the heat production of the plant was 6 GWh, primarily from the two biomass boilers. The boilers are operational during the winter, while the demand during summer is met by one boiler in operation.
Through multi-cyclones, the flue gases from the biofuel boilers are purified, and the plant has a reserve power equipped in it, to prevent the heat supply being affected by power outage. For the 1MW boiler, the biofuel used is wood fuel and wood chips with a 30-45% moisture content. Approximately 40 m3 of this fuel is consumed per day, when the system is running at maximum load, giving a fuel consumption of about 10,000m3/year. For the 0.7MW boiler, the fuel consumption is about 600tons/year and about 4tons/day, at full load (Växjö Energi, 2021).
Figure 4.1: Aerial view of the VEAB Ingelstad facility (Växjö Energi, 2021).
The process position for the measurement of operations for the VEAB, Ingelstad facility is shown in Figure 4.2 for February 2021. The heat realised from the different boilers in operation are represented in the diagram. The wood fuel boiler, with a capacity of 1MW is represented as FP1, while the Teem pellet boiler with a 0.7MW
capacity is represented in the figure as FP3. The initial process measurement at NDA10 CT201 of 81.1℃ represents the temperature reading from the FP1 boiler at that point, while the measurement at NDA20 CT202 of 100.4℃ represents the temperature reading from the FP3 boiler at that point. The FP2 boiler was at the time, not in operation. The point measurement for the heat temperature after the boilers and after the application of pressure of 2bar at NDA01 CT201 is 93.6℃. The heat temperature from the combination of FP1 and FP3 gives 75℃ at NDA01 CT203, which is the supply temperature to be sent to the distribution network at that point.
The difference between the actual heat temperatures at the boilers and the supply temperature is due to the heat utilized to reheat the return water from the return networks, which at the point of measurement, as represented in this figure is 37℃ at NDB01 CT201. The heat utilized for this operation leads to a temperature drop of the heat that goes into the distribution networks from the boilers.
Figure 4.2: Process operation at VEAB, Ingelstad facility, February 2021 (Source:
VEAB, Ingelstad 2021)
The supply and return temperature of the district heating system, as well as the ambient temperature for the facility during February 2020 – February 2021 are presented in Figure 4.3, for one-year period. The temperature point measurements were made at 3-hours intervals, and for the different days and months. This
information will be used to calculate the amount of electricity that can be produced, when a cogeneration facility is incorporated with this heat production.
Figure 4.3: Supply and return temperatures for VEAB, Ingelstad, Feb 2020-Feb 2021 (Source: VEAB, Ingelstad 2021).
4.2. Small scale Organic Rankine Cycle Engine
An ORC engine manufactured by Againity is considered in this study. Againity is an ORC system specialist company based in Norrkoping, Sweden. The Againity technology is based on converting low grade heat to power, as an ORC system, as shown in Figure 4.4. The technology involves a steam turbine set in motion, due to the pressure from hot steam. Electricity is then produced by a generator, driven by the rotating turbine. Electrical efficiency for these systems can be up to 20%, in good conditions. The turbine used in Againity’s ORC system is a compact unit with few moving parts, which results in a reliable turbine with long life. For the Againity ORC system, a heat source with a temperature from 90℃ can be utilized, to heat up the organic fluid to steam (Againity, 2021a). For small scale district heating systems where the heat produced from the boiler is usually close to that temperature value, this serves as an excellent way to co-produce heat and power to meet demand.
The ORC modules are typically of different capacities, from the AT50, which is the 50-kW capacity engine to the AT2500, which is the 2.5 MW capacity engine. The capacity of the ORC engine to be installed is usually dependent on the potential electricity to be generated, which is in turn dependent on the heat demand of the heat source, in this case the Ingelstad district heating system.
-40 -20 0 20 40 60 80 100
Temperature (℃)
DH supply temperature(℃) DH return temperature (℃) Ambient temperature (℃)
Figure 4.4: Againity ORC system connected to a district heating system (Againity, 2021a).
The Againity ORC engines have a typical lifetime of 20 years and can be applied for energy purposes in different ways. Some of these ways include: In recovering waste heat from gas turbines and diesel generators and converting to electricity, converting heat from incineration of waste to energy, conversion of heat from power plants to electricity. Given the low number of moving parts in the Againity system, the need for service and maintenance is minimized, and this significantly reduces the payback time to a shorter time (Againity, 2021a). The organic working fluid used in the Againity ORC system is R1233zd. This is a type of new generation organic fluid for ORC systems. The organic fluid is a hydrochloflouroolefin type of fluid with molecular structure of C3Cl H2F3 (Eyerer et al., 2016), with low ozone depletion potential (ODP) and low Global warming potential (GWP) of about 4 (Againity, 2021a). These characteristics of the organic fluid used in the system, shows it meets the criteria for a suitable fluid for the Organic rankine engine process. The fluid’s low ODP and low GWP properties gives it a sustainability advantage, and in extension gives a sustainability advantage to the Againity ORC system.
4.3. Assumptions and Calculations
4.3.1. Total Fuel Consumption and Fuel cost Calculation
The total fuel consumption was gotten as the summation of the monthly fuel consumption for the gross electricity production for each month. The values for the Net electricity production for each month was gotten from Againity. However, the
