Nici Bergroth, Fortum, Finland
Fortum applied in February 2009 for a political permission (decision-in-principle) from the Finnish government to construct a new nuclear power plant unit (Loviisa 3) at its existing Loviisa nuclear power plant (NPP) site. The application concerns a nuclear power plant unit ranging from 2800MWth–4600 MWth. The location of the Loviisa NPP site, at the southern coast of Finland approximately 75 km east of the Helsinki metropolitan area (i.e. Helsinki, Espoo and Vantaa) with one million inhabitants, offers a good opportunity for large-scale district heat generation for the region from the Loviisa 3 unit.
By combining heat and power generation into the design specification, Loviisa 3 unit could generate about 1000 MWth of district heat for the Helsinki metropolitan area. This is an attractive alternative investigated, for its great potential to reduce Finland’s carbon dioxide emissions. Nevertheless, the project would be a challenge, partly due to the large district heat generation output envisaged, and partly due to the rather long transportation distance of the district heat required.
NUCLEAR DISTRICT HEAT REFERENCES
Combined heat and power (CHP) generation in nuclear power plants, for example for district heat generation, is nothing new, although its use in the world is presently not extensive. Today, only a small percentage of the heat generated in nuclear reactors worldwide is used for industrial purposes; desalination and district heating. With the nuclear renaissance and the vast new build programmes in-sight all over the world this will hopefully change. By replacing heat generated with fossil fuels with heat generated by nuclear power climate change can be mitigated even further.
Nuclear energy has been and is used for district heating in several countries both in dedicated nuclear heating plants and in CHP plants. Most extensively, nuclear CHP generation is used today in Russia, where also several new nuclear district heating applications are planned. One application is currently under construction at the Leningrad NPP phase two. District heat is also produced along with electricity at the moment in nuclear power plants, for example in Bulgaria, Hungary, Slovakia, Ukraine and Switzerland. The heat generation capacity of the existing plants is, however, usually rather limited, with plant specific outputs ranging from 10 MWth to 250 MWth. Set against this, the Loviisa 3 unit’s 1000 MWth district heat generation capacity is much more ambitious.
WIDE-RANGING ECOLOGICAL BENEFITS
The idea to transport district heat from Loviisa NPP to the Helsinki metropolitan area is not new. The possibility was investigated back in the 1980s, after Loviisa NPP’s two existing VVER-440 pressurized water reactors were commissioned in 1977 and 1981, but it proved impractical at the time. Since then, the world has changed. Today, global warming, greenhouse gas emission rights and carbon dioxide reduction targets are a reality. Climate change imposes tough requirements and challenges on heat and power generation. In this context, the idea is much more attractive today.
A view of the Loviisa site, with the proposed new plant as an artist’s impression
By replacing district heat generated with fossil fuels in the Helsinki metropolitan area, large-scale district heat generation in Loviisa 3 unit would reduce Finland’s annual carbon dioxide emissions by as much as 6 per cent, or by up to 4 million tonnes. In addition to large carbon dioxide emission reductions, large-scale CHP generation would increase the overall efficiency of Loviisa 3 unit significantly, and hence reduce also the environmental impact on the local marine environment by cutting heat discharges into the Gulf of Finland.
DISTRICT HEATING IN THE HELSINKI AREA
The heating energy in the Helsinki metropolitan area is mainly based on district heating, but only in the city of Helsinki is the share of district heat over 90 per cent. The district heat consumption in the region is on average 11 TWh to 12 TWh per year. Typically, consumption varies in the region from a minimum of 400 MWth during the summer to 3000-3500 MWth peak loads during the winter. For a typical 12 TWh annual consumption, the consumption is higher than 1000 MWth for approximately 60 per cent of the year. Consequently, Loviisa 3 unit could ideally provide a very significant portion of the district heat base load needs of the region.
District heat for the Helsinki metropolitan area is currently produced mainly by coal and natural gas fired CHP plants producing 5–7 million tonnes of carbon dioxide emissions per year. In the Helsinki metropolitan area there are currently three separate district heat networks; those of Helsinki, Espoo and Vantaa. The networks are interconnected, but they are owned and operated by different district heat producers.
The district heat network of Helsinki is the largest one, with over 13 000 real estate consumers connected to the network. The majority of the consumers are residential buildings. The district heat consumption is in Helsinki alone over 6 TWh per year. The total length of the district heat network of Helsinki is around 1200 km and it covers almost the entire city. Over 20 km of new district heating network is built each year. The district heat network of Helsinki is constructed so that the heat can be distributed to the consumers via several different routes. Due to this, the availability of district heat is very high.
In Helsinki, the total installed capacity is around 1150 MWe and
3600 MWth, with the largest baseload CHP power plants located in Salmisaari, Hanasaari and Vuosaari. The Vuosaari power plant is connected to the central city area, by an approximately 30-km long tunnel, which is the longest continuous district heating tunnel in Europe. For peak load and reserve capacity, heating plants with capacities ranging from
56 MWth to 336 MWth fired with natural gas and oil are connected to the network in various locations in Helsinki.
The heat and power generation arrangements in Espoo and Vantaa are in general similar to those in Helsinki, with total installed capacity of around 360 MWe and 1300 MWth and approximately 200 MWe and
1000 MWth respectively.
LOVIISA 3 CHP CONCEPT
The Loviisa 3 CHP concept is based on a completely separate, closed district heating circuit, where the district heating water is heated up to approximately 120 °C using steam extracted from the turbine. The heating steam is extracted from the turbine in a way that is good in terms of overall efficiency. The heated water is then pumped along a dedicated underground pipeline to the Helsinki metropolitan area, where the heat is transferred to the region’s local district heat network via heat exchangers. The approximately 60 °C return water is pumped back to the Loviisa NPP site, where it is reheated, and the cycle is started over again. The temperature levels are preliminary and based on the reference levels from the district heating networks in the Helsinki metropolitan area.
Loviisa 3 unit would be based on advanced light water reactor technology, either pressurised water reactor (PWR) or boiling water reactor (BWR) technology. The unit will be designed from the start and optimized to generate both electricity and district heat in large-scale with high efficiency. The changes required, are mostly limited to the turbine and turbine plant, in order to enable them to handle both electricity and district heat generation needs with high efficiency and the very large district heat generation capacity.
Optimizing the turbine and the turbine plant processes to be capable of generating as much electricity as possible during different operation modes, including full condensing mode, is of central importance. When steam is extracted from the turbine for heating up the district heating water, the electrical output at the generator terminals will consequently drop. For Loviisa 3 unit, the target is that the electrical output would drop by less than around one-sixth in proportion to the generated district heating power, i.e. 1000 MWth district heat generation would reduce the electrical output by around 160-180 MWe.
Large scale CHP generation would, however, increase the unit’s overall efficiency significantly. During full district heat generation, the efficiency would in the Finnish cold cooling water conditions increase from around 39 per cent to approximately 55-65 per cent, depending on the considered plant alternative.
The starting point is that the CHP generation design has to be such that it will not impair the nuclear safety of the unit. Furthermore, both plant types will encompass two physical barriers in order to reliably prevent the spreading of any radioactive material from the unit to the district heat transport system under any conditions. As the turbine plant processes in a PWR plant are isolated from the radioactive reactor circuit during normal operation, the district heat transport system can be connected directly to the turbine plant processes via heat exchangers. A BWR plant needs a separate intermediate circuit, because of the radioactivity inherently present in its turbine plant processes during normal operation.
DISTRICT HEAT TRANSPORT SYSTEM
One of the biggest technical challenges in the Loviisa 3 CHP concept is the district heat transport system between Loviisa NPP site and the Helsinki metropolitan area. Two major alternatives are being investigated; one where the district heating pipelines would be installed in an approximately
85-km long tunnel excavated out of the bedrock and one where the pipelines would be installed in an approximately 100-km long ditch excavated from the ground surface.
In both options, pipes measuring approximately 1.2 metres in diameter are required, in order to restrict the flow velocity and pressure losses. For the preliminary parameters, the total heat losses with standard insulation are around 20 MW, which is relatively small compared to the transported heat load and distance. Between four and seven pumping stations, with a total pumping capacity of 40-50 MW, are needed along the route to maintain sufficient pressure in the system. In general, the energy inserted to the system by pumping compensates roughly the heat losses. Pumping stations with two parallel pumps, each with a 100 per cent capacity, in both lines, will ensure high reliability of the district heat transport system.
The district heat transport system will be connected to the district heating network of Helsinki, from where the heat can be distributed further to Espoo and Vantaa. The existing interconnections between the networks in the region might need strengthening. By taking into account the capacity of the main lines of the district heating network of Helsinki and distributing the district heat load from Loviisa 3 unit in more than one location, the existing large CHP power plants in Helsinki can also generate heat to the network when needed. The connection of the district heat transport system to the district heating network of Helsinki requires in addition to heat exchanger stations, a large heat accumulator for short-term storage and buffer.
THE UNDERTAKING REQUIRES CO-OPERATION
District heat generation in Loviisa 3 unit for the Helsinki metropolitan area cannot be realised by Fortum alone, it requires co-operation amongst all the district heat producers in the region as well as the political will. The solution has to be such that it is economically profitable for all parties. The connection of the district heat transport system to the Helsinki metropolitan area must eventually be designed in co-operation with all the district heat producers in the region. All the district heating networks, heat generating power plants and heating plants must be considered as a whole, taking into consideration necessary reserve capacity arrangements.
An independent study carried out by Pöyry Management Consulting, on assignment by Fortum, shows that large-scale district heat generation by Loviisa 3 unit is clearly the most economical and environmentally sound alternative for the future heat generation in the Helsinki metropolitan area. Despite this, the implementation of the district heat transport system between Loviisa NPP site and the Helsinki metropolitan area is currently ambivalent politically. However, Fortum is committed to design and build the Loviisa 3 unit intended for large-scale district heat generation for the Helsinki metropolitan area, to be realized either immediately or at a later stage.
Unfortunatley, the Finnish Government decided in late April that it would give a negative ‘decision-in-principle’ on Fortum’s application concerning the construction of the new Loviisa 3 nuclear power plant unit.
Nonetheless the above article does offer a useful viewpoint on how to maximize the operation of a nuclear power plant by using its waste heat for district heating. This is especially true if the anticipated new nuclear build takes off in both Europe and globally.
The article was first published in our sister title, Cogeneration & On-Site Power Production, May–June 2010.