Thermal stores including phase change materials have the potential to store larger amounts of thermal energy within smaller temperature ranges when compared to water-based thermal stores. Due to the low thermal conductivity of many PCMs, poor rates of thermal diffusion within the PCM can reduce significantly the nominal storage system charge and discharge heat transfer rates.
Due to the versatility in store geometry possible with encapsulated packed bed systems they appear to be a feasible option for integrating PCMs into conventional domestic space heating systems. Inserting the PCM filled spheres into standard water buffer tanks can be achieved without major technical challenges. The lower PCM volume fraction achievable when compared to more compact store designs such as the tube in tube, can limit the increase in heat storage capacity; however, the increase in specific heat transfer area reduces the effect of having low thermal conductivity.
The numerical models developed to evaluate the replacement of conventional gas fired boilers with heat pumps coupled with a PCM thermal store to offset heat pump operation from peak electrical demand periods, predicted that CO2 emissions for all the three dwelling studied could be reduced yearly by 51.8% using a packed bed of PCM spheres and 57.3% using the compact latent heat store with current grid emission values, as shown in table 6.20.
Table 6.20: Comparison of the results obtained for all the systems studied. Yearly energy used [kW h/year] Yearly CO2emitted [kgCO2/year] Average
energy savings [%]
Average CO2savings
[%] Terraced Semi detached Detached Terraced Semi detached Detached
Reference 7717 10520 8878 1574 2146 1811
Buffer tank system 2305 2819 2546 763 1153 939 71.5 51.2
Packed bed system 2262 2741 2489 385 606 496 72.2 51.8
Compact system 2066 2656 1985 212 346 304 74.8 57.3
Economically, the integration of PCMs into domestic hot water systems is a costly solution, using current energy prices, the best levelized cost of heat obtained with the compact latent heat storage system was 127.77£/M W h. A carbon tax added to current natural gas consumption of 11£/tonCO2 is required to make the levelized cost of energy supplied using natural gas equal to that using the compact latent heat storage system, assuming a 20 years life cycle, this value is still less than the assumed 17£/tonCO2 by the UK government for 2016. If 31% of the initial capital expense is supported by the government, replacing current gas boilers with air source heat pumps with a compact latent heat store had an average return on investment for a 20-year cycle around 60% for all the dwellings modelled.
Shifting domestic hot water and space heating loads into off peak periods of the national grid by using compact thermal energy storage systems will allow further integration of heat pumps, so that its use does not adversely affect the electrical supply system.
Chapter 7
The potential of phase change
energy storage to integrate
industrial waste heat into district
heating networks
District heating (DH) systems are comprised of insulated piping networks which deliver heat via steam or hot water to serve the space and water heating demands of multiple buildings [28]. They are generally seen as a convenient, economic and environmental-friendly way to supply heat to a large amount of buildings.
While the technical components of DH are relatively mature [193], their inte- gration into the UK’s space and water heating market is somewhat unsuccessful (around 2%, compared to 47% in Denmark and 55% in Sweden in 2012 [28]).
There are two main limitations to significantly scale up DH installations in the UK. First, while DH is an inherently local infrastructure, it is competing in a lib- eral and private market, where the main players international scope challenges the development of locally-specific systems [193]. Secondly, shifts in the role of local government, from service provision to enabling others to provide services led to an increase in public and private service providers, reducing the in-house capabilities of local authorities to plan, design and operate technically and financially viable schemes [193].
Integrating industrial waste heat into a district heating network can offer the possibility to provide cost effective hot water and space heating, improving also the industrial process efficiency, although the process industry heat source distance to the DH network often minimizes available heat and economic gains due to increased piping heat losses and costs. Phase change materials could improve the economic feasibility of such systems by providing a portable thermal storage solution, charged with industrial process waste heat and discharged to nearby DH networks.
7.1
Overview of the district heating networks in
the UK
There are 81 district heating schemes implemented in the UK as of the end of 2016 according to [194]. In practical terms, a DH network should operate between 58-
82◦C [195], in order to obtain relatively low flows, hence minimizing the district heating pipe cross section and its heat losses. A brief description of some of those schemes will be made in the next paragraphs.
7.1.1
Aberdeen heat and power Ltd.
Aberdeen heat and power ltd. was created in 2002 by the Aberdeen city council to develop Combined Heat and Power (CHP) schemes for the city to investigate the potential cost reductions in providing communal heat and reduce the council CO2 emissions. It has four projects running (Seaton, Hazlehead, Stockethill and Tillydrone).
In Seaton, a gas engine sells 2.1M We to the electrical grid whilst providing 7M Wth (backed up by a gas boiler) to a district heating network composed of 14 multi-storey blocks consisting of 1243 flats, 5 public buildings and a swimming pool. The second project installed was the Hazlehead energy centre, with a CHP pro- ducing 212kWeand distributing 350kWthto four multi-storey blocks comprising 209 flats, a sheltered housing scheme, Hazlehead academy, public swimming pool and sports pavilion. Due to the different heat profiles of the public buildings and the blocks of flats, the CHP can run throughout the year.
Stockethill energy centre has 2 steam boilers producing steam to a 212kWe steam generator and distributing 300kWth to 4 blocks with a total of 268 flats. Due to the load being only domestic flats, the steam generators do not operate 24/7 due to the heat profiles of the domestic users. The site has been extended to provide heat to 4 more blocks comprising 350 flats and a bigger generator is being installed.
7.1.2
Thameswey Energy Ltd.
Thameswey energy Ltd was created in 1999 by Woking borough council, part of the Greater London urban area. The company developed a joint venture with a Danish company to develop a CHP with DH network unit in Woking and one in Milton Keynes.
Woking CHP unit comprises a Deutz DK-TBG620 V16K gas engine providing 1.4M We and 1.4M Wth and 2 backup gas boilers with 1.5M Wth capacity, providing district heating to 2000 residential buildings and 170 commercial offices.
Milton Keynes CHP unit comprises two GE Jenbacher J624 gas engines that provide 3.2M We and 3M Wth each, and a 10M Wth backup gas boiler to supply district heating to nearly 3000 residential buildings and over 1100 commercial offices.
7.1.3
Birmingham Distric Energy Company Ltd.
In 2006, the Birmingham city council developed a partnership agreement with the energy services company ENGIE, to construct a CPH with district heating network under a 25-year “concession contract”.
Birmingham district heating network comprises 3 individual schemes: Broad street scheme, Aston University and Birmingham children’s Hospital. The broad street scheme comprises one CHP unit with a capacity of 3.6M We and 3.5M Wth, backed up by a gas boiler with 11.8M Wth of capacity. It provides district heating to 5 public buildings, one hotel and to Cambridge and Crescent residential towers, with a total daily heat consumption of 55.6MWh.
The Eastside scheme, completed in 2009, has 2 gas engines in Aston Univer- sity (one with 2.0M We and one with 1.0M We) with a total electrical capacity of 3.0M We and a thermal capacity of 2.9M Wth, backed up by a combined capacity of 14.3M Wth backup/top-up gas boilers (two with 3.5M Wth and one with 7.3M Wth). A new energy centre was added to Birmingham children’s hospital and connected to the East side heat network with a 1.6M We gas engine and two 4.5M Wth back up gas boilers. The heat network provides 4 public buildings (Aston University, Birmingham children’s hospital, new Birmingham magistrate’s court and Lancaster circus) and 3 clusters of buildings (Bagot street, Woodcock street and Masshouse block M), with a daily heat consumption of 54.8MWh The 2 heat networks were connected in 2015, with the addition of a new gas engine providing 1.6M We and 1.55M Wth in Birmingham new street station.
All of the example district heating networks briefly detailed in the paragraphs above are agnostic to the heat source being used; hence the indirect integration of industrial waste heat via a portable latent heat store could have some economic feasibility, and potentially avoid more CO2 emissions by reducing the use of the backup gas boilers.