Sustainable Carbon Footprint Reduction by Integrating Renewables Into Total Sites

The majority of industrial, residential, service and business, agriculture farms and production settlements energy systems are still dominated by the fossil fuels as primary energy sources. They are mostly equipped with steam and/or gas turbines, steam boilers and water heaters (running on electricity or gas) as energy conversion units. The challenge to reduce the Global Warming impact can be efficiently addressed by increasing the share of renewables in the primary energy mix, leading to smaller carbon footprint of the related activities. This requires integrating solar, wind, biomass, as well as some types of waste together with the fossil fuels.

The current work analyses integrating renewable heat into Total Sites comprising industrial processes, residential areas, building complexes (hospitals, universities, hotels, business centres) and farms. The variability in energy demands and supplies are taken into account.

1. Introduction

Renewable resources are usually available on smaller scale distributed over a given area. Their availability, with the exception of biomass, is usually well below 100 %. The resource availability varies significantly with time and location. This is caused by the changing weather and geographic conditions. The energy demands (heating, cooling and power) of the considered sites vary significantly with time of the day and period of the year.

Therefore, designing energy conversion systems for utilisation of renewable resources is more complex than when using just fossil fuels. By combining the supply and demand streams of the individual users, such systems may serve industrial plants as well as residential customers and the service sector (hotel complexes, hospitals).

A novel approach has been extending the Heat Integration methodology to including waste and renewables (Perry et al. 2008) for a given steady state of supply and demand.

2. Issues and concepts for resolving them

2.1. Combining consumer sites and resources

The energy demands vary with the types of end users as well as with the time schedules. However, whenever it is geographically and safety- feasible, the simultaneous integration of Industrial sites, residential areas and service buildings with renewable energy sources can bring significant benefits. The main reasons for this are that the application demands vary in different ways, potentially complementing each other, as well as the possibility for inter-process heat integration.

2.2. Demand characteristics

Both the supply and demand for energy vary with time and location. To simplify the initial analysis, a given fixed a set of locations is assumed.

The time variations of energy demands have been subject to research both in industrial and residential contexts. An example is a study investigating the variation of residential energy consumption for heating, electricity and hot water (Bance, 2008). The results show two types of trends ? hourly variations during each day and seasonal variations during the year. For the hourly variations there are nearly steady periods during the usual office hours and two consumption peak intervals in the morning and in the evening. The seasonal variations are relatively smooth with more substantial space heating demands from October until April.

For their efficient exploitation it is necessary to assess renewables' overall availability and variability with time. Some of them are close to the performance of fossil fuels and can be well stored for continuous energy generation. An example is biomass, where the supply varies by year seasons and by bio-waste availability. However, sufficient storage could be made available. The availability of other renewable sources as wind and solar varies faster ? in hours and even minutes.

These types of variation present an integration challenge where the time horizons of the changes are diverse. From the given examples, for biomass, the time slices needed would last on the order of months and at smallest ? weeks. For wind and solar energy, the time slice durations will obviously be much shorter. This brings a necessity to extend the Total Site methodology (Dhole and Linnhoff, 1993; Kleme? et al., 1997) to deal with the described variations.

2.3. Integrating the total sites including renewables

The Heat Integration methodology has been traditionally dealing with industrial plants where most effort has been made to achieve the stable steady-state production (Linnhoff and Hindmarsh, 1983). It was summarised by Smith, (2005) and more recently by Kemp (2007). The second step has been to integrate more industrial plants into Total Sites (Dhole and Linnhoff, 1993; Kleme? et al., 1997), where more than just industrial plants may be included (Perry et al. 2008).

The maximum availability of renewables is usually limited and cannot be controlled by the energy conversion system. To integrate renewables as fixed and not belonging to the degrees of freedom is obviously not correct. What can be used as a degree of freedom is the fraction of the renewable resources to be harvested, compared with their overall availability.

3. Integration approach

To account for the variation of the demands, the renewables availability and simultaneously maximising the heat recovery, it is necessary to apply Total Site integration and when needed to consider heat storage possibilities. This is performed by constructing Total Site Profiles and Total Site Composite Curves for a set of time intervals, referred to as ?Time Slices? and maximising the heat recovery within each Time Slice. This formulation extends the methodology developed previously for batch processes (Kleme? et al., 1994; Kemp and Deakin, 1989).

A very important issue related to the system design is the need for heat storages. If this is available at given time and required capacity with feasible cost it could considerably increase the system efficiency. The energy storage is complicated and demanding issue, which is still waiting for major breakthrough. The Heat Integration methodology could contribute to this problem solution by providing targets which are supposed to be achieved.

4. Conclusions

The inclusion of renewables with their changing availability requires extensions of the traditional heat integration approach. The problem becomes more complicated and has several more dimensions. Revisiting some previously developed Process Integration tools and their further development enables solving this extended problem. The presented contribution has been a step in this direction summarising the problem and suggesting some options for its solution. A demonstration case study illustrates the heat saving potential of integrating various users and using heat storage. The advanced tools based on the suggested methodology have been under development.

References

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