Design of small cogeneration system for public buildings in the town of Matera

Population growth and economic development increases energy consumption.

An analysis of Italian National Energy Budget in the last decades shows an important role of the civil sector and of the impact of fossil fuels in air conditioning systems. The predominance of buildings using conventional system causes a high consumption of fossil fuels [1]. This problem causes the necessity to find new renewable energy sources characterized by a constant production of thermal and electricity energy. Photovoltaic, solar thermal and wind energy are discontinuous energy sources and, for this reason, they need the integration with other fossil sources to produce thermal and electric energy.

Most of the world’s energy is currently produced and consumed faster than earth is able to restore and this compromises its sustainability. Moreover, the energy resources in the world decreased while energy needed by humanity increased. As it is stated by [2], there are two ways to deal with these problems. One of these is finding and improving new renewable energy sources such as solar or wind energy systems. The other way is to improve conventional energy converting systems for using existing energy source more efficiently and for longer time, such as cogeneration systems [2]. Cogeneration system applied to the case studies can guarantee us some proven, reliable, applicable and cost-effective solutions which could give an important contribution in global heat and electric demand reduction. Most of electric energy generated in Italy and in the world comes from thermal power plants, in which high-temperature heat is before converted into mechanical energy and then into electric energy by means of electric generators.

Cogeneration systems, also known as CHP (Combined Heat and Power), give us some significant advantages thanks to their environmental advantages. In this system, there is an increase in efficiency and a decrease in fuel use compared to other conventional processes for thermal and electrical energy production which, normally, yields a large reduction in greenhouse gas emissions.

Sustainability of cogeneration systems can be obtained using this heating system together with some renewable energy conversion systems like solar or heat pumps system. Cogeneration system can also be used together with different systems to save more energy. An example is given by the study of Burer et al. [3] and named "multi-criteria optimization of a district cogeneration plant integrating a solid oxide fuel cell–gas turbine combined cycle, heat pumps and chillers". The conversion of heat into mechanical energy, which is the most complex transformation, is performed through a thermodynamic cycle: Rankine and Hirn cycles and the Brayton-Joule cycle in turbo-gas installations [4].

Cogeneration, using the same fuel for a combined production, uses primary energy more efficiently.

Generally, CHP systems are formed by a primary motor, a generator, a heat recovery system and electrical interconnections. The primary motor is any engine used for converting fuel into mechanical energy, the generator converts it into electric energy, while the heat recovery system collects and converts energy contained in the primary motor discharges into usable thermal energy.

As is stated in [5], combined production can increase the efficiency of use of fossil fuels of more than 80%. This permits lower costs and lower emissions of pollutants and greenhouse gases compared to separate electricity and heat productions.

Unlike power plants, CHP needs small systems that are able to generate heat and electricity for large structures (i.e. hospitals, hotels, etc.) or small urban centre.

In small-cogeneration system, combustion reaches savings up to 40% in the use of primary energy sources.

Figure 1 illustrates the comparison between conventional and combined production; in the case of the combined production the total efficiency is higher than a separate production.

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Figure 1. Separate production of electric and thermal energy versus cogeneration production [5]

As is shown in Figure 1, in separate electric and thermic productions, we need about 148 units respect to 100 units in cogeneration production. A CHP system with thermal and electrical production processes requires 35% less fuel than the electricity and heat production separate configuration.

The consequential evolution of an integrated system of cogeneration is tri-generation. It is possible, using a single source of primary energy, to obtain a production of electricity (or mechanical energy) and the heating for the cooling process.  These systems present a lot of advantages especially in dairy industries when there is the maximization of operating hours, in which happens self-consumption of heat and electricity cogenerated, increases the investment convenience. [6]

As stated by [6] a district heating service, even if it has undoubted advantages from the energy point of view, it is often less efficient compared to the modern technologies. Nowadays, in fact, condensing boilers and heat pumps provide energy efficiency higher than the “traditional” district heating (steam turbines and gas). From the environmental point of view, the comparison on pollutant emissions is even more surprising, since, thanks to the many technological improvements that the boilers have had in recent years of development (low-NOx burners, modulating, premixed), their use generates a much lower environmental impact than the district heating plant (including combined cycles). To overcome this important gap, cogeneration plants should be equipped with systems to reduce pollutants much more effective than the today used [7].

The main benefit of a CHP system respect others system is its efficiency. Environmental Protection Agency (EPA) defines “simple efficiency of a single system” as the ratio of the net electrical output and the amount of fuel consumed.

The global efficiency is given by the ratio of the sum of the useful thermal energy and the total electrical/mechanical energy produced and the fuel energy input into the cogeneration energy production system (see Eq. 1) [5].

$\eta =\frac{{{Q}_{el}}+{{Q}_{th}}}{{{Q}_{ch}}}$     (1)

The first advantage of cogeneration is clearly economic: a well-designed cogeneration system provides significant energy savings of 25-40%.

Energy performance is equal to 70-85%, a really significant value compared to the 50-60% of the new combined cycle plants for electricity production only.

To design a CHP system, it is necessary to define:

•  Climate data;

•  Thermal energy needs for heating and hot water;

•  Electric energy request;

•  Calculation interval (month);

•  Performance data of cogeneration section.

The cogeneration plant was designed on electricity consumption of the entire district because electricity production of a CHP system is only of 32.5% respect to the heating production. If we designed CHP system on heating request, we obtained a very high production of electricity, which cannot be used by users.

The estimated cogeneration power is 90 kW_el, so it is a small-cogeneration plant as established by [14] because it is less than 1 MW [14]

In Figure 6, the authors analysed the covering of CHP electricity production on electricity demand for the entire district. Annual electricity demand is covered by 92%.

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Figure 6. Electricity demand and CHP electricity production

To eliminate or to reduce the need for a timely coincidence between the thermal power output and the incoming request to the distribution system, we considered an inertial heat accumulation system. Performance under actual operating conditions will therefore be influenced by the correct sizing of the system.

In our case we installed a storage system of 5000 litre with a maximum flow temperature of 75 °C and an average return temperature of 45 °C. To have a perfect system sizing it is necessary to verify the congruity index α defines by UNI/TS 11300-4 [15] (see Eq. 2) as:

$\alpha =\frac{{{Q}_{CG,s}}}{{{Q}_{CG,s,design}}}$   (2)

- if α >1 storage is oversized but it is considered congruous

- if α =1 storage is perfectly sized

- if 0 < α <1 storage is present but no congruous

Since the demand for thermal energy is variable in the different calculation periods (month), the congruity index cannot be equal to "1" throughout the year, so if you want to have a more reasonable sizing (α ≥1), it should be designed on the most demanding requirement.

In heating period congruity index α is always no congruous because its value is 0.62. This value is acceptable because system is design to cover the maximum electricity demand but it does not cover the global thermal energy demand.

Regarding heating energy request (Figure 7), the cogeneration system designed is able to cover the 28% of yearly thermal energy needs (Table 8).

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Figure 7. Heating energy request and CHP heating production

Another analysis was conducted in case Public Administration decided to improve the energy performance of envelope in order to reduce the district heating energy demand.

The authors assumed to consider only interventions that not compromise architectural appearance of buildings.

The assumed energy improvement measures are:

• Insulation of flat roof with inner coat made with thermal insulation wall, composed of a plasterboard sheet thickness mm 10 coupled to an insulating layer consisting of panel hemp fibre with thermal conductivity λ=0.030 W/mK;

• Installation of windows with double or triple low-emissivity glass, with cavity filled with inert gas and the wooden frame with finishes and colours compatible with those of the opaque walls.

Heating consumptions are estimated through the dynamic method using Design Builder interface based on Energy Plus code [13].

In the case of combined intervention on envelope and heating system, thermal request has a high reduction (Figure 8). Annual heating energy request is covered for 68 %.

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Figure 8. Heating energy demand and CHP heating production in the case of combine intervention (CHP and improving on envelope)

The assumptions to improve energy performance are in accordance with the parameters laid down by the Decree Minimum Requirements for 2021 [16].

Thanks to the reduction of energy requirement, the cogeneration system is allowed to cover heating energy demand most of the heating period.

The analysis conducted on the assumption of a district cogeneration system used by school buildings highlights and confirms that it is a great potential source of energy savings in the building-plant system.

The case study described acts as a driver for the next replicability of the intervention proposed in other similar contexts, since it is configured as a reference criterion, applicable to almost all of the public buildings belonging to the national heritage.

The hypothesis of a unique installation for several buildings stands as a starting point for a proposal for reduction of energy consumption that unifies and reduces the number of thermal plants currently present.

This would allow, thanks to the innovative generation systems, to reduce demand for primary energy and lower maintenance costs borne by the Public Administration.

These actions, in addition to providing solutions to the needs of management optimization of the buildings, also meet the transposition duties of current European and National legislation.

Through the cogeneration system designed we can guarantee an energy savings of 53% in the case of only replacement of plant, and of 86% in the case of combine intervention on envelope and CHP installation.

Regarding the pay-back time, it is constant even in the case of an increase of price cost rate and it is between 3-4 years in the case of only CHP installation and 6-7 years in the case of combine intervention on envelope and CHP installation.