Cogeneration and photovoltaic, the union that works

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Combined heat and power (CHP) continues to be the most effective technological solution for making energy efficient energy intensive processes while achieving environmental and economic benefits. Even more so if you apply it in combination with renewable sources of energy production, such as solar photovoltaics. To say so are the numbers.

For years, cogeneration has been the technology that best meets the needs of large energy-consuming enterprises, and the advantages it promises are attractive in several respects. First and foremost, it is a huge help in bringing down energy costs and enabling energy intensive companies to remain competitive in the marketplace-especially in Italy, where energy costs are higher than in other countries. Then it is an environmental sustainability choice and, as a result, helps companies corroborate their eco-sustainability in terms of image; finally, it offers a proactive contribution to countering the energy crisis.

Despite all this, other energy solutions are considered better than CHP in all the aspects just considered. The need to make the ecological transition is established and shared, however, in order to make truly effective choices in a perspective of reducing climate-changing gas emissions and reversing the effects of climate change, it is essential to evaluate with real and scientific data which energy sources are really convenient to use. And Marco Cuttica, senior sales engineer at AB Energy, did just that – during the “White Focus 2023” webinar organized by White Energy group – showing what the advantages of cogeneration are for industry, in particular, when integrated with photovoltaics.

Cogeneration efficiency data

First, Cuttica highlights an initial comparison, between traditional energy supply and cogeneration. With the former, a company has to purchase electricity from the grid with which to power its industrial process and possibly supply chillers for refrigeration, while with gas taken from the grid it will supply boilers for hot water production or higher-temperature heat carriers needed for the production process.

With cogeneration and trigeneration, on the other hand, an industry can feed, with gas taken from the grid or possibly with biogas, a generator with which to produce hot water, while with the flue gas it can produce the thermal carrier appropriate to its needs-for example, hot water, superheated water, diathermic oil, steam-or feed hot water to an absorber that produces chilled water to be sent later to the production process.

Comparing data from two plants using the two different modes of energy supply provides interesting information. The numerical data give the real weight of the difference between the two conditions and more importantly, Cuttica points out, how efficient cogeneration is. “If, for example, you introduce 100 kW of primary energy in the form of gas into a cogeneration plant, you will have more or less a 15 percent loss but an electrical efficiency around 41 percent – a very conservative value of the latter, calculated on average plant sizes, as larger engines can actually get as high as 45 percent or 46 percent efficiency – and then on average a 44 percent thermal energy.”

With a traditional energy supply system, on the other hand, “to have 41 kWe of electricity, it is necessary to introduce 102 kW of primary energy, from which, however, 61 kW of losses would also inevitably be obtained. And again, to have 44 kWt of thermal energy would require the introduction of 52 kW of primary energy.” This means a total primary energy requirement of 154 kW, compared with the 100 kW required with CHP to have the same thermal and electrical carriers. The first result from this is obvious and interesting in itself: “with cogeneration it is possible to reduce the primary energy introduced by about 1/3”.

Cogeneration, says Cuttica, “in addition to being an energy virtuosity is also an economic virtuosity,” especially in countries like Italy where the cost of energy significantly affects a company’s operating costs. Indeed, the data show how cogeneration can help companies remain internationally competitive, because by reducing thermal costs and thanks to the benefits of TEEs – energy efficiency certificates or white certificates – the costs that remain to produce the electric kilowatt-hour are very low and much less than the European average.

Biofuels and derived benefits

It can be done even better, however, says Cuttica. The reference is to the possibility, which technology now allows, of exploiting as a resource what until recently was simply waste and, therefore, a cost. “It is possible,” he says, “from industrial waste, from purifiers, from food production waste, from digesters to produce biogas and with this to produce electricity.” A possibility applicable primarily to the food sector but also to sectors such as pharmaceuticals, chemicals, and paper (Figure 3).

Once the biogas is produced, there are two paths to choose from: either feed directly into a cogeneration plant or produce biomethane. “A farmer who produces biomethane from waste with a digester can then feed the biofuel into the grid, issuing the guarantees of origin; so a company, anywhere in Italy, will be able to purchase biomethane from the grid with the relevant guarantees with which to feed the cogeneration plant, which will then become completely “green” because it is made with a fuel produced from renewables.”

The benefits to be gained are many. First, an economic advantage because the company remains competitive; second, an energy supply safe from crises brought about by geopolitical upheavals can be guaranteed to companies, and the cost of primary energy is reduced. Not to mention the benefit to the environment. Finally, out of all this, the company can also gain interesting marketing leverage.

Comparison with photovoltaics

Despite the obvious advantages of CHP, there are still companies that choose technologies that ensure their energy sources are considered more “green.” However, thinking that one technology is better and more efficient than the other a priori is a mistake not to be made. Cases must be evaluated individually, and the answer is not always as obvious as one would think.

To explain this better, Cuttica compares two technologies that AB Energy deals with: cogeneration, precisely, and solar photovoltaics. And he does so using numbers, which, he stresses, should always guide a company’s choices.

“First of all, we know that CHP is recommended for companies that work at least two shifts, five days a week, forty-eight weeks a year, so about 5,000 hours a year. In contrast, PV is also perfect for those who work one or two shifts. Second, CHP requires simultaneous thermal consumption, whereas PV obviously does not require simultaneous thermal and electrical demand. And again, while in CHP companies often have a trade-off between daytime and nighttime load, PV is perfect for those with daytime bells.

We can say that photovoltaics is a “technology for everyone” however, it has a big limitation in the actual annual working hours. We know that in principle a photovoltaic system works around 4 thousand hours per year, following the natural bell of the Sun, so it produces very little the closer we get to sunrise and sunset. Generally speaking, statistics in Italy say that in the north a photovoltaic plant works a time of 1,100 equivalent hours in a year. This means that it is like working at full power for 1,100 hours per year. The equivalent hours rise to 1,300 in central Italy and 1,500 in the south.

A cogeneration plant, on the other hand, can be turned on at will, so if a paper mill works on a continuous cycle, it can be kept on for as many as 8,500 hours a year.

This huge difference in the number of hours of use cannot be forgotten when you want to do environmentalism, otherwise you risk making a ‘gut’ choice and not a ‘head’ choice.”

If one considers, then, the savings given by the two technologies, one will notice that, going back to the values seen earlier, CHP is more efficient. And in fact, “with cogeneration to produce the 41 kW of electricity mentioned above, we manage to avoid the introduction of 54 kW of primary energy; so if we assume the use of cogeneration for an average of 6,300 hours per year-which for a paper mill could become as much as 8,500-the primary energy savings will be around 340 thousand kWh per year of non-introduced energy-that is, 54 kW x 6,300 h = 340,200 kWh.”

Taking into consideration, on the other hand, the use of solar panels, “to make the same 41 electric kW of cogeneration, with photovoltaics one would avoid – as seen from the calculations made earlier – the introduction of the 102 kW introduced in the thermoelectric plant. Then considering the average hours of use, for example, in northern Italy – where, it was said, there are about 1,100 – the primary energy savings would be around 112 thousand kWh/year – that is, 102 kW x 1,100 h = 112,200 kWh.”

This means that, numbers in hand, “cogeneration allows a primary energy saving that is even three times that of photovoltaic systems, in northern Italy.

So,” Cuttica stresses, “thinking that installing a photovoltaic system is a more sustainable choice, simply because with cogeneration you burn methane, does not make sense, and companies that have the characteristics to be able to install and take advantage of a cogeneration system and do not take advantage of it are not doing a gift to the environment at all.”

How to combine the two technologies

How, then, can the maximum benefit be derived from both technologies? There is no single answer, because it must be considered on a case-by-case basis. Certainly, integration between the two technologies can be a viable solution. And to explain this, the engineer gives the example of different application cases.

The first concerns the case of a dairy farm, which is an example of ideal integration. In this specific situation, “photovoltaics goes perfectly with cogeneration, as it covers the idle hours of the thermal. Without photovoltaics, therefore, the company would have had to buy energy from the grid on Sundays, when there is no thermal production, or for the integration part; instead, photovoltaics, practically, allows it to go into self-consumption, covering most of the surges in demand during the day and instead selling energy to the grid when it is in excess”.

In contrast, the second case involves a paper mill in northern Italy, a type of industry that is perfect for cogeneration and for whose needs could be addressed in two ways.

“Assuming a 2 MWe cogeneration plant and a photovoltaic plant of the same size,” says Cuttica, “one could choose two paths: during weekdays one could modulate with cogeneration the electrical power according to the photovoltaic production, in essence using the operation of a cogeneration plant in an electric load-tracking set-up”. On the other hand, the second possibility would involve “combining the two, selling the excess weekday portion of electricity to the grid.” 

The combination with the application of full-power cogeneration and grid release of excess electricity turns out to be the most virtuous solution, economically and environmentally. “Excess energy is enhanced with grid release. You continue to buy a small amount of energy from the grid, but you get the maximum environmental benefits of both CHP and PV, because you make a contribution to the national grid and you add the CO2 saved by the CHP system – amounting to 2,893 tons per year – to the CO2 saved by PV – or 1,173 tons per year. And it remains advantageous from an economic point of view as well, if one considers that the marginal cost of the energy sold is for certain lower than the remuneration from the grid, so it allows one to margin, albeit a little, from that as well.”

Certainly these are considerations that need to be made on a case-by-case basis, bearing in mind also that the rules on the possibility of ceding energy to the grid in the future, with the transformation of the system and an increasing presence of renewables, may change and be more facilitated.

Compact energy

AB Energy’s Ecomax cogeneration plants are characterized by a structure that allows for space-saving while ensuring ease of use. The module is compact, but inside it contains all the components of a cogeneration plant for combined heat and power production. It was designed by taking care of its insulation, air conditioning and soundproofing to reduce dispersion and lower noise levels generated by the engine.

It is precisely its modularity that allows it to be tailored to the needs of the production site it is to serve, thus integrating effectively into different plant situations and any operational scenario. It also does not require the industrial reality that installs it to make major-and often costly-interventions in the energy supply chain. Finally, the plant can also be managed remotely through the integrated system, set up by AB, which monitors every element of it.