During the next 30 years, electricity consumption will increase, and renewable energy sources will reach the same or even larger share as coal has nowadays in power generation.
Solar and wind energy capacity will increase most out of renewable energy sources by 2050, because of growing investment in these sources. (British Petroleum 2020, 7.)
High-temperature heat pumps currently have a temperature limit of 180 °C, which might limit their drying efficiency compared to dryers using electricity directly (Sintef 2021). For example, new porous biobased fiber structures require novel approaches for drying. In prac-tice, suitable drying methods could be impingement drying for indirect electricity use and microwave drying for direct electricity use. Primary energy consumption by source in 2018 and 2050 (estimated) globally is illustrated in Figure 9. It is noticeable that renewable energy consumption is estimated to increase over ten times growing from 27 EJ (2018) to 277 EJ (2050). This makes it the highest consumed energy source. Hydro power consumption is estimated to increase about 20 EJ. Oil consumption is estimated to drop in half from 2018 to 2050 and coal consumption is estimated to be only 15 % of 2018 consumption in 2050.
Natural gas will remain its consumption in the same level while nuclear power will be almost doubled.
Figure 9: Global primary energy consumption by source in 2018 (left) and estimated 2050 consumption (right).
(British Petroleum 2020, 64.)
5.2 Impingement drying
The air impingement drying method that uses high-velocity hot air jets to dry products makes it possible to use indirect electricity as an energy source (Karlsson et al. 2000, 73). One possible technique that could be exploited is high-temperature heat pumps. These heat pumps can use low-temperature waste heat to produce high-temperature process heat by us-ing electricity as an added energy source. In the case of impus-ingement dryus-ing, for example solar energy could be used as an indirect electricity source to heat impingement air with waste heat received from some cooling process. The coefficient of performance (COP) de-scribes the efficiency of a pump and is the ratio of heat output to electrical input. These high-temperature heat pumps can have COP of 2-5. (Ahrens 2021.) COP value’s theoretical max-imum for the heat pump operating with constant temperatures can be determined with the Carnot process. Carnot process is described in the Equation 9 (Ahrens et al. 2021, 4.):
𝐶𝑂𝑃carnot = 𝑇sink
𝑇sink−𝑇source (9)
Where 𝐶𝑂𝑃carnot is theoretical maximum for coefficient of performance [-], 𝑇sink is process heat temperature [K] and 𝑇source is waste heat temperature [K].
To calculate true COP, Carnot efficiency must be considered. If COP of 5 is wanted when produced process heat is 180 °C and Carnot efficiency is 50 %, waste heat temperature of
~135 °C is needed. If Carnot efficiency is increased to 60 %, waste heat temperature can be
~126 °C.
A simple illustration of the most common heat pump process, the vapor compression cycle, is shown in Figure 10. In the vapor compression cycle, refrigerant circulates in a closed circuit. State of refrigerant changes during the circuit giving out heat to the low-temperature waste heat converting it to high-temperature process heat. First, refrigerant is in a low tem-perature state at low-pressure side and is heated to low-pressure vapor by waste heat in the evaporator. Then added electricity is used to run the compressor increasing the vapor pres-sure and temperature making it high-temperature vapor. High-temperature vapor is then transferred through the condenser giving out heat to heat up the waste heat to process heat.
After the condenser, the circulating refrigerant condenses back to a liquid state and it is ex-panded back to low pressure and temperature state in the expansion valve. (Ahrens 2021.)
Figure 10: High-temperature heat pump process. (Ahrens 2021.)
5.3 Microwave drying
The microwave drying process that uses electricity directly as a source of power has variety of possibilities to use amongst renewable energy sources. When process steam is not needed, it can use any type of electricity. As renewable energy sources take over most of the energy consumption in the future (Figure 9), they will probably be primary energy sources for
microwave drying as well. Energy system transition to lower carbon system opens possibil-ities to use energy more versatile, which allows consumers to have more options. Energy markets will be more localized, which is advantageous for energy sources like solar and wind power that are under high development. Localized energy markets enable smaller areal production amounts, which is also beneficial for solar and wind power. This also favors microwave drying because of its low operating heat loads and high process efficiency. Mi-crowave drying has an on-off heating nature with rapid heating and fast controllable output power adjusting, which makes it a highly suitable drying method, powered by renewable electricity, in the future. (British Petroleum 2020, 7; Schiffmann 2014, 293.)
EXPERIMENTAL PART
6 MATERIALS AND METHODS
To achieve the most energy-efficient and cost-efficient process for the manufacturing of thick porous fiber structures, the drying part of the process must be optimized. The quality of the product must be considered when the drying process is been designed. This means that structural or visual damage to the product during drying is to be prevented.
In this study, two different drying methods were investigated experimentally on a laboratory scale. Drying methods using conductive heat transfer were ruled out, so one drying method using convective heat transfer and one using radiative heat transfer were chosen. These dry-ing methods were impdry-ingement drydry-ing and microwave drydry-ing. Experiments took place in VTT Jyväskylä, Central Finland. Foam forming was used as a technology to produce porous samples for drying experiments. Used fiber raw materials were Metsä Pine (AKI) pre-refined BSKP pine pulp from Metsä Fibre Oy, Äänekoski Bioproduct Mill Finland, and bleached spruce chemi-thermomechanical pulp (CTMP, CSF 600 ml) from Rottneros AB, Rottneros Mill, Sweden. Drying was examined by observing the mass change of the samples during drying in the form of removing water. Also, temperature measurements were made.