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Aim of this work is to define a dynamic model developed in the Trnsys software environment where a biomass boiler and two micro-CHP gasifiers are considered as heat source for a detached house. The selected case study is a detached house of about 240 m2 near Bologna, Northern Italy, where the HVAC system is currently equipped with a diesel oil boiler with 40 kW of thermal power. The building is connected to a farm and the total electrical consumption is about 13850 kWh/year. The thermal energy demand was calculated and validated in Trnsys using measured diesel oil consumption data. Two different retrofitting solutions for the heating system have been investigated: biomass boiler and micro-CHP gasifiers. Results show the capability to use biomass boiler and micro-scale gasifiers as heat source for the building with a wood consumption of 23 and 86 ton/year respectively. The gasifier electricity production is greater than the electrical demand and an extra energy of 64800 kWh/year is injected into the grid. The cost-benefits economical assessment of these retrofitting solutions is reported considering subsidies for renewable energy production.
gasification, trnsys, combined heat and power, dynamic simulation, biomass
Wood biomass as heat source for residential building HVAC plants is widely diffused [1]. Low cost and environmental impact reduction in terms of CO2 balance are the main drivers that facilitate the extensive use of wood feedstocks such as wood logs, chips and pellets [2]. Several studies report the behavior of residential wood stoves and boilers [3], while literature shows few studies about micro-CHP gasifier for residential use [4,5]. On the other hand, micro-scale gasification systems are arising interest due to their economical and sustainability advantages [6-7]. These plants are used as bio-energy investment and they are normally used to produce a constant electrical power for 7500 hour/year [8-10]. Cost and maintenance of these plants is significant and yearly biomass consumption is high in order to achieve faster economical return, with the help of subsidies [11]. Nominal electrical power output of these plants ranges between 45 kW and 1 MW [12].
In general, in gasification power plants the thermal power is taken from the engine coolant and engine exhaust and it is often used to dry the biomass feedstock into rotative dryer or for district heating [13]. Excess heat is high especially in summer when the biomass moisture is lower and this heat is commonly disposed through cooling towers.
In this paper, a further application of biomass gasifiers is presented through this case study: a small-scale gasifier power plant for heat and power production for a residential building located on the hills near Bologna, Northern Italy. The gasifier is a commercial All Power Labs PP20 [14] able to produce about 15 kW of electrical power and 20 kW of thermal power.
The simulations have been carried out using TRNSYS 17 dynamic thermal modeling software [15]. TRNSYS contains dynamic simulation models that allow to accurately evaluate the buildings energy performances. The inputs of such models are normally geographical, geometrical and thermal properties of the case study as well as information about the HVAC system, occupant’s behavior and internal gains. Furthermore, TRNSYS integrates large number of sub-systems (called types) with the final aim to understand the effect and the output of the interaction. Sub-systems can describe boilers, renewable energy systems, district heating etc. [15].
The building here presented has a diesel oil boiler as thermal sources. First, a simulation with this fuel solution was done in order to validate the dynamic model. Second, the application of a biomass boiler instead of the diesel oil boiler was simulated. Finally, two micro-CHP gasifiers were evaluated as retrofitting solution. In this case, the electricity production is used by electrical loads in the house and in the farm and the surplus is injected into the grid. Peak thermal power demand to the gasifier results in about 40 kW, requiring the use of two PP20 gasifiers in parallel.
An economical assessment was performed considering the use of wood logs as fuel for the biomass boiler and wood chips as fuel for the for the gasifiers. In the biomass boiler scenario, the return of the investment is guarantee by the cost reduction of the biomass fuel compared to the diesel-oil fuel and the 65% tax deduction applicable for this solution [16]. With gasifiers, the return of the investment is given by the cost reduction of the biomass fuel compared to the diesel-oil fuel, in addition subsidies for the electrical power injected into the grid are taken into account.
2.1 Trnsys dynamic model of the building
The case-study is a typical Italian detached house built in 1975. It is composed by four floors: the basement and the attic are unheated zones while the first and second floors are the location of two independent apartments with central heating system. The building has a total heated usable area of about 239,04 m2.
External walls are composed by a brick layer, while the floor and ceiling towards unheated zones are non-insulated concrete layers. The windows are all wooden-framed and single glazed. Thickness and thermal transmittance of the envelope components are shown in Table 1.
Table 1. Characteristics of the building envelope components.
|
Component type |
Length [m] |
[W/(m2K)] |
|
Wall |
0.265 |
1.644 |
|
Window |
- |
5.68 |
|
Floor/Ceiling |
0.295 |
1.796 |
The existing heating system is a conventional hydronic, composed by one diesel-oil boiler and radiators often positioned under the windows. There is a manual regulation; all the heated system has been installed during the construction of the building. The sub-systems performance is shown in Table 2 (the values have been estimated according to UNI TS 11300 [17]).
Table 2. Performance of the subsystems [17]
|
Subsystems |
Thermal efficiency |
|
Emission |
ηem = 0.88 |
|
Distribution |
ηdis = 0.90 |
|
Regulation |
ηreg = 0.88 |
The simulations were conducted over the 2015-2016 heating season. The meteorological data used in the simulations are given by Trnsys weather database [15]. The hourly weather data includes horizontal global radiation, dry bulb temperature, wind speed and relative humidity. Monthly billings were used to calibrate the dynamic simulation model of the building.
Internal gains, such as lighting, equipment and occupants have been gathered with audits. This study evaluates the hourly heating power demand of the building QH,dem [kW] considering the thermal dissipation of the building elements, the thermal power dissipated by ventilation and thermal gains as suggested by UNI TS 11300 standard [17]. The hourly thermal power consumption of the building QH,con [kW] for the three simulation cases (changing the generation sub-system performance) is evaluated with the following formula:
$Q_{H, c o n}=\frac{Q_{H, d e m}}{\eta_{g e n }\eta_{ e m} \eta_{d i s} \eta_{r e g}}$ (1)
where $\eta_{g e n}$ [-] is the nominal efficiency of the thermal generator. This value changes for every simulation depending of manufacturer data.
2.2 Diesel-oil boiler
The diesel-oil boiler is actually used as heat source of the HVAC system. It is an UNICAL EXOCELL 2-41 [18] with a maximum thermal power of 58.1 kW. The nominal thermal efficiency of this machine is about 85%. In the simulation, the lower heating value of the diesel oil is assumed to be 10 kWh/liter and this cost is 0.95 €/liter [19]. Other assumptions are an annual regular maintenance cost of about 300 € and a not-ordinary maintenance cost of about 5% of the boiler cost every 5 years.
2.3 Biomass boiler
The biomass boiler used in this work is a Mescoli GLUP 38 HT LP/200 [20] depicted in Figure 1. It is a reversed flame biomass boiler with 91.3% of thermal efficiency. The biomass is loaded through the front door. It burns high quality wood logs with a maximum length of 0.5 m that have a cost of about 152 €/ton [21]. About 60 kg of wood logs can be loaded in the combustion chamber. About 1% of the inlet biomass becomes ash and it is disposed with a cost of 50 €/ton [21]. This boiler version has also an extra pellets combustor and pellets container of about 200 l. The pellets are burned automatically if wood logs finish to burn. In this paper only wood logs as fuel are considered and the lower heating value of this biomass is set to 4.5 kWh/kg that it is the minimum value suggested by the standard [22]. Assumptions about annual regular maintenance and not-ordinary maintenance cost are similar to the ones chosen for the diesel-oil boiler scenario.
Figure1. Mescoli GLUP 38 HT LP/200 [11]
2.4 Wood chips gasifiers
The gasifier simulated in this work is a PP20 of the Californian company All Power Labs [14]. The plant is showed in Figure 2. It is an Imbert type downdraft gasifier with internal heat recovery. Wood chips are loaded in the hopper that has a volume of about 300 l. The chips are moved to the gasification reactor by an auger and they are converted in wood gas and biochar. About 5% in mass of the inlet biomass is converted into biochar. In this paper, the biochar is disposed with a cost of about 50 €/ton [21]. The wood gas is cooled down and purified in the filter. During this process, a tarry condensate is produced and this need to be disposed as special waste with a cost of and 500 €/ton [21]. The condensate yield is about 2% of the inlet biomass. After the filtration, the wood gas is mixed with air and it is burned into the engine that drags an alternator to produce electrical power. The engine is equipped with a heat exchanger that extracts about 20 kW of thermal power form the engine coolant when the electrical generator produces 15 kW of electrical power. System specification are summarized in Table 3. About biomass fuel, conventional wood chips P16 W10 with a lower heating value of 4.5 kWh/kg are considered [21]. In this case, a regular maintenance cost of 0.03 € every electrical kWh produced is take into account; the not-ordinary maintenance is similar to the other scenarios (5% of the gasifier cost every 5 years of working).
Figure 2. All Power Labs PP20 [10]
Table 3. Specification of the All Power Labs PP20 with CHP auxiliary subsystem
|
Continuous electrical power rating |
15 kWel @50 Hz |
|
Continuous thermal power rating |
20 kWth at 15 kWel |
|
Biomass P16 W10 consumption |
1.2 kg/ kWel |
|
Biomass moisture content |
5-30% dry basis |
|
Electrical efficiency with P16 W10 |
18.5 % |
|
Thermal efficiency with P16 W10 |
24.6 % |
|
Installed foot print |
1.36 x 1.36 m |
|
Run time with hopper fill |
3 hours at 15 kWel |
3.1 Energy and efficiency results
Figure 1 reports the thermal power demand of the buildings during the standard heating period of 4368 hours from the 15th of October to the 15th of April. The maximum thermal power demand is about 25 kW and the total energy required for heating the building is 60429.2 kWh. This energy needs to be provided through the HVAC system. Results concerning the simulation with diesel oil are summarized in Table 4. Emission, regulation and distribution losses afflict the global thermal energy efficiency that it is about 54%. The maximum thermal power required to the boiler is lower than the nominal thermal power of the diesel-oil boiler. The Trnsys simulation estimates an annual diesel consumption of 11342 liter very similar to the value given by the building owner of 11757 liter. This result validates very well the Trnsys building model.
Figure 3. Building thermal power demand during the heating period
Table 4. Simulation results with diesel oil boiler as thermal generator
|
Global thermal energy efficiency |
53.86 % |
|
Maximum boiler thermal power |
40.40 kW |
|
Annual boiler energy consumption |
112322 kWh |
|
Annual diesel-oil consumption |
11342 lt. |
|
Annual diesel-oil cost |
10775 € |
Results of the simulation with biomass boiler as heat source are reported in Table 5. The global efficiency is 57.80%, this value is higher in comparison to the value obtained using the diesel oil. This is due to the higher biomass boiler thermal efficiency of 91.3% respect diesel oil boiler. Therefore, the maximum boiler thermal power required is 40.4 kW, slightly higher than the nominal thermal power of the biomass thermal boiler of about 38 kW. This difference is low and it can be neglected. About 23 tons of biomass are consumed during this period with a cost of 3532 €. This is about 1/3 of the cost of the diesel-oil.
Table 5. Simulation results with biomass boiler as thermal generator
|
Global thermal energy efficiency |
57.80 % |
|
Maximum boiler thermal power |
40.40 kW |
|
Annual boiler energy consumption |
104572 kWh |
|
Annual wood logs consumption |
23238 kg |
|
Annual wood logs cost |
3532 € |
The electrical energy consumption is depicted in Figure 4. These data are back calculated using a load curve typical of a residential building [23] and the monthly electrical consumption reported in the invoice of the electrical service company. For every month, a typical daily electrical load was simulated using the method suggested by Caffarelli et al. [23]. In the figure, the minimum, maximum and average daily electrical load is reported for the whole simulation time. The building electrical load is completely covered by the gasifiers. Figure 5 reports the electrical power not self-consumed by the building and injected into the grid. This happens because the gasifiers set-points is heat demand driven and the electrical power consumption is much lower than the thermal power consumption. Anyway, the subsidies for renewable electrical power production are calculated considering the injected electrical power, so high electrical injection increase the revenues of this kind of plant.
Figure 4. Building daily electrical load during the heating period
Table 6 resumes the results of the simulation with the micro-CHP gasifiers. The global thermal efficiency is low as results of the low thermal efficiency of the gasifier (24.6%). Thermal and power maximum electrical powers are similar to the maximum powers of two gasifiers that work in parallel. The electrical power production is huge (71868 kWh) and 5 times higher than building electrical consumption. This is due by the heat demand driven and by the low thermal efficiency of the machine. In fact, the thermal heat recovery is applied on the engine coolant, the heat of the exhaust it recovered internally in order to pyrolyze the biomass that enters in the reactor. This permits to use biomass with a maximum moisture of 30%.
The annual biomass consumption is greater (about 4 times) than the scenario with the biomass boiler, anyway the specific cost is lower and the fuel feeding annual cost is 2 times the cost of biomass boiler scenario and half the cost of the diesel-oil scenario.
Figure 5. Electrical power injected into the grid
Table 6. Simulation results with micro-CHP gasifiers
|
Global thermal energy efficiency |
15.58 % |
|
Maximum gasifiers thermal power |
40.40 kW |
|
Maximum gasifiers electrical power |
30.41 kW |
|
Annual gasifier energy consumption |
388105 kWh |
|
Annual electrical energy production |
71868 kWh |
|
Annual electrical energy self-consumption |
7069 kWh |
|
Annual electrical energy injection |
64800 kWh |
|
Annual wood chips consumption |
86245 kg |
|
Annual wood chips cost |
6900 € |
3.2 Economical analysis
The study compares the existing diesel boiler system with two retrofitting solution:
Case 1) Installation of two micro-CHP gasifiers that covers the thermal and electric energy demand and foresee the sale of the electrical energy production excess. The economic analysis compared two cases: the sale of electrical energy exploiting public subsidies (0.21 €/kWh [11]) and without them (electric price 0.0923 €/kWh [24]). Net present value curves are depicted in Fig. 6-7-8.
Case 2) Installation of Biomass Boiler. The economic analysis compared two cases: the benefit with deduction of 65% from initial cost in 10-year time and the benefit without deduction. Net present value curves are depicted in Fig. 9-10-11.
All the analysis is estimated with a discount rate “r” that change between 1% at 5%.
Figure 6. NPV Micro-CHP with subsidies
Figure 7. NPV Micro-CHP without subsidies
Figure 8. NPV Micro-CHP with/without subsidies
Figure 9. NPV Biomass Boiler with 65% Tax Deduction
Figure 10. NPV Biomass Boiler without 65% Tax Deduction
Figure 11. NPV Biomass Boiler with/without Deduction 65%
Table 7. Simulation results with micro-CHP gasifiers
|
Generator |
r=0% |
r=1% |
r=5% |
|
CHP – YES Inc. |
5.84y |
6.02y |
6.94y |
|
CHP – NO Inc |
13.44y |
14.3y |
>20y |
|
Biomass B.-YES 65% |
1.42y |
1.44y |
1.52y |
|
Biomass B.-NO 65% |
1.57y |
1.59y |
1.59y |
Figure 12. Comparison of cumulative costs trend
Table 7 shows the investments return time (years). Biomass boiler has the faster payback time, therefore the NPV of this solution at 20 years is about 140000 €, lower than the micro-CHP solution that has a NPV at 20 years of about 200000 €.
Figure 12 shows the comparison of cumulative costs trend, in particular the investments return time can be individuated where the line of diesel boiler crosses the other lines.
The results obtained from this study shows that the replacement of the existing diesel boiler with a biomass boiler is very convenient in the short period compared with the micro-CHP gasifier. In the first case the investments return time is about 1.5 years (considering public subsidies), while using the micro-CHP system it is approximately 6.5 years).
However, in the long period (in this case after 13 year time from installation of the new system of generation), the CHP is more convenient than Biomass Boiler. On the other hand, by using Micro-CHP Gasifier System there is the possibility to gain money from the electricity sales as well as heating the house without any other cost. Both the biomass boiler and the micro-CHP Gasifier system are therefore interesting retrofitting solutions for buildings in locations where natural gas is not available.
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