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Analysis and Performance of a Residential Indoor Vertical Plant Production Unit.

INTRODUCTION

Human population is expected to approach 10 billion by the year 2050, with an estimated 80% of the population living in or very near to a city. Sustainably feeding this population will require significant innovation in current agricultural practices. Producing food directly inside the city offers several advantages over conventional farming, including reduced transportation costs, improved food freshness, and improved food quality. In addition, the prospect of repurposing industrial buildings for indoor vertical plant production offers advantages of environmental control for optimized production, year-round production, reduced land footprint, elimination of agricultural runoff and soil degradation, water savings, and socio-economic opportunities (Despommier 2011; Despommier 2013).

Several questions remain however related to integration of such systems into buildings. Indoor food production is energy intensive with significant loads related to lighting, humidity control, thermal control, and ventilation. Water and nutrient inputs also represent large production costs. Lettuce for example, can require up to 30 mol/[m.sup.2]/day of photosynthetically active radiation (PAR, wavelength 400-700 nm) (Brechner et al. 2012). In field and greenhouse production this is supplied by natural solar radiation, however in the indoor environment such radiation levels require approximately 6.6 and 4.6 kWh/[m.sup.2]/day for fluorescent and LED lighting respectively. Energy conversion via photosynthesis is on the order of 5 % meaning that lighting also generates significant heat within the space which must be managed (together with water vapor from transpiration) by the building HVAC system. While commercial indoor food production operations continue to emerge both within the United States and around the world, publications in the scientific literature documenting full-scale performance are relatively scarce, leading to questions about long-term viability given the challenges mentioned (Goldstein 2013; Kozai 2013). There is therefore the need to document the efficiency of indoor food production systems and methods.

This paper investigates the potential of a small-scale and low-cost indoor vertical plant production module for residential applications and examines its relationship to the indoor environment. An experimental unit is built and tested. Energy efficiency is evaluated considering loads for lighting and climate control. Water efficiency is also determined. The potential for such a system to supply year-round production of salad greens to a typical household is discussed along with financial viability.

METHODS

Production Unit

A low-cost indoor vertical production prototype was assembled as shown in Figure 1(a). The unit has a footprint of 1524 mm (5ft) by 914 mm (3 ft) with a height of 1829 mm (6 ft). Bays with clearance of 305 mm (1 ft) are stacked above one another in vertical shelving arrangement. Each bay is equipped with four 1219 mm (4 ft) linear fluorescent F32T8 daylights (3500K) providing illuminance of 8180 lux, or PAR of 114 [micro]mol/[m.sup.2]/s given the spectrum of the source.

Test Conditions

The unit was used for production trials of arugula (Eruca sativa L.) and lettuce (Lactuca sativa L.). Plants were grown in soil blocks measuring 51 x 51 x 51 m[m.sup.3] (2 x 2 x 2 [in.sup.3]) consisting of soil, plant-based compost, perlite, and peat in relative parts of 1:2:2:3. Ammendments of bloodmeal (12% N), rock phosphate (3% P2O5, 1% Fe), greensand (0.1% [K.sub.2]O), and dolomitic lime (20% Ca, 12% Mg) were added to the growing medium. Blocks were closely spaced on trays and seeded at densities of 2.1 and 2.4 seeds per [in.sup.2] (0.33 and 0.37 seeds per [cm.sup.2]) for arugula and lettuce respectively. Seeds were held under dark conditions for 3 days to promote germinataion after which they were subject to 13 h photoperiods, giving integrated PAR of 5.4 mol/[m.sup.2]/day. Mist irrigation was provided from municipal water source. Temperature and relative humidity were controlled by the residential HVAC system, and were maintained at 18 [+ or -] 2 [degrees]C (64.4 [+ or -] 3.6 [degrees]F). Trials were run for 21 days from May 29, 2017 to June 19, 2017 to produce loose leaf salad greens. Figure 1(b) shows plants at day 21.

Measurements and Calculations

Plant yield was evaluated at day 21 as plants were harvested. Fresh mass was measured along with dry mass, after heating samples at 37.8 [degrees]C (100 [degrees]F) for 8 h.

The water efficiency of the system was defined as the ratio of water used by the plants relative to the water supplied to the system (Ohyama et al. 2005),

[[eta].sub.H2O] = ([m.sub.fresh]-[m.sub.dry]) / [m.sub.in] (1)

The mass of water stored within the plant biomass is the difference between the plant fresh mass mfresh and dry mass [m.sub.dry]. [m.sub.in] is the mass of water supplied to the plants via irrigation, which was monitored using a flow meter. The difference between these two defines the mass of water that is evaporated to the indoor environment.

[m.sub.evaporation] = [m.sub.in] - ([m.sub.fresh] - [m.sub.dry]) (2)

Under dry climate conditions this moisture is desireable, however under humid climate conditions this moisture must be removed from the indoor space. Energy consumed by the dehumidification unit to handle this load is a function of the efficiency factor k1, assumed as 1.2 l water vapor per kWh electricity.

[E.sub.dehumidifier] = [m.sub.evaporation] [k.sub.1] (3)

The energy efficiency of the system was evaluated by comparing the chemical energy stored in the plant biomass relative to electric energy inputs (Ohyama et al. 2005),

[[eta].sub.E] = [E.sub.plant] / ([E.sub.light] + [E.sub.hvac] + [E.sub.dehumidifier]) (4)

Elight is the energy used by electric lighting, which consumes power at rate of 92 W/[m.sup.2] (= 32 W/lamp x 4 lamps/bay / 1.393 [m.sup.2]/bay). Eplant is the plant chemical energy. This is proportional to fresh mass mfresh via energy density factor [k.sub.2] equal to 1.03 kJ/g (7 cal/oz) and 0.59 kJ/g (4 cal/oz) for arugula and lettuce respectively.

[E.sub.plant] = [k.sub.2] [m.sub.fresh] (5)

The difference between [E.sub.light] and [E.sub.plant] is radiated to the space as heat. Under warm climate conditions this heat represents a load that must be handled by the HVAC system. To handle this load the HVAC system will consume power [E.sub.hvac] equal to the heat divided by the coefficient of performance (COP). On the other hand, under cool climate conditions, heat generated by the plant production unit is desireable and will displace power consumed by the HVAC system. Displaced power will be equal to heat divided by COP. Energy consumed by the dehumidification unit is also radiated to the space as heat and must be similarly handled, depending on climate conditions. Total energy consumed by the HVAC unit is therefore,

[E.sub.hvac] = ([E.sub.light] - [E.sub.plant] + [E.sub.dehumidifier]) COP (6)

The mass and energy balance described by equations (1)-(6) is summarized in Figure 2.

RESULTS

Yield of arugula and lettuce from the vertical production unit is reported in Table 1 in terms of both fresh and dry mass. Variability in mass readings from trials is also reported as uncertainty. Yield of both plant types is approximately the same (within the range of uncertainty). Given that the typical serving size for leaf vegetables is 85 g (3 oz) the vertical production unit can produce approximately 0.25 servings/[m.sup.2]/day. The unit includes total growing space of 8.358 [m.sup.2], and can therefore accommodate total daily production of 178 g (6.28 oz) or approximately 2 servings.

The efficiency at which the system uses water is reported in Table 2. Irrigation is supplied to the unit at an average rate of 1.375 l/[m.sup.2]/day and is taken up by the plants with efficiency of 1.38 and 1.45 % for arugula and lettuce respectively. The balance of the water is evaporated into the indoor environment and must be handled by the dehumidifier system.

The energy efficiency of the system is reported in Table 3 for two assumed scenarios, namely warm and humid climate conditions and cool and dry climate conditions respectively. The HVAC power consumption during cooling mode and power savings during heating mode are reported assuming COP of 4. The energy efficiency of the system is 0.199 and 0.113 % during warm climate conditions and 0.644 and 0.366 % during cool climate conditions for arugula and lettuce production respectively. This is consistent with values reported elsewhere in the literature (Koontz et al. 1986; Ohyama et al. 2005).

The up-front cost of the unit is approximately $630. This includes lamps, fixtures, structure, and automated irrigation as listed in Table 4. Annualized over the life expectancy of the components, this translates to $46/year.

Table 5 summarizes the initial investment, operational costs, and value of food produced under both warm/humid and cool/dry climate conditions. Note that under cool/dry climate conditions the heat generated by the plant production unit displaces power that would have been consumed by the HVAC system and is therefore considered as dollar savings. The value of lettuce, electricity, and water are assumed to be 15.00 $/kg, 0.10 $/kWh, and 2.00 $/[m.sup.3] respectively. Labor for seeding and harvest averages less than 5 min/day, and the associated cost is neglected on the assumption that the household members can operate the unit.

The unit as a whole offers annual savings of $34 and $647 under warm/humid and cool/dry climate conditions respectively. In the first case the savings are negligible, however in the second case the savings are significant and would recoup initial investment within the first year. The discrepancy between the two scenarios shows the sensitivity of the system to climate conditions and to target indoor ambient conditions. Assuming lower efficiency equipment for heating, cooling, and dehumidification would further increase the discrepancy between each scenario.

CONCLUSION

A small-scale and low-cost indoor vertical food production unit has been built and demonstrated. Yield, energy efficiency, and water efficiency were evaluated for 21-day production trials of arugula and lettuce for loose-leaf salad use. With growing area of 8.353 [m.sup.2], the unit has capacity to produce approximately 2 vegetable servings per day providing a significant dietary contribution to typical household. Net annual savings generated by the unit can reach nearly $650 under favorable cool/dry climate conditions. Under less favorable warm/humid climate conditions the unit's annual savings are negligible. Normalizing results over growing area of the unit prodives insight into the potential for commercial scaling, with net revenue of up to 77 $/yr per [m.sup.2] of growing space, depending on climate conditions.

Yield and efficiency values can be significantly improved via optimization of systems and methods. Among the primary challenges is to reduce the energy load and associated production costs of electric lighting. Lighting loads can be reduced with more efficient technologies including light emiting diodes (LED). It is likely however that the increased capital cost of such upgrades will be of more interest to commercial operations than to home-scale producers. Yield can also be significantly increased by optimizing irradiance levels and photoperiod length. For example, exposing plants to the same daily integrated PAR levels (i.e. same daily energy use) distributed over longer photoperiods has been shown to increase yield relative to shorter photoperiods (Koontz et al. 1986; Ohyama et al. 2005). Such modification requires no additional equipment or investment and can be easily implemented. The use of hydroponic (soil-less) cultivation practices also offers many advantages, with water savings reported on the order of 80-90 % when compared to soil-based cultivation methods. While these potential improvements will increase the viability of indoor food production systems, results suggest that even the simple prototype proposed here offers typical households an economically viable system that can be scaled to provide on-site fresh vegetable production under certain seasonal conditions.

REFERENCES

Brechner, M., and A. J. Both. 2012. Hydroponic lettuce handbook. Cornell University Controlled Environment Agriculture: 48p.

Despommier, D. 2011. The vertical farm: Controlled environment agriculture carried out in tall buildings would create greater food safety and security for large urban populations. Journal of Consumer Protection and Food Safety 6(2):233-236.

Despommier, D. 2013. Farming up the city: The rise of urban vertical farms. Trends in Biotechnology 31(7):388-389.

Goldstein, H. 2013. The indoor farm: Urban Organics plans to grow fish, greens, and maybe the whole indoor aquaponics industry. IEEE Spectrum 50(6):59-63.

Koontz, H.V., and R.P. Prince. 1986. Effect of 16 and 24 hours daily radiation (light) on lettuce growth. HortScience 21(1):123-24.

Kozai, T. 2013. Resource use efficiency of closed plant production system with artificial light: Concept, estimation and application to plant factory. IEEE Spectrum 89(10):447-461.

Ohyama, K., Manabe, K., Omura, Y., Kozai, T., and C. Kubota. 2005. Potential use of a 24-hour photoperiod (continuous light) with alternating air temperature for production of tomato plug transplants in a closed system. HortScience 40(2):374-377.

Jonathan Maisonneuve, PhD

Associate Member ASHRAE

Lianqing Zhu, PhD

Pouyan Pourmovahed

Student Member ASHRAE

Mingli Dong, PhD

Jonathan Maisonneuve is an assistant professor and Pouyan Pourmovahed is a PhD student in the Department of Mechanical Engineering, Oakland University, Rochester, Michigan. Lianqing Zhu and Mingli Dong are professors in the School of Instrument Science and Optoelectronic Engineering,, Beijing Information Science & Technology University, Beijing, China.
Table 1. Plant Yield

Plant Type        Fresh              Mass             Dry Mass
            (g/[m.sup.2]) (*)  (g/[m.sup.2]/day)  (g/[m.sup.2]) (*)

 Arugula     423 [+ or -] 58   20.1 [+ or -] 2.8    33 [+ or -] 10
 Lettuce     448 [+ or -] 46   21.3 [+ or -] 2.2    22 [+ or -] 2

Plant Type                 Servings
               (/[m.sup.2]) (*) (/[m.sup.2]/day)

 Arugula    4.98 [+ or -] 0.68 0.237 [+ or -] 0.032
 Lettuce    5.27 [+ or -] 0.54 0.251 [+ or -] 0.026

(*) After 21 day trial

Table 2. Water Balance

Plant Type      Irrigation       Water in Plants    Evapotranspiration
            (ml/[m.sup.2]/day)  (ml/[m.sup.2]/day)  (ml/[m.sup.2]/day)

 Arugula           1375           19 [+ or -] 2      1356 [+ or -] 2
 Lettuce           1375           20 [+ or -] 2      1355 [+ or -] 2

Plant Type      Efficiency
                   (%)

 Arugula    1.38 [+ or -] 0.15
 Lettuce    1.45 [+ or -] 0.15

Table 3. Energy Balance

Plant Type       Chemical            Lighting        Dehumidification
            (kJ/[m.sup.2]/day)  (kJ/[m.sup.2]/day)  (kJ/[m.sup.2]/day)
                                 Warm and Humid C   limate Conditions

 Arugula    20.8 [+ or -] 2.8          4300          4068 [+ or -] 6
 Lettuce    11.8 [+ or -] 1.2          4300          4065 [+ or -] 6
                                Cool and Dry Climate Conditions
 Arugula    20.8 [+ or -] 2.8          4300                 0
 Lettuce    11.8 [+ or -] 1.2          4300                 0

Plant Type         HVAC              Efficiency
            (kJ/[m.sup.2]/day)          (%)


 Arugula      2087 [+ or -] 2   0.199 [+ or -] 0.027
 Lettuce      2088 [+ or -] 2   0.113 [+ or -] 0.011
              Cool and Dry Climate Conditions
 Arugula     -1070 [+ or -] 2   0.644 [+ or -] 0.087
 Lettuce     -1072 [+ or -] 2   0.366 [+ or -] 0.037

Table 4. Capital Cost

   Item              Cost       Life             Cost/Life
            ($)  ($/[m.sup.2])  (yr)  ($/yr)  ($/yr/[m.sup.2])

Structure   250       30         25     10          1.20
 Fixtures   240       29         15     16          1.90
  Lamps      40        5          4     10          1.20
Irrigation  100       12         10     10          1.20
  Total     630       75                46          5.50

Table 5. Cost Analysis

                                     Quantity
                  (/[m.sup.2]/yr)    (/yr)

                  Warm and Humid Climate Conditions
     Yield          7.77 kg            64.9 kg
    Lighting      435.8 kWh          3642 kWh
Dehumidification  412.5 kWh          3445 kWh
    Cooling       211.7 kWh          1768 kWh
   Irrigation       0.502 [m.sup.3]     4.19 [m.sup.3]
   Equipment
      Net
                   Cool and Dry Climate Conditions
     Yield          7.77 kg            64.9 kg
    Lighting      435.8 kWh          3642 kWh
    Heating      -108.4 kWh          -906 kWh
   Irrigation       0.502 [m.sup.3]     4.19 [m.sup.3]
   Equipment
      Net

                  Value
                  ($/[m.sup.2]/yr)  ($/yr)

                  Warm and Humid Climate Conditions
     Yield        + 117              + 974
    Lighting       - 44              - 364
Dehumidification   - 41              - 345
    Cooling        - 21              - 177
   Irrigation       - 1                - 8
   Equipment        - 6               - 46
      Net           + 4               + 34
                  Cool and Dry Climate Conditions
     Yield        + 117              + 974
    Lighting       - 44              - 364
    Heating        + 11               + 91
   Irrigation       - 1                - 8
   Equipment        - 6               - 46
      Net          + 77              + 647
COPYRIGHT 2018 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE)
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Author:Maisonneuve, Jonathan; Zhu, Lianqing; Pourmovahed, Pouyan; Dong, Mingli
Publication:ASHRAE Conference Papers
Article Type:Report
Date:Jan 1, 2018
Words:2718
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