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Renewable for Heat & Power Generation and in

National Laboratory Gail Mosey, Laura Supple

Greenhouses Considered as Summary of Findings Examples in the Study Supporting widespread growth of the agricultural industry requires (1) Altman Specialty Plants, a 1.4 innovative solutions to meet the unique energy challenges and demands of each million ft2 (32 acre) specialty plant farm with sustainable and cost-effective strategies and . This study nursery for ornamental succulents examines renewable energy for heat and power generation and storage at four with on-site -fired hot greenhouses located in Colorado. Results outline key considerations for energy water plants for radiant heating demand characteristics and the renewable energy technologies and strategies that account for the vast majority of available to meet energy needs more sustainably, reliably, and economically and natural gas usage at the facility are broadly applicable to greenhouses across the world. (2) Gunnison Gardens, a cold- climate single-gable roof greenhouse Snapshot designed for energy efficiency and minimal heating and cooling inputs • This study considered four greenhouse farms in Colorado as examples to understand to support year-round production of energy needs and opportunities in controlled environmental (CEA). seasonal crops. • Energy audits from 2016 to 2020 for each of the facilities were made available (3) Welby Gardens, a wholesale to understand the unique profiles of each operation. greenhouse operation consisting of several round roof gutter-connected • Although some commonalities can be found across the studied farms, greenhouses and hoop houses no “one-size-fits-all” solution should be applied across all cases. totaling 350,000 ft2 for year-round An appropriate combination of technical and nontechnical propagation of annuals, perennials, solutions to sustainably meet energy demands with ornamental grasses, organic clean and renewable sources should consider how vegetables, and herbs characteristics like the crop system, climate 2 (4) Zapata Seeds, a 7,020 ft covered control technologies, cultivation practices, and growing operation where 81% of greenhouse structural design influence energy use goes toward maintaining energy demand and opportunities to a 60oF indoor temperature for seed potato production optimize solutions for each unique context. Joint Institute for Strategic Energy Analysis

Joint Institute for Strategic Energy Analysis Introduction of CEA may be required to overcome unavailable or unreliable (Hassanien inhospitable conditions and enable et al. 2016). Many greenhouse crops Agricultural greenhouses could more reliable, high-quality yields are particularly sensitive to sudden improve food system resilience (McCartney and Lefsrud 2018). changes in the environment; therefore, in the face of climate change, ensuring stable, reliable, and resilient farmland degradation, population However, due to the degree of control energy to protect against outages and growth, water scarcity, and other needed for greenhouses, they can be potential crop failure is a key priority challenges. This type of controlled extremely energy-intensive compared (Esmaeli and Roshandel 2020). environment agriculture (CEA) can to traditional open-air agricultural significantly improve water and land systems (Gorjian et al. 2021). Most While floriculture maintains a dominant use efficiency for food production of this energy demand is currently portion of the greenhouse industry, by sheltering crops from adverse served by fossil . The growing greenhouse cultivation for food weather conditions and pests; carefully environmental and economic costs production is growing across the United and precisely controlling water associated with energy use raise States. In a five-year span, from 2012 to and nutrient cycles; and optimizing concerns about the sustainability of 2017, the total number of greenhouses internal environmental conditions to greenhouse cultivation in the future nationwide growing food increased by maximize crop productivity (Esmaeli (Gorjian et al. 2021). Energy access nearly 34%. In the same time frame, and Roshandel 2020). In remote and and reliability can also be a substantial the overall number of agricultural extreme environments, including arid, challenge for many greenhouses in greenhouses increased by nearly 8%. arctic, and urban locations, some form rural areas, where grid power may be By 2017, the land area for greenhouse

Figure 1. Number of greenhouse farms in Colorado by type in 2012 (yellow bar) and 2017 (blue bar). State-wide data from Table 39. Floriculture and Bedding Crops, Nursery Crops, Propagative Materials Sold, Sod, Food Crops Grown Under Glass or Other Protection, and Mushroom Crops: 2017 and 2012 of the 2017 Census of Agriculture (USDA 2019).

2 | Renewable Energy for Heat and Power Generation and Energy Storage in Greenhouses food production reached 3,600 farms grew by 70% from 105 to 179 environmental harms associated with acres, and the land area overall for all (USDA 2019). greenhouse energy could be reduced greenhouse crops reached 32,000 by more sustainable and resilient To keep pace with this growth and acres. Nursery crops, including nursery energy solutions for these key areas of realize the potential benefits of stock and aquatic plants, accounted demand (Gorjian et al. 2021). CEA, solutions are needed to meet for the single largest increase in the unique energy challenges and greenhouse land area, adding nearly Temperature demands of each farm with sustainable 1,150 acres from 2012 to 2017 (USDA Temperature regulation typically and cost-effective strategies and 2019). accounts for most of the primary technologies. Several interdependent energy demand for agricultural The greenhouse industry in Colorado factors impact greenhouse energy greenhouses. Conventional materials is also growing with a 47% increase requirements including shape and used in greenhouse facades are in the total number of agricultural size, building materials, designed to maximize solar light greenhouse farms from 2012 to 2017 utilization, crop selection, and transmittance to crops at the expense (USDA 2019). The largest growth in cultivation methods (Esmaeli and of thermal performance, creating the land area occurred in sectors producing Roshandel 2020). Sustainable need for auxiliary heating and cooling cuttings and seedlings, which grew greenhouse design can be used to systems to maintain suitable internal by 17.5 acres in 5 years. During the optimize greenhouse productivity and climate conditions (Gorjian et al. 2021). same time, the number of greenhouse energy consumption efficiency with the Thermal regulation can be achieved tomato farms more than doubled from appropriate combination of strategies, through active, passive, or hybrid 90 to 184, and the number of other technologies, and systems to meet systems. vegetable and fresh herb greenhouse the unique energy and environmental demands of the location. The plants, soil, and equipment involved in crop production also Passive or active Energy Demands in influence the heating and cooling technologies and strategies Agricultural Greenhouses needs of greenhouses. The absorption of incoming sunlight and artificial can be used to regulate Energy demands in agricultural light by plant leaves; latent and light, temperature, humidity, greenhouses center around convective heat exchanges between maintaining optimal internal and carbon dioxide (CO2) crop surfaces and the ambient air; microclimatic conditions for plant levels in greenhouses. and heat exchanges between plant, growth, crop productivity, and off- soil, and building surfaces all influence season cultivation (Ezzaeri et al. 2018). Passive strategies operate the overall energy balance of the Microclimate controls can include greenhouse. Characteristics such as the without any external various passive and active technologies leaf area index, crop spacing, indoor , often using and strategies to maintain a stable and outdoor temperature, humidity, aspects of building design indoor environment. The electrical and light availability can significantly to capture and utilize “free” energy consumption characteristics impact these heat exchange rates. Peak of a greenhouse will vary according . heating and cooling energy demands to several factors, including the for agricultural greenhouses will geographic location, climate, crops, Active strategies rely upon therefore be influenced by the growing cultivation practices, structural design, external technologies and systems and operation parameters and technologies employed in the of the greenhouse, which may vary devices to increase capture greenhouse (Hassanien et al. 2016; daily or even hourly, depending and utilization beyond what Shen et al. 2018; Yano and Cossu 2019). on environmental conditions and can be achieved through With most of this energy demand management decisions (Talbot and passive strategies. currently served by fossil fuels, the Monfet 2020). high costs, supply vulnerabilities, and

3 Lighting crops, shade curtains or other means CO2 Lighting is an important aspect of of reducing sunlight intensity may be Along with light availability, ambient greenhouse energy management. required to prevent overheating temperature, and relative humidity, Plant growth and fruit production and damage (Hassanien et al. 2016). ambient CO2 concentrations within the depend on the rate at which plants Humidity greenhouse directly influence plant photosynthesize, which depends on the photosynthetic rate. Ideal CO2 levels, amount of photosynthetically active Within the controlled environment as with other internal microclimatic radiation (PAR, 400–700nm wavelength of a greenhouse, relative humidity parameters, are partially a function range) reaching plant leaves (Yano typically increases as temperature of the crop and horticultural system and Cossu 2019). However, excessively decreases, and vice versa. When in use. Lettuces, for instance, do best high light intensity can also disrupt relative humidity drops too low, many with CO2 concentrations between photosynthesis (Hassanien et al. 2016). crops respond by closing stomata 700–1,000 ppm, which can be Crops are typically classified as high, to prevent excessive moisture loss, maintained with forced air circulation medium, or low light-demanding, restricting gas exchange between and CO2 enrichment systems that, depending on the average daily light plant leaves and the environment, and together, facilitate optimal gas requirements for optimal growth and negatively impacting photosynthesis exchange at the leaves (Talbot and Monfet 2020). Maintaining these production. Fruit-producing horticultural and other key physiological processes. optimal gas exchange rates to manage crops such as cucumber, sweet Excessively high humidity, however, plant metabolism and development pepper, and tomato tend to be in the can lead to fungal diseases, inhibit involves a dynamic balance of internal high light-demanding category, while flower pollination, and promote environmental parameters, including ornamental and floricultural crops such development of certain types of crop temperature, relative humidity, airflow as poinsettia and kalanchoe tend to be pests (Ezzaeri et al., 2018). Particularly and ventilation rates, light intensity, and in the low light-demanding category. in cold climates, a failure to regulate photoperiod (Ezzaeri et al. 2018; Talbot Even small deviations above or below temperature and humidity can result and Monfet 2020). these optimal lighting levels can in condensation on leaves, fruits, and affect crop yields (Cossu et al. 2020). the internal surfaces of the building, Mechanical Equipment Growers also need to regulate the contributing to crop damage and Mechanical equipment constitutes a day length, or photoperiod, to control disease and potentially damaging the fourth major component of agricultural plant physiological and morphological structural integrity of the greenhouse. greenhouse energy consumption. developments, such as the onset Maintaining ideal humidity levels Depending on the crop and cultivation of flowering and fruit production is therefore a critical function of system, these may include irrigation (Hassanien et al. 2016). greenhouse heating, ventilation, and pumps and sprayers, mechanical Both natural and artificial lighting (HVAC) equipment harvesters, transplanting and potting systems are used in agricultural (Talbot and Monfet 2020). machines, product packaging and greenhouses to maintain optimal light labeling equipment, autoclaves, levels for crop growth and production. Crop-air interactions, along with certain refrigerators or coolers, and other Greenhouses often use supplemental greenhouse design and management farm equipment and facilities. While lighting during low-light conditions to decisions, including crop selection, these energy demands are typically extend the growing season, regulate moisture-holding capacity of interior small compared to microclimatic plant development, and maintain surfaces, light and temperature control systems, utilizing renewable optimal photosynthesis rates. In regulation, and irrigation practices can energy to supply all or part of the some settings and conditions, energy significantly impact relative humidity energy demands for these smaller demands and operational costs for and need to be considered when applications can be an environmentally supplemental lighting can rival heating selecting and sizing both passive and and economically attractive option, particularly in areas where grid and cooling costs (Yano and Cossu active humidity regulation and HVAC is unreliable or unavailable 2019). When solar radiation levels systems (Talbot and Monfet 2020). exceed optimal light intensities for (Hassanien et al. 2016).

4 | Renewable Energy for Heat and Power Generation and Energy Storage in Greenhouses Solar Technologies to Meet strategies alone­—both solar PV or solar and potentially impacting crop yields Energy Demands in Solar thermal technologies are available (Cossu et al. 2020). Conversely, Greenhouses (Gorjian et al. 2021). greenhouses in hot climates can benefit from shading and cooling Solar energy technologies and Solar PV provided by roof-mounted solar PV strategies could sustainably and Solar PV systems convert sunlight modules, which can improve indoor reliably meet the energy demands directly to electricity, which can greenhouse microclimatic conditions, of agricultural greenhouses, reduce then be used to power mechanical reduce cooling demand loads, and operating costs, improve farm and electrical systems within the positively impact crop yields (Gorjian competitiveness, and alleviate greenhouse or other on-site facilities, et al. 2021). competing land development pressures sold back to the electric grid, or for energy and food production To balance trade-offs between energy stored in battery cells for later (Hassanien et al. 2016; Cossu et productivity and crop productivity, PV use. Greenhouse solar PV systems al. 2020). Both passive and active cell type, module design, and module include both on-grid and off-grid solar technologies can be used by installation patterns on the rooftop configurations. agricultural greenhouses. should be considered. Dominant On-grid systems are connected to the PV cell types used in greenhouses Passive Solar local power grid and can either access include opaque and semi-transparent Passive solar greenhouses use grid-supplied power when greenhouse PV cells. Opaque cell types are a more conventional technology for siting and design strategies to energy demands exceed PV electricity rooftop solar, but they result in more maximize efficient capture, storage, generation or sell excess power back shading effects and typically require and utilization of incoming solar to the grid. Off-grid systems are more other system configuration and/or energy. The amount of solar energy common in settings where a utility operational strategies to mitigate any absorbed depends on factors such connection is not available, such as adverse impacts to crop productivity. as latitude and season; greenhouse remote or isolated locations, and Replacing some or all of the PV typically incorporate other backup size, dimensions, and orientation; local modules with semi-transparent PV power sources, such as fossil-fueled climate and other characteristics of the cells can help optimize the shading generators or battery banks, to ensure installation site; and the materials used impacts of PV modules to balance stable and reliable energy supplies in exterior walls and facades. Passive crop photosynthetic requirements, (Gorjian et al. 2021). solar greenhouses can also include , and thermal collection units to capture and Wall- and roof-mounted PV modules management of interior microclimatic store excess incoming solar radiation allow for co-production of energy conditions, in some cases improving in heat storage mediums and minimize and crops without incurring some both crop and energy productivity by diurnal temperature fluctuations. While of the negative land development capturing both incident and ground- many passive strategies are primarily pressures and ecological consequences reflected radiation (Cossu et al. 2016). available for new construction, some associated with large ground-based Other system design parameters strategies can be incorporated into solar farms in rural and agricultural include the use of sun-tracking systems existing structures. areas (Castellano 2014). However, there to produce dynamic shading and are energy production and crop yield improve electric generation efficiency Active Solar trade-offs to consider. For each 1% (Gorjian et al. 2021), the installation In active solar greenhouses, solar increase in the PV cover ratio (the ratio pattern of PV panels on the roof, and technology systems, such as of projected area of PV panels on the the total PV cover ratio (Cossu et al. photovoltaic (PV) panels or solar ground to the total greenhouse floor 2020). The optimal combination of thermal collectors, are used to produce area), the cumulative global radiation these various parameters depends electricity and/or improve thermal within the greenhouse decreases by upon characteristics of the local performance beyond what can be 0.8%, limiting the amount of sunlight climate, crop production method, and achieved through passive design available to support plant development greenhouse design.

5 Photovoltaic solar panels installed on a greenhouse. Photo from iStock 1207270991

Solar Photovoltaic Technologies

Technology / Grid-connected (on-grid) solar PV system Approach Description Excess energy generated by PV modules is fed into the power grid Example This is the most common system configuration in agricultural greenhouses. Notable exceptions include Applications remote and/or geographically-isolated locations where grid power may be inaccessible or unreliable. In some parts of the world, such as Italy and China, greenhouse-generated electricity makes up a substantial portion of renewable energy on the local grid Benefits Potential revenue generation opportunity for greenhouses with excess solar insolation Grid backup power reduces the need for additional backup generators or batteries Drawbacks Requires utility connection and policies (e.g. ) to allow sale of excess power to the grid

Technology / Semi-transparent / bifacial panels Approach Description PV modules with PV cells on the front (sky-facing) and back (ground-facing) sides to capture incoming sunlight and ground-reflected irradiance Example Technology to mitigate shading impacts of rooftop PV Applications Balancing light demands for energy and crop production Benefits Well-established technology Allows partial transmittance of sunlight to crops Drawbacks Higher capital and O&M costs Total power production varies depending on the amount of light transmitted by the panels and the amount of light reflected from the ground

6 | Renewable Energy for Heat and Power Generation and Energy Storage in Greenhouses Technology / Organic PV (OPV) cells Approach

Description PV cells using molecular or polymeric absorbers rather than silicon

Example Incorporated in semi-transparent solar blind systems, bifacial modules, and hybrid PVT systems as an Applications alternative to conventional Si-PV

Benefits Smaller life-cycle environmental impacts and shorter energy and carbon payback times compared to Si-PV Thin and lightweight Low production costs Can be designed to create colored or transparent PV modules that use non-PAR wavelengths for electricity generation and transmit PAR wavelengths to crops

Drawbacks New technology is difficult to obtain Further efficiency improvements are needed to make OPV cost-competitive with traditional Si-PV

Technology / Dynamic PV shading controls Approach

Description Movable PV modules such as venetian blind-type systems or sun-trackers

Example Incorporated in semi-transparent solar blind systems, bifacial modules, and hybrid PVT systems as an Applications alternative to conventional Si-PV

Benefits Sun-trackers prioritize electricity production, blind-type systems can be automatically or manually adjusted to meet crop or energy production needs

Drawbacks Higher capital and operations and maintenance costs relative to stationary modules; an area of ongoing research and design optimization

Technology / Near-infrared (NIR)-reflective coatings Approach

Description NIR-reflective greenhouse roof coating allows PAR wavelengths to reach crops and reflects NIR portion of sunlight to a PV system positioned to capture the reflected light

Example Prototype greenhouse with circular trough reflector integrated in the greenhouse roof constructed in 2007 in the Applications Netherlands

Benefits Allows simultaneous utilization of sunlight for energy and food production NIR reflection prevents overheating of greenhouse interior

Drawbacks Requires solar tracking and complex custom greenhouse design Curved roof structures accumulate snow

7 Technology / Checkerboard PV configurations Approach

Description Opaque or semi-transparent PV modules alternated with conventional clear roof panels in a checkerboard pattern to create more uniform shading of the greenhouse interior

Example Creation of more uniform shading pattern; Applications High light-demanding crops (tomato, cucumber, sweet pepper) or medium light-demanding crops (asparagus, basil, lettuce, strawberry, spinach, chrysanthemum, rose, ficus) requiring PV cover ratio < 60%

Benefits Simple design approach compatible with conventional technologies; more uniform shading patterns can improve greenhouse microclimate and crop yields compared to unshaded greenhouses

Drawbacks Tradeoff between energy and crop production

Solar Thermal PV systems by efficiently transferring concentrators using reflectors or Solar thermal technologies convert waste heat away from the PV panels refractors to focus sunlight from a incoming sunlight to heat, which toward productive low- to medium- wider collection area onto a single can be used for space and water temperature applications, cooling the point or line, such as a fluid-filled heating, or transferred to a storage panels and preventing overheating in pipe, achieving higher thermal and medium for later use. These systems the process (Gorjian et al. 2021). Hybrid optical efficiencies and making them can achieve high energy conversion systems combining thermal collectors particularly attractive options for efficiency and energy storage density with heat pumps can more sustainably cold climates or situations where at relatively low cost, making them a and reliably meet greenhouse heating competing demands for light and/ promising alternative to fossil - demands (Hassanien et al. 2018; Awani or space are constraints (Gorjian fired heating systems (Gorjian et al. et al. 2015). et al. 2021; Wu et al. 2019). Non- 2021). Systems typically consist of a concentrating flat-plate collectors and solar collector to absorb incoming Thermal Collectors evacuated tube collectors can supply solar radiation and convert it to heat, Thermal collectors can be categorized a substantial portion of greenhouse and a storage unit to according to the collector type heating demands with short payback deposit excess heat for colder periods. and medium. Non- periods in certain settings, especially in When used alongside PV panels in concentrating collectors include flat- moderate climatic conditions or when hybrid PV-thermal systems, solar plate and evacuated tube collectors using large collector areas (Gorjian et thermal technologies can improve or concentrating (e.g., parabolic and al. 2021; Hassanien et al. 2018). the electric conversion efficiency of concentrators). Solar

8 | Renewable Energy for Heat and Power Generation and Energy Storage in Greenhouses Solar Thermal Technologies

Technology / Hybrid photovoltaic-thermal (PVT) systems Approach

Description Combined PV modules for electricity production with thermal collectors to capture excess heat for low- to medium- temperature applications

Example Greenhouse solar dryers Applications PV modules at risk of overheating Space- and/or light-constrained systems

Benefits More efficient in both heat and electricity production than either system alone Cooling medium extracts excess heat from PV modules, improving overall electric efficiency and capturing usable heat

Drawbacks Reflectors, lenses, and solar tracking mechanisms increase system complexity and cost More research is needed on controlling the temperature provided by concentrators

Technology / Latent (LTES) Approach

Description Storage of excess heat in phase-change materials (PCMs) that transition between solid and liquid states as energy is absorbed and released

Example Heating and cooling applications Applications High temperature (e.g., concentrated ) and low temperature applications Seasonal and short-term storage Frequently integrated with other thermal technologies to enhance overall energy utilization efficiency Can be integrated in the greenhouse structure or exist in stand-alone storage tanks

Benefits Large amounts of energy can be stored and released without changing the temperature of the storage medium PCMs can be selected based on the desired thermochemical characteristics

Drawbacks Most systems are more expensive and complex compared to sensible thermal energy storage systems

9 Technology / Sensible thermal energy storage (STES) Approach Description Storage of excess heat generated during the day in common materials with high thermal capacity, typically rocks, water, and/or soil. The heated materials then dissipate the heat at night or during cloudy / cooler periods Example Heating and cooling applications Applications High temperature (e.g., ) and low temperature applications Seasonal and short-term storage Frequently integrated with other thermal technologies to enhance overall energy utilization efficiency Can be integrated in the greenhouse structure or exist in stand-alone storage tanks Benefits Inexpensive design and simple operation Long lifetime and short repayment period Widely available materials with high thermochemical and mechanical stability Positive secondary impacts on greenhouse microclimate, pest suppression, and crop yield Drawbacks Lower energy density than phase-change materials System efficiency may be dependent on weather conditions

Technology / Concentrating solar collector Approach

Description Modules use curved reflectors or refractors to focus solar energy on a single point (point-focus) or line (line-focus) where the heat transfer medium absorbs and transmits the collected Example collectors Applications Fresnel lens collectors Parabolic dish collectors Compound parabolic collectors Benefits Result in the most thermally and electrically efficient hybrid PVT systems when integrated with PV technology Less crop shading impact for rooftop systems compared to flat-plate collectors Drawbacks Reflectors, lenses, and solar tracking mechanisms increase system complexity and cost More research is needed on controlling the temperature provided by concentrators

Technology / Non-concentrating solar collector Approach Description Systems collect heat in air or liquid transfer mediums without the use of concentrating reflectors or refractors Example Flat-plate collectors Applications Evacuated tube collectors Benefits Simple, inexpensive design Easier control of operating conditions compared to concentrating solar collectors Flat-plate collectors can more easily be mounted on greenhouse facades or tilted roofs, offering beneficial shade protection Drawbacks Supplying the full thermal demand of the greenhouse may require increased collector area, auxiliary heat supplies, and/or integration of thermal energy storage

10 | Renewable Energy for Heat and Power Generation and Energy Storage in Greenhouses Colorado Greenhouse Technology Considerations The four greenhouse farms considered challenges or address the greatest other energy resources. Careful as examples for this study are Altman energy consuming areas of the consideration of the different Specialty Plants, Gunnison Gardens, greenhouse identified through benefits and challenges of various Welby Gardens, and Zapata Seed the audit. Further study and solar energy options, as well as the Company. They span a wide range assessment should be conducted potential synergistic benefits of of cultivation techniques, existing before implementing any of these different combinations of active and on-site energy resources, structural technology suggestions. The passive solar power and solar thermal designs, and climate control strategies, appropriate combination of technical solutions, could deliver multiple collectively demonstrating a diverse and nontechnical solutions will vary energy and non-energy benefits to set of energy needs and challenges. according to the unique characteristics help reduce operating costs, improve Each of these greenhouses received of each operation, including current farm competitiveness, and improve an energy audit from an outside state of operations and factors such the sustainability and resilience of agency and the results of the audit as light, water, and temperature agricultural yields from controlled were used as the basis to suggest requirements of crops, the use of environment agriculture (in the face solar energy technologies that different passive and active climate of a changing climate and evolving could be implemented at each of control technologies, and the site’s industry. the greenhouses to meet energy access to sunlight, grid power, and

Non-concentrating solar hot water collector system. Photo by Dennis Schroeder, NREL 48515

11 Greenhouse Name Primary Challenges/ Solar Suggestions from Case Study and Characteristics Greatest Energy Consuming Areas Identified through Energy Audits

Altman Specialty Space heating a. Repair/ replace existing equipment, opting for energy-efficient upgrades where Plants • Four 9,300kW boilers possible to reduce overall electric and heating loads. Repair curtain controls to Location: Peyton, CO (18,600kW per greenhouse) regulate light on-demand, consider centralizing controls for irrigation, roof vents, curtains, and heating system to facilitate more dynamic energy and water use Size: 1.4 million ft2 Water pumps regulation. (32 acre) • Five 10 horsepower well b. Replace boilers with liquid-type photovoltaic thermal system to produce both Main Crop: Ornamental water pumps (27.7 million hot water and to power the radiant heat system. Concentrating Succulents gallons per year) Photovoltaic Thermal collectors (e.g., parabolic trough collectors or Fresnel lenses) Current Heat & Power • Four 20 horsepower offer higher thermal and electric efficiencies (Gorjian et al. 2021), but partially Source: Electricity and irrigation pumps covered flat plate water collectors can also be suitable for hot water production Natural Gas Other mechanical systems (Sultan and Ervina Efzan 2018). Audit conducted by: • Small (<1 horsepower) c. Incorporate thermal energy storage to improve overall heating system efficiency Nexant motors for roof vents, and reduce diurnal / seasonal indoor temperature fluctuations (see Vadiee and Audit date: Nov. 19, conveyor system, curtains Martin 2013 for an assessment of different TES options to cover seasonal and daily 2020 heating and cooling demands). d. Advanced option: replace thermal curtain with solar PV or Photovoltaic Thermal blind system using bifacial or semi-transparent PV panels as blind slats. Blinds can be operated manually or with automated controls regulated by indoor air temperature, humidity, and lighting conditions (see Li et al. 2020, Vadiee and Yaghoubi 2016 for example configurations).

Greenhouse Name Primary Challenges/ Solar Energy Technology Suggestions from Case Study and Characteristics Greatest Energy Consuming Areas Identified through Energy Audits

Gunnison Gardens Space heating These recommendations are based on an energy audit performed in 2016, some Location: Gunnison, CO • Two 1.5kW electric radiant upgrades may have already occurred since then. Size: single gable roof space heaters in seed starting a. Replace electric radiant space heaters with a solar Photovoltaic Thermal greenhouse room system to provide space heating for the seed starting room and electricity for fans, lighting, and other equipment. A free-standing system with concentrating Main Crop: Seasonal Fans collectors (e.g., parabolic trough collectors) could be an efficient use of impervious Crops • 1.36kW climate battery fans area adjacent to the two primary site structures (see Sultan and Ervina Efzan 2018 Current Heat & Power • 1 horsepower exhaust for a review of various Photovoltaic Thermal options). Source: Electricity fan for summer ventilation b. Upgrade fluorescent grow lights to high efficiency LED grow lights to reduce (planned) Audit conducted by: overall electricity demand. GDS Associates, Inc. Lighting c. Incorporate thermal energy storage for the seed starting room to reduce Audit date: Dec. 29, • Twenty-one 68 W grow nighttime and wintertime heating demand. Due to the significant energy demand 2016 lights spikes in winter, seasonal storage in underground thermal energy storage may be • 156 W and 90 W appropriate (Vadiee and Martin 2013). fluorescent lights (seed d. If wintertime electricity demand continues to exceed supply from Photovoltaic starting room) Thermal system, consider a residential battery storage system to capture and store • Five 51 W LED flood lights any excess electricity produced during warmer and sunnier periods. (storage area) • 66 W LED string lights (storage area)

12 | Renewable Energy for Heat and Power Generation and Energy Storage in Greenhouses Greenhouse Name Primary Challenges/ Solar Energy Technology Suggestions from Case Study and Characteristics Greatest Energy Consuming Areas Identified through Energy Audits

Welby Gardens - Greenhouse heating These recommendations are based on an energy audit performed in 2017, some Westwoods • Central steam boiler upgrades may have already occurred since then. Location: Arvada, CO (natural gas) a. Replace (or supplement) boiler with efficient rooftop solar Photovoltaic Thermal Size: 350,000 ft2 • Several hydronic Modine system to provide wintertime space heating for the gutter-connected greenhouses and hoop houses as well as electricity to power fans, cooling pad pumps, walk-in Main Crop(s): unit heaters coolers, and other equipment. Concentrating Photovoltaic Thermal collectors (e.g., Propagation of Fans parabolic trough collectors or Fresnel lens collectors) could be used to maximize Annuals, Perennials, • 142 exhaust fans (1.1 kW solar thermal energy capture while still allowing sufficient light penetration into Ornamental Grasses, each) the greenhouse (Gorjian et al. 2021). A feasibility study including an assessment of Organic Vegetables, • 17 horizontal air flow fans available and sufficient solar resource should be conducted before installing any and Herbs (0.1 kW each) concentrating solar power systems. Current Heat & Power b. Conduct a feasibility study to see how much of the annual heating demand Source: Electricity and could be met by solar thermal, especially if investing in concentrators and/or Natural Gas thermal energy storage to improve the overall efficiency and overcome some Audit Conducted by: of the mismatch between peak demand times and peak solar thermal energy GDS Associates, Inc. production times. Audit date: May 31, 2017 c. Incorporate seasonal soil heat storage units with greenhouse solar photovoltaic thermal systems to reduce seasonal thermal energy demand fluctuations, serve wintertime heating demand, and improve overall Photovoltaic Thermal system efficiency (see Awani et al. 2017 and Zhang et al. 2015 for potential system designs with and without a ground-source heat pump, respectively). d. Advanced option: install thermal curtains on greenhouses with solar PV or Photovoltaic Thermal blind system using bifacial or semi-transparent PV panels with flat-plate collectors as blind blades. Blinds can be operated manually or with automated controls regulated by indoor air temperature, humidity, and lighting conditions to reduce nighttime heat losses, provide shading while generating electricity during sunny periods, and prevent overheating in the summer (see Li et al. 2020, Vadiee and Yaghoubi 2016 for sample configurations. Note that this included modeling for a photovoltaic thermal system with flat plat collectors on a much smaller greenhouse with single polycarbonate glazing; other critical site data such as roof pitch and load capacity might preclude rooftop PV systems).

Solar photovoltaic panel built on the roof of aerial vegetable greenhouse. Photo from iStock 1131751289

13 Greenhouse Name Primary Challenges/ Solar Energy Technology Suggestions from Case Study and Characteristics Greatest Energy Consuming Areas Identified through Energy Audits

Zapata Seed Company Greenhouse heating These recommendations are based on an energy audit performed in 2017; some Location: Hooper, CO • Modine overhead liquid upgrades may have already occurred since then. Size: 7,000 ft2 propane gas unit heaters in a. Replace greenhouse overhead liquid propane gas unit heaters with rooftop each greenhouse or ground mounted photovoltaic thermal system to provide space heating for Main Crop: Seed greenhouses and attached headhouse, west headhouse, and second office as well Potatoes Lighting as electricity to power lights, fans, evaporative cooling pad pumps, and walk- • 45 high-pressure sodium Current Heat & Power in cooler. Concentrating photovoltaic thermal modules (e.g., parabolic trough greenhouse grow lights (465 Source: Electricity and collectors or Fresnel lens collectors) generally offer the greatest optical and W each) Liquid Propane thermal efficiencies (Gorjian et al. 2021) Audit conducted by: • 68 fluorescent overhead b. Incorporate seasonal soil heat storage (SSHS) unit for long-term storage of light fixtures (90-180 W GDS Associates, Inc. excess thermal energy generated during warm months to use for wintertime each) Audit date: Dec. 6, 2017 greenhouse heating (see Awani et al. 2017 and Zhang et al. 2015 for potential • Three CFL fixtures in walk- system designs with and without a ground-source heat pump, respectively). in cooler (18 W each) Short-term (diurnal) rock-bed or water thermal energy storage units can also Fans help reduce daily temperature fluctuations and improve overall thermal efficiency • Eight 0.2 kW greenhouse (Gorjian et al. 2021) HAF fans c. Upgrade grow lights and fluorescent lights to high efficiency LED lights to • Four 1.6 kW greenhouse reduce overall electricity demand. exhaust fans d. Advanced option: install thermal curtains on greenhouses with solar PV or • 0.7 kW head house exhaust photovoltaic thermal blind system using bifacial or semi-transparent PV panels fan with flat-plate collectors as blind blades. Blinds can be operated manually or with automated controls regulated by indoor air temperature, humidity, and lighting Misc. heating, cooling, conditions to reduce nighttime heat losses, provide shading while generating motors, etc. electricity during sunny periods, and prevent overheating in the summer (see Li • 1.5 kW ductless split unit et al. 2020, Vadiee and Yaghoubi 2016 for sample configurations. Note that this (front office) included modeling for a Photovoltaic Thermal system with flat plat collectors on a • Furnace style forced air much smaller greenhouse with single polycarbonate glazing; other critical site data system (grow room / head such as roof pitch and load capacity might preclude rooftop PV systems). house backup heating) • Overhead liquid propane unit heater (west head house backup heating) • Two 0.1 horsepower greenhouse vent motors • Three 1 horsepower cooling pad / well pumps • 12.1 horsepower autoclave • 5 horsepower condensing unit, evaporator coil and standard motors (walk-in cooler

14 | Renewable Energy for Heat and Power Generation and Energy Storage in Greenhouses Conclusion Solar energy technologies represent of these technologies were suggested and greenhouse structural design. a promising solution to sustainably as possible solutions to address energy While there are no one-size-fits-all and reliably meet energy demands challenges and requirements at four solutions to the energy challenges of of agricultural greenhouses. The greenhouses in Colorado but could be indoor agriculture, these technologies technologies described in this study applied in greenhouse farms across the and strategies create promising represent some of the most promising world. The appropriate combination of opportunities to reduce operating costs, options for sustainable, reliable, and technical and nontechnical solutions improve farm competitiveness, and economical heat and power generation will depend upon characteristics alleviate competing land development in an expanding and diversifying like the crop system, climate control pressures for energy and food agricultural greenhouse industry. Some technologies, cultivation practices, production.

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This work was authored by the Joint Institute for Strategic Energy Analysis (JISEA), operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE- AC36-08GO28308. Funding provided by Colorado Dept of Agriculture and Colorado Energy Office. The views expressed herein do not necessarily represent the views of the DOE, the U.S. Government, or sponsors.

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JISEA is operated by the Alliance for Sustainable Energy, LLC, on behalf of the U.S. Department of Energy’s National Renewable Energy Laboratory, the University of Colorado- Boulder, the Colorado School of Mines, Colorado State University, Massachusetts Institute of Technology, and Stanford University.

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