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Journal of Resources and Ecology >

2022 , Vol. 13 >Issue 3: 382 - 393

DOI: https://doi.org/10.5814/j.issn.1674-764x.2022.03.004

Ecological Community Management

An Ecological Compensation Mechanism based on the Green Productive Area of Cities

  • YANG Yuanchuan , 1 ,
  • ZHANG Yukun , 1, * ,
  • ZHENG Jie 1 ,
  • HUANG Si 1 ,
  • ZHAO Man 2 ,
  • HONG Long 3
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  • 1. School of Architecture, Tianjin University, Tianjin 300072, China
  • 2. Tianjin Huahui Engineering Architectural Design Co., Ltd, Tianjin 300384, China
  • 3. Jinke Property Group Co., Ltd., Chengdu 610051, China
*

YANG Yuanchuan, E-mail:

Received date: 2020-11-24

  Accepted date: 2021-03-29

  Online published: 2022-04-18

Supported by

The National Natural Science Foundation of China(51978443)

The National Natural Science Foundation of China(52078322)

The Youth Program of National Natural Science Foundation of China(51708395)

The Tianjin Graduate Research and Innovation Project in 2009(2019YJSB175)

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Abstract

China's urban expansion, food security, and energy transition are in a critical situation. One solution is to tap into the green production potential of the built urban environment and explore new ways to save land space and alleviate ecological pressure through food and solar energy production. This paper differs from previous ecological compensation studies, which mostly focus on key ecological functional areas or fiscal compensation mechanisms, in that it innovatively establishes an ecological compensation mechanism within the urban space. In this paper, we propose the "green productive area" of cities as a way to measure the ecological carrying capacity improvement potential of cities from the perspective of urban ecology, and it is based on converting the green resource income of cities into the ecological footprint area they could save under the same conditions. First, a typological approach was used to establish a compensation strategy for green production. Second, a spatial inventory was taken of all elements of the built environment and an analysis of their green production potential was carried out. Finally, it was necessary to establish a unified accounting standard for the ecological land saving benefits of different green production options, which could be converted into green productive land area indicators. In the case of Xuefu Street in Nankai District, Tianjin, the available rooftops and idle land were used for green production, which supplemented the ecological carrying capacity provided by the natural land occupied by 12% of the buildings in the district.

Key words： ecological compensation; green productive area; land-saving benefits; urban agriculture; solar energy potential

Cite this article

YANG Yuanchuan , ZHANG Yukun , ZHENG Jie , HUANG Si , ZHAO Man , HONG Long . An Ecological Compensation Mechanism based on the Green Productive Area of Cities[J]. Journal of Resources and Ecology, 2022 , 13(3) : 382 -393 . DOI: 10.5814/j.issn.1674-764x.2022.03.004

1 Introduction

Since the economic reform and opening up, China's urbanization rate has increased from 19.99% in 1979 to 60.60% in 2019, an increase of more than 30% in 40 years. While urbanization has improved people's living standards, it has also triggered a series of problems, such as the conflict between land supply and demand, declining food security, and the energy crisis, which have seriously restricted China's future sustainable development.
(1) Urban sprawl and idle land
Urban areas occupying 2% of the world's land and consume 67%-76% of the global energy and emit 71%-76% of the greenhouse gases (IPCC, 2015), and their ecological footprint far exceeds the natural ecological carrying capacity of the cities themselves (Rees et al., 1996). The expansion of global urbanization has also had a tremendous impact on the global arable land area. In 40 years, China's urban construction land grew nearly eight-fold (Qie, 2014), and the per capita arable land area fell from 0.155 ha per person to 0.097 ha per person (NBSC, 2019a), which is less than half of the world average (0.206 ha). China's urbanization rate was 58.52% at the end of 2017, and is expected to consume approximately 29857 km2 of land if it is to reach the level of developed countries (80%) (Zhang et al., 2019). In addition, due to China's extensive land development and management model, the rapid expansion of urban areas is also accompanied by numerous idle land problems. Idle land will bring a financial maintenance burden to local governments, while causing community decline, environmental pollution, urban space fragmentation, and other adverse consequences for the city's economic, social, and environmental well-being, as well as other negative impacts (Pan, 2009).
(2) Arable land shortage and food safety
Urban dwellers rely on inner-city and peri-urban food production and distribution systems for their basic subsistence needs, yet in recent years food availability has become a growing problem worldwide. The number of hungry people in the world reached 821 million in 2018, an annual increase since 2015 that has broken a decade-long slowdown (Liu, 2019). The food problem has been linked to land occupation, ineffective urban management and related socioeconomic issues (Mendes et al., 2008). In 2016, China's arable land decreased by a total of 3.045×105 ha (Zhu, 2019) due to land occupation for urban construction, natural disaster damage, ecological reclamation, and regional agricultural restructuring. It is expected that urbanization will reduce the global arable land area by 1.8% to 2.4% by 2030, 80% of which will occur in Asia and Africa (Li, 2019). China's food production and demand gap will reach 137 million tons in 2030, accounting for 46% of the global trade volume (FAO, 2019). In the post-pandemic era, the instability of the international market will seriously affect China's political, economic and social stability. In addition, the transportation industry has extended the distance between food production and consumption, exacerbating the functional differences between urban and rural areas and even indirectly causing the phenomenon of over-consumption (Grewal and Grewal, 2012). A globalized food system that relies too much on industry is 4-17 times more energy-intensive, resource-wasting, and environmentally polluting than a local food system (Pirog et al., 2009).
(3) Energy crisis and green transition
The shortage of arable land resources not only threatens food security, but also becomes a bottleneck in the energy transformation of cities. China is now in a special period of rapid economic development, rapid energy consumption, and worsening environmental problems, while facing the harsh reality of dealing with global climate change and getting rid of external energy dependence. Therefore, the transition from fossil fuels to renewable energy is the best way to achieve sustainable development in both urban and rural areas of China. However, the vigorous development of solar farms and biomass energy production bases will take up a lot of land. According to the National Energy Administration's 13th Five-Year Plan and the National Forestry Biomass Energy Development Plan (2011-2020), the construction target of 150 million kilowatts of total installed photovoltaic capacity and the planting target of biomass raw materials will be achieved by 2020. These efforts are expected to consume 170797 km2 of land area, thus increasing tensions with farmers over land. There is an urgent need to find another way to achieve renewable energy production and ecological compensation in cities.
Therefore, the challenge for cities in the future will be to increase the carrying capacity of their systems. Cities will need to exploit their green production potential and modify their physical and functional structures to produce most of the resources needed by their inhabitants, mainly space, food and energy (Guallart, 2014).

2 Materials and methods

2.1 Inventory space development model

In 2014, China promulgated the Regulations on Land Conservation and Intensive Utilization, enforcing the strictest system of farmland protection and land conservation and intensive utilization. The current arable land conservation and intensive land use strategies had emerged to reduce the land taken up by construction. They improved the efficiency per unit of land by means of compact layout and mixed functions, “underground-surface-above” high-intensity development, etc. (Dong, 2008; Li, 2008), effectively relieving the pressure on urban land use. However, most of these land-saving methods only met the spatial needs of urbanization, but it was difficult to fundamentally compensate for the productive land functions occupied by urban construction.
Urban ecological overload has seriously plagued national ecological security and sustainable development, and China is one of the countries with the most serious ecological overload in the world. According to the 2019 Ecological Footprint Report, China's ecological overload day has been advanced to 14 June 2019, and China's ecological footprint has reached 3.8 times its ecological carrying capacity. Among the components, the urban ecological footprint accounts for about 63%-75%, which is the main reason for the rapid increase of the country's total ecological debt (Fig. 1). In Tianjin, for example, the city's urban plot ratio increased from 1.01 to 1.035 between 1990 and 2014, but the ecological deficit increased by 4.5-fold (Fig. 2). Thus, under the pressure of the increasing contradiction between land supply and demand, and the limitations of existing land-saving strategies, China is in urgent need of finding a new land- saving model.
Fig. 1 Relationship between urbanization rate and per capita ecological footprint

Note: Data source: Global Footprint Network, 2019.

Fig. 2 Relationship between urban plot ratio and ecological deficit in Tianjin

Note: Data source: NBSC, 2019b.

In 2015, the United Nations Conference on Sustainable Development included food supply (Goal 2) and clean energy (Goal 7) among the 17 sustainable development goals for 2030. Subsequently, the Chinese government proposed the concept of “Ecological Restoration and City Betterment” for sustainable development, accelerating the transition from “incremental” to “stock” renewal. Combining these two, the spatial resources available for green production in the urban built environment consist mainly of vacant land and building roofs.
Around the end of the 20th century, the United States, the United Kingdom, Canada and other countries had begun to pay attention to the lack of brownfield management and environmental issues (Roger, 2008; Deng, 2010), marking the beginning of a systematic study of urban idle land. Beginning in 2010, China successively issued the Further Strengthening Real Estate Land and Construction Management and Control (2010), the Disposal of Idle Land (2012), and the Improving the Mechanism of “Incremental and Inventory Linkage” of Construction Land (2018) (Department of Natural Resources Website, 2018), etc., which gradually promoted the green development of urban idle land.
Urban roofs generally account for 21%-26% of the built- up area (Nadal et al., 2018), and even as high as 35% in some areas (Peck et al., 2009). Assuming that the average roof area in China's urban areas was 25%, and the built-up area of China's cities in 2016 was 54300 km2 (MHURD, 2015), then about 13600 km2 of roofs would be available for photovoltaic power generation. If 50% of them were flat roofs, and the availability of roofs was 64.1%-99.2% (Li et al., 2012), then at least 0.43 million km2 of idle roofs could be used for rooftop planting.

2.2 Urban ecological compensation strategies

In China, ecological compensation is generally regarded as a system for regulating economic interests among the protectors, beneficiaries, and destroyers of ecosystems (Wang, 2014). The establishment of the ecological compensation system has opened up a new phase of ecological protection in China. After many years of efforts, key ecological assets have been effectively protected; the deterioration of ecological conditions has been initially curbed; and ecosystems in key governance areas have been improved (Xie et al., 2015). However, traditional ecological compensation strategies have mostly focused on natural ecosystems (forests, grasslands, wetlands, watersheds, and other key ecological functional areas), and they have seldom addressed ecological compensation in urban areas. Since ecological compensation is based on the value of ecosystem services, conservation costs and opportunity costs of development, it is mainly a government-led economic compensation through financial, vertical and horizontal transfers and market mechanisms. In addition, the scarcity and fragmentation of natural land, and the complexity of quantifying ecological values, have led to a long-term lack of attention to the ecological service functions of urban ecosystems.
However, the “ecological compensation” described in this paper was designed to compensate for the land resources occupied by non-ecological functions by making the urban built environment accommodate as many green productive functions as possible, and to save land indirectly. Previous compensation measures had been limited by funds, institutions, methods, regulations, etc., resulting in inadequate coverage, implementation effects, public awareness, flexibility, etc. (Xie et al., 2015). In short, passive compensation through economic means has been easily misunderstood as antagonistic to urban development, while open-source compensation through secondary excavation of physical space and active green production is the future trend of sustainable development.
Ecological footprint theory suggests that most resources are produced on ecologically productive land, so that every source of human consumption can be measured in terms of the area of land/water needed to produce it. Under this premise, the question of the supply and demand of urban resources is translated into the question of “How much ecologically productive land is needed to sustain the needs of a given population?”. Therefore, increasing the amount of green productive land in cities is the key to increasing the ecological carrying capacity of cities.

2.3 Built environment green production

In light of the above realities, it is urgent to break the mindset of “nature as producer, city as consumer”. Cities need to take the initiative to “open source” on the basis of “cut costs” and take on the role of producer in order to fundamentally solve the problem of sustainable development. In combination with the theories of “spatial production” and “ecological compensation”, the efficiency of space utilization in the urban built environment was re-examined.
In fact, urban construction occupies natural land which lost the ecological service value that it was originally intended to carry, but it is possible to reclaim the land and change the underlay of the built environment to make it capable of carrying the appropriate production or ecological functions (Gong, 2017). The urban underlay is a three-dimensional concept rather than just the ground, such as roofs, roads and open spaces of different elevations, and even elevations and slopes, and is largely underutilized. Green production is a necessary means to achieve sustainable urban development, which mainly covers industry, agriculture, energy, water, waste recycling, etc. This paper focuses on the food and solar energy that are directly related to urban ecological land saving, such as urban agriculture, photovoltaics, or complex green production integrated with urban elements. While the ecological carrying capacity provided by urban ecosystems is certainly not fully equivalent to that of natural ecosystems, the complex calculations of social, economic and environmental benefits could be translated into the green productive area of cities and used as a figurative measure to improve the carrying capacity of cities, drawing on the expression of the ecological footprint.
The development of the ecological compensation mechanism based on the green productive area of the city was divided into three steps. Firstly, a green production compensation strategy was formulated, which summarized the spatial reconstruction of green production and the built environment based on typical cases. The second step was to analyze the potential of the built environment, mainly including urban spatial information acquisition and land use inventory. The last step was to quantify the ecological land-saving benefits, mainly by converting the ecological carrying capacity of the green productive area.

3 Results

3.1 Green production compensation strategies

The urban built environment elements can be typologically categorized into buildings and structures, external spaces, and other infrastructures, and analyzed by combining existing domestic and international research and typical cases (Table 1). The spatial integration of agriculture, photovoltaic (PV) and built environment elements includes the use of existing space (placement of planting containers), overlay of productive functions (solar pavement), replacement (replacement of landscape with productive landscape), integration (integration of PV with buildings), and reorganization (PV greenhouses).
Table 1 Summary of the strategies for built environmental elements and types of green production
Built environmental elements Urban agriculture Photovoltaic power Generation
multifunctional		 Other
Buildings and structures Roofs
Open air planting1
Roof covering2
PV greenhouses3
Roof wind energy4
Balconies
Elevation
(Walls,
windows)
Use of balcony5
Facade integration6
Side by side7
Green algae blinds8
Courtyards
Basements
Indoor planting9
PV atrium10
Ecological atrium11
Indoor planters12
External spaces Piazza
Greenfield
Water
Kiosks13
Water surface PV14
Replacement15
Wind energy tree16
City roads
Parking lots
Vegetable beds17
Parking shed18
Integrated units19
Wind energy lights20
Idle land
Use of open space21
Courtyard22
Negative space23
Under the bridge24
Infrastructure Roads
Railways
(Stations)
Highline park25
Roadway PV26
Integrated corridor27
Road wind energy28
Landfills
Water plants
Storage
Vertical farm29
Aqueduct PV30
Composite plant31
Biomass32

Note: 1. Brooklyn Navy Yard, https://www.brooklyngrangefarm.com/; 2. Vatican Solar Power (Italy), https://www.solaripedia.com/13/88/vatican_solar_power (italy).html; 3. ZICER Building, http://www.bipv.ch/index.php/en/administration-s-en/item/592-zicer; 4. Venger Wind, http://inhabitat.com/venger-wind-unveils-worlds-largest-rooftop-windfarm-in-oklahoma-city/; 5. Parkroyal Collection Pickering, https://www.panpacific.com/en/hotels-and-resorts/pr-pickering.html; 6. FKI Tower, http://smithgill.com/work/fki/; 7. Rooftop Garden Idea, https://decoratorist.com/modern-underground-home-rooftop-garden/; 8. The Clever Treefrog, http://inhabitat.com/splitterwerk-architects-design-worlds-first-algaepowered-building-for-germany/; 9. Pasona Urban Farm, http://konodesigns.com/urban-farm/; 10. El Centre del Mon, http://www.bipv.ch/index.php/en/other-sen/item/1177-elcentredelmon-eng; 11. Wooden Orchids, https://vincent.callebaut.org/object/150527_woodenorchids/woodenorchids/projects; 12. INFARM, https://breakthrough.unglobalcompact.org/briefs/infarmfarming-in-the-aisles/; 13. Kiosks, https://inhabitat.com/delightful-pop-up-camping-restaurant-brings-the-great-outdoors-to-busy-buenos-aires/; 14. Adopted from Saxena (2018). 15. Serra fotovoltaica Venlo, http://www.idrotermserre.com/portfolio-item/serre-fotovoltaiche/; 16. Aeroleaf, https://newworld wind.com/en/; 17. Railroad Park, https://www.flickr.com/photos/sitephocus/5522882903/in/photostream/; 18. Parking shed, https://treelion.com/promo/sustainabletransportation/; 19. Canopea House, http://www.designboom.com/architecture/solar-decathlon-winner-2012-canopeahouse-by-rhone-alpes-team/; 20. Tur-bine Light, https://www.engadget.com/2010/02/06/eco-shocker-turbine-light-concept-useswind-to-light-highways/; 21. Le 56-Eco- interstice, https://www.foodurbanism.org/le-56-ecointerstice/; 22. Masdar City Center, http://www.l-a-v-a.net/projects/masdar-city-centre/; 23. Adopted from Viljoen et al. (2005). 24. Solar Wind Bridge, https://www.evolo.us/solar-wind-bridge-efficient-reuse-of-highways/; 25. High Line, https://www.thehighline.org/; 26. Solar Panels Roads, https://www.slideshare.net/mkanth/solar-panels-on-roads; 27. Broadwater Parklands, https://landezine.com/southport-broadwater-parklands-by-aecom-design-and-planning/; 28. The Highway Turbine, https://www.mywindpowersystem.com/2018/04/03/amazing-wind-turbines-designs/; 29. Adrofarms, https://www.aerofarms.com/; 30. Solar panels covering canals, https://tekdeeps.com/put-solar-on-the-canals/; 31. Heizplan Solar Park, http://www.bipv.ch/index.php/en/industrials-en/item/960-heizplansolarpark-eng; 32. Quiver, 2nd Place Winner, LAGI 2014 Copenhagen, http://www.landartgenerator.org/competition2014.html.

Among these options, there are many cases of urban agriculture and photovoltaic power generation on rooftops and idle land in cities, so this paper focuses on the ecological compensation potential of green production in such spaces. Urban rooftops in China are complex and involve planning, technical and economic feasibility. Idle land includes undeveloped urban construction land, unmaintained green parks, decaying infrastructure (parking lots, open squares), and abandoned industrial land. This paper focuses on open-air agriculture, hydroponic greenhouses, rooftop photovoltaic and composite models based on green production compensation strategies, and assesses the suitability of green production and ecological compensation benefits for a given area.
Current domestic and international research on the intensive use of rooftops and idle land includes two main levels of spatial assessment at the geographic level and comprehensive assessment at the system level. Therefore, when introducing green production strategies in the built environment, it is equally necessary to analyze both aspects, and the green productive area is the quantitative index that combines the assessments of both spatial potential and ecological benefits (Fig. 3).
Fig. 3 Assessment framework for spatial potential and ecological benefits

3.2 Analysis of the built environment potential

First, it is necessary to construct a green production potential assessment system for rooftops and unused land, which consists of three main steps. 1) Reviewing the cases of existing built environment inventory work in the relevant domestic and international literature for the screening of assessment factors. On the basis of the previously proposed criteria, the specific assessment indexes corresponding to each criterion were partially modified and improved, and the initial set of green production suitability assessment factors was compiled (Table 1). 2) Based on the evaluation of stakeholders' perceived barriers to green production implementation obtained from the research, a group decision model was established in Yaahp software with the help of hierarchical analysis to assign weights to the assessment factors. To obtain data statistics and facilitate the analysis, the quantitative hierarchy of each indicator in the assessment system was determined by referring to the quantitative design of domestic and foreign inventory assessment indicators and relevant regulations (Table 2). 3) The final step is constructing a three-level hierarchical assessment system, in which the suitability of roofs and idle land is used as the target level, the planning, technical and economic aspects are used as the criterion level, and the 6 primary factors (11 secondary factors) are used as the indicator level.
Table 2 Evaluation criteria and references for green production suitability of roofs and idle land
Criteria Indicators References
Planning Field location Roof accessibility (I1) Nicole, 2014; Wang et al., 2016; Lv, 2017
Traffic accessibility (I2)
Functional
properties		 Architecture (I3) The People's Government of Beijing Municipality, 2011; Sbicca, 2019
Land (I4)
Technology Basic
conditions		 Roof material, age, height and carrying capacity (I5) The People's Government of Beijing Municipality, 2011; Esther, 2015; Wang et al., 2016
soil quality (I6)
Production conditions Sun shade (I7) Esther, 2015; Nadal et al., 2017; Saha et al., 2017
Irrigation source (I8)
shape (I9)
Economy Usable area (I10) Esther, 2015; Smith et al., 2017
Legal entitlement (I11) Wang et al., 2016
Planning criteria. Population density and road network patterns dictate that urban centers are more necessary and appropriate for building urban agriculture in terms of consumer demand and logistical distances (Wang et al., 2016). Rooftop accessibility (I1) is usually determined by the vertical traffic of buildings transporting people and materials; traffic accessibility (I2) contributes to the formation of urban local production for consumption, thus significantly reducing food miles. Buildings (I3) and sites (I4) of a mixed functional and public nature are more ready for development and easier to plan and manage than individual private spaces due to their openness and accessibility, which can effectively serve as a typical model for educational outreach (Lv, 2017).
Technical criteria. The basic conditions of the roof and site (I5 and I6) determine the feasibility and ease of implementation, such as whether the roof materials and age have the structural strength needed to meet the load-bearing capacity requirements of production activities, facilities and equipment; temperature, moisture and wind vary with increasing roof height; sites with safety hazards such as pollution, fire and traffic are not suitable for urban agricultural activities, etc. Sun and shade (I7) directly affects the efficiency of photovoltaic power generation and the variety andyield of agricultural cultivation. The total annual average sunlight radiation (5154.84 MJ m-2) in Tianjin meets these requirements, but the shading of the surrounding environment also needs to be considered. Proximity to water sources (I8) for easy cleaning of panels and irrigation of crops, flat or sloped roofs, site slope, as well as square and regular or narrow and fragmented shapes (I9) will determine the form of green production (i.e., agriculture only, PV only, or a combination of both).
Economic criteria. Although urban agriculture and photovoltaics can be carried out at different scales which match the size of the site, the available area (I10) determines the general standard scale for different green production strategies in order to ensure economic feasibility. Rooftops and idle land with different tenure status (I11) are related to possible business purposes, production themes and modes of operation (Esther, 2015).
The quantitative grading descriptions of the indicators in Table 3 are taken from the references listed in Table 2, with specific data descriptions as follows.
Table 3 Grading scores and weights for green production suitability of roofs and idle land
Indicators Quantitative rating descriptions of indicators Weight
1 2 3 4
Roof accessibility (I1) Climbing ladders Outdoor stairs Elevator or stairs Floor 0.09
Traffic accessibility (I2)1
(Walking/public transit)		 >500 m 300-500 m 100-300 m <100 m 0.04
Architecture (I3) Affordable facilities Industrial buildings Residential buildings Service-oriented public buildings 0.07
Land (I4) Filter reserved, protected and prohibited areas based on land use planning -
Roof material (I5-1) Asbestos, wood Brick, steel tile Concrete, steel Concrete, steel 0.04
Age (I5-2) Protect buildings >20 yr 10-20 yr <10 yr 0.04
Height (I5-3)2 >12 stories 7-12 stories 4-6 stories 1-3 stories 0.09
Carrying capacity (I5-4)3 0.7 kN m-2-2 kN m-2 ≥2 kN m-2 ≥2 kN m-2 ≥2 kN m-2 0.14
Site permeability (I6-1)4 <20% 20%-49% 50%-79% >80% 0.09
Soil quality (I6-2) Filtering of land for safety hazards based on current land use status -
Sun shade (I7)5
(Winter solstice 6 h)		 <33% 33%-66% >66% Unobstructed 0.12
Irrigation source (I8) Scoring sites based on ease of access to water and cost 0.09
Slope, shape (I9)6 Gradient>10°
>15°		 Gradient ≤10°
6°-15°		 Narrow flat roof
2°-6°		 Square flat roof
Plain		 0.08
Usable area (I10)7 <15 m2 15-100 m2 100-500 m2 >500 m2 0.09
Legal entitlement (I11) Based on public or private property, individual or collective owners -

Note: Superscripts 1-7 in the table are explained in the text below.

1. According to the experience of Portland urban walking system, the distance is more comfortable at 100-300 m, while more than 500 m makes people feel tired. Therefore, the road spacing in China's Urban Residential Planning and Design Standards should not exceed 300 m.
2. Although there are cases of high-rise rooftop agriculture and photovoltaic in high-density cities (Hong Kong and Singapore), it is recommended to refer to the building heights of domestic green roofs: Beijing and Chengdu green roofs stipulate 12 floors and less than 40 m, and Shanghai stipulates 6 floors and less than 18 m.
3. Green roof production needs a load bearing capacity of not less than 2 kN m-2 for roofs that can be manned and not less than 0.7 kN m-2 for roofs that cannot be manned.
4. Boston filters out sites with a permeability of less than 20%, and Portland recommends no less than 80%.
5. Greenhouses need to meet the standard of 6 h sunshine on the winter solstice, and open-air agriculture requires at least 6 h sunshine on the summer solstice, and the sunshine shading area is divided into three categories: full, large and small.
6. The slope of green roof production is recommended to be 0.5%-2.0%, and according to the slope for roof drainage of existing buildings in China, flat roofs are generally less than 5% and sloped roofs are generally more than 10%. Site slope: The five grades of slope of cultivated land classified by the Technical Regulations of Current Land Use Survey are ≤2°, 2°-6°, 6°-15°, 15°-25°, and >25°, respectively.
7. Referring to the modern greenhouse scale of 400-9300 m2 in North America, Europe and East Asia, 500 m2 is suggested as the minimum greenhouse area for economic reasons, while the scale of a citizen farm unit is 15-100 m2.
Secondly, after the construction of the production suitability assessment system for rooftop and idle land was completed, further inventory and site selection studies were carried out. It mainly consisted of three steps. 1) Acquisition phase. High-definition remote sensing map collection, open data crawling to obtain the city's basic geographic information, and GIS were used to establish a database of rooftop and idle land attributes. 2) Screening phase. In ArcMap software, the initial inventory of rooftops and idle land obtained from the inventory was overlaid with the screening attribute factor data layer for analysis. 3) Evaluation phase. This used the weighted sum analysis tool to perform the calculations of the raster overlay data with the following formula:
$K=\sum\limits_{i=1}^{n}{{{x}_{i}}\times {{a}_{i}}}$
where, K is the total score of each raster cell; i is the i-th evaluation factor; xi is the score of the i-th evaluation factor; and ai is the weight of the i-th evaluation factor.
Finally, this study considers the example of Xuefu Street in Nankai District, Tianjin. Nankai District has a total area of 40.64 km2, is located in the southwest of the downtown area, and has 12 streets under its jurisdiction, of which Xuefu Street has an area of 4.7 km2. The boundary of this street is the main city road, which is equivalent to four “fifteen-minute living circles”, and the scale is suitable for calculation as an urban green productive area. Most of the existing buildings in the area were reconstructed after the 1976 Tangshan Earthquake, and there are abundant building types that can provide a comprehensive assessment environment.
The base dataset of the area was obtained from Google HD image maps downloaded from Local Space Viewer software, and vector data (buildings, roads and parcel outlines) were downloaded from Arceyes software in 2018. Based on the software screening and overlay analysis of the GIS platform, the available areas and distribution locations of rooftops and idle land in the area to be evaluated were obtained by visual interpretation combined with field survey corrections. The results show that there are 260 suitable rooftops (244135 m2) in Xuefu Street, accounting for 24.4% of the total rooftop area (1002129 m2), and the total area of available idle land is 27818 m2. Then, based on the scores calculated by the above formula (Eq. 1), the appropriate types of green production compensation strategies were selected from those proposed above (Table 4), and the distribution of short-, medium- and long-term and unsuitable spaces for the development of green production in the region was mapped (Fig. 4).
Table 4 Technical selection of suitable rooftops and idle land in Xuefu Street
Space Short term development Medium and long term development
Roof 41258 m2 (Open-air agriculture, S1) 102673 m2 (Hydroponic greenhouse, S2)
31991 m2 (Photovoltaic power generation, S3） 68212 m2 (Composite mode, S4)
Idle land 17031 m2 (Open-air agriculture S5)
10787 m2 (Composite mode S6)

Note: Idle land does not need to distinguish between short term and medium and long term development.

Fig. 4 Distribution of short, medium and long term and unsuitable spaces for the development of green production in Xuefu Street in Nankai District, Tianjin.

3.3 Quantifying ecological land-saving benefits

First, the production potentials of three cases of agriculture, photovoltaic, and composite model were calculated. The calculations in this paper used the following values:
(1) Urban agriculture with an annual yield of 6 kg m-2 (open-air) and 50 kg m-2 (hydroponic greenhouse) of vegetables (Zhao, 2018).
(2) Photovoltaic power generation was based on polysilicon PV modules with a conversion efficiency of 18.5%, with a 30° inclination and full coverage (Gong et al., 2018).
(3) The greenhouse in the composite mode with a light- transmitting photovoltaic film roof was installed with modules which have a conversion efficiency of 10% with a 30° angle and 25% coverage, with a staggered spacing between the upper and lower rows, in order to take into account both planting needs and power generation efficiency (Wei, 2015).
Various green production scenarios were available for both flat roofs and most idle land, except for sloped roofs which were only suitable for PV generation. The results were then estimated using PVsyst software and the production equations shown in Table 5, where, Sua and Spv are the actual production areas; S is the available area for each production type in Table 3; an is the production per unit area of urban agriculture; and bn is the PV module generation per unit area.
Table 5 Methodology and results for estimating green production potential
Green production type Formula for yield estimation of the actual production area Annual production
Agriculture Open-air agriculture ${{S}_{ua}}=S\times {{a}_{1}}$ (S1+S5)×6=349734 kg
S2×50=5133650 kg
Hydroponic greenhouse
Photovoltaics Flat roof and idle land
(Double pitched roof × 0.5)		 ${{S}_{pv}}=\frac{S}{\text{cos}30{}^\circ }\times {{b}_{1}}\times 18.5%$ 10.05×106 kWh
Composite mode Hydroponic greenhouse
Thin film photovoltaics		 ${{S}_{ua}}=S\times {{a}_{2}}$
${{S}_{pv}}=\frac{S}{\text{cos}30{}^\circ }\times 25%\times {{b}_{2}}\times 10%$		 3949950 kg
10.03×106 kWh

Note: where, Sua and Spv are the actual production areas; S is the available area for each production type in Table 3; an is the production per unit area of urban agriculture; and bn is the PV module generation per unit area.

Next, an accounting standard based on the ecological carrying capacity calculation model was developed. Taking the Xuefu Street (470 ha) as an example of the ecological carrying capacity improvement potential of an urban area, we calculated the amount of electricity and crop yield per ha as the ecological compensation increment of the green productive area, and obtained the equilibrium factor of the green productive area by converting the crop yield and fossil energy provided by the natural land with urban agriculture and photovoltaic power generation.
Finally, a uniform conversion was performed in terms of urban ecological land-saving benefits (Table 6). This conversion was based on the fact that since most of the current forms of green resource income in cities (food, energy, water) are generated by urban agriculture and solar systems, it is possible to estimate the ecological carrying capacity under the same conditions that would be saved by such sustainable production. It relates multiple and complex energy flows to green production and translates them into a very easily understood Green Productive Area (GPA). The basic formula for its calculation is as follows.
$GPA=N\times \sum\limits_{i,j=1}^{n}{{{r}_{j}}\times \frac{{{P}_{i}}}{{{P}_{j}}}}$
Table 6 Calculation of the green productive area equalization factors and land saving benefits
Type Yield per ha Conversion formula Equilibrium factor (rj) Land saving benefit (m2)
Open-air agriculture 744 kg ${{r}_{c}}=\frac{P}{35730}\times 2.19$
${{r}_{f}}=\frac{P\times 0.66}{360000}\times 1.38$		 0.05 (S1+S5)×0.05=2914.45
Hydroponic greenhouse 10923 kg 0.67 S2×0.67=68790.91
PV power generation 21383 kWh 0.05 S3×0.05=1599.55
Composite mode 8404 kg
21340 kWh		 0.52
0.04		 (S4+S6)×0.56=44239.44
Total - - 1.33 117544.35

Note: In the table, P is the yield per ha; and rc and rf are the equilibrium factor formulae for converting green production areas to cropland and forest land, respectively. Agricultural output (kg) uses the rc formula, and photovoltaic power generation (kWh) uses the rf formula. S1-S6 are the same as S1-S6 in Table 4.

where GPA is the total green productive area; N is the total buildable space area; rj is the equilibrium factor; Pj is the yield of traditional land types; Pi is the yield of the i-th green productive type; i is the green productive type; and j is the buildable space type.
According to the literature (Editorial Committee of China Agricultural Yearbook, 2018; Lin et al., 2019), the annual production of vegetables per unit area in China is 35730 kg ha-1, the forest absorbs 360×103 kg ha-1 yr-1 of CO2, power generation emits approximately 0.66 kg kWh-1 of CO2, and the equilibrium factors are 2.19 and 1.38 for common cropland and forest land.
According to the calculation results, the total green productive area that can be provided by adopting the above green production types in Xuefu Street, Nankai District, Tianjin is 117544.35 m2, which is equivalent to the ecological carrying capacity that can be provided by replenishing 12% of the natural land occupied by the buildings in the district. The ecological land saving benefits for the Tianjin urban area can be estimated by calculating the total buildable area of available roofs and idle land (N) multiplied by the overall green production equilibrium factor (1.33) to estimate the green productive area (GPA) that can be provided in this area. In the table, rc and rf are the equilibrium factor formulae for converting green production areas to cropland and forest land, respectively. In addition, although using the Xuefu Street community as the base unit of the city has the property of approximate expansion, the accuracy of the land-saving benefits calculation is limited by factors such as urban spatial data, the scale of living neighborhoods and the overall appearance of buildings. Therefore, when expanding to a larger area or other regions, the equilibrium factor would need to be corrected according to the local suitable green production types, rooftops and idle land, so as to obtain a more accurate quantification of the ecological compensation.

4 Conclusions

Since the Industrial Revolution, cities have been overloaded resource-consuming machines (UN-DESA, Population Division, 2014), and fossil fuel-driven consumer cities urgently need to be transformed into productive ones. This study breaks the limitations of the solidified nature of the original urban land use and re-evaluates the potential of converting the current built environment into ecologically productive land. Based on the green productive city concept of “open source and cut costs”, we studied the theories, strategies and methods of urban agriculture, renewable energy and other green production in the future development of urban space for ecological and land-saving benefits. This paper shows that the existing city can be regarded as a brownfield site to be developed, and the ecological potential within the city can be tapped under the premise of zero growth of urban land. Through the spatial integration of green production and urban systems, we can effectively increase the area of ecologically productive land, thus improving the ecological carrying capacity of cities, realizing the sustainable development of complementary production space, reducing energy consumption and improving the ecological environment.
This paper innovatively proposes an ecological compensation mechanism based on the green productive area of cities, aiming to form a set of quantitative methods and operational procedures to quantify the potential ecological carrying capacity of the built environment, based on the transfer of resource pressure by economic means in the past. Based on the cost, technical feasibility, and ecological service value of urban green production, we adopt appropriate approaches such as urban agriculture, urban solar energy, and combined agricultural and solar energy production. The aim is to provide direct compensation for the ecological carrying capacity of urban ecosystems, providing theoretical and technical support for the development of green production industries, diversification of compensation methods, and evaluation of ecological benefits, which is of great significance for the sustainable development of cities and the improvement of ecological carrying capacity.
In addition, there are some shortcomings in this paper that can be improved in subsequent studies. For example, in constructing a system for assessing the green production potential of rooftops and idle land, the quality and quantity of the urban sample, as well as the subjective perceptions and judgments of individuals, would influence the weighting of indicators and the evaluation results, thus influencing the selection of green production technologies. In addition, only the positive benefits of agricultural cultivation and photovoltaic power generation were considered in the ecological land-saving benefits, and subsequent studies will need to add the full life-cycle cost, and environmental and social benefits. The study of ecological compensation mechanisms also requires more accurate information on the built environment, richer types of green production, and a more comprehensive eco-efficiency analysis.

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