4.3.1 Power generation potential

Publically available commercial data was used to estimate all generating capabilities for gas turbines, steam turbines, and wind turbines as follows: two 7.9 kW gas turbines operating at 30.6% efficiency [47] driving a single 750 kW gas turbine [48] raising the total efficiency to 50.2% and 1 kW wind turbines [49]. All power generation was calculated on an hourly basis and balanced with the hourly load as much as possible. Renewable energy sources were given precedence. Annual average daily data for wind speed at 50 meters aboveground [50], sunrise and sunset [51], and average solar irradiance in kWh/m<sup>2</sup> [50] were obtained for each city of interest. As the wind speed and irradiance data were daily averages, they were applied for all 24 hr in each given day. Sunrise and sunset data were used to "turn on" and "turn off" the solar component of the system.

There are multiple methods for determining the efficiency of trigeneration systems [52–55]. For this study, general estimates based upon these methods will be used. The fast-start capability of modern turbines was utilized to estimate cogeneration outputs with one gas turbine operating at all times. If hourly load minus available renewables exceeded the capacity of one gas turbine, the second turbine was started. If the hourly load still exceeded the capacity of both gas turbines, the steam turbine was included. Solar generation was calculated on an hourly basis by multiplying the irradiance by the total panel area (total roof area) at a 15% conversion efficiency and a 75% transmission efficiency. For wind energy, the manufacturer's power generation curve was used [49]. The power curve, with a cut-in at 6 miles per hour of wind speed, was applied to the hourly average wind speed to determine the kW delivered by the posited 295 turbines. Usable waste heat from cogeneration (as well as input fuel needs) was calculated on an hourly basis. Input energy in kW was calculated as hourly output divided by hourly efficiency of the cogeneration set, 30.6% for gas-only generation and 50.2% for combined generation. Gross waste heat was obtained by subtracting this number from generated power (Eq. 1):

ð Þ kWh=hr <sup>=</sup>Efficiency � ð Þ kWh=hr <sup>=</sup>0:0002931 BTU=kWh <sup>¼</sup> Gross BTU (1)

Usable waste heat was calculated by obtaining the ideal thermodynamic efficiency of the system (Eq. 2) [56] and multiplying this by the results of Eq. 1 (Eq. 3): Micro-Grids - Applications, Operation, Control and Protection

$$(\mathbf{T\_{high}} - \mathbf{T\_{low}})/\mathbf{T\_{high}} = \mathbf{0.61} \tag{2}$$

<sup>1</sup>:<sup>81</sup> � <sup>10</sup><sup>8</sup> kWh Annual Electrical=:<sup>272</sup>

This number was then multiplied by 0.17 (17%) to provide annual hot water

4.4 � <sup>10</sup><sup>7</sup> BTU/hr. for hot water (Eq. 6). This was applied to every hour of the year

<sup>2</sup>:<sup>27</sup> � <sup>10</sup><sup>12</sup> BTU=year � <sup>0</sup>:<sup>17</sup> <sup>=</sup>8760 hr:=year <sup>¼</sup> <sup>4</sup>:<sup>4</sup> � <sup>10</sup><sup>7</sup> BTU=hr:hot water

Hourly structural heating power was calculated at 0.558 (55.8%) of total energy divided by actual hours of heat usage in New York. This number of 2.96 � 108 BTU/

<sup>2</sup>:<sup>27</sup> � <sup>10</sup><sup>12</sup> BTU=year � <sup>0</sup>:<sup>558</sup> <sup>¼</sup> <sup>1</sup>:<sup>27</sup> � 1012 BTU=year building heat (7) <sup>1</sup>:<sup>27</sup> � <sup>10</sup><sup>12</sup> BTU=year<sup>=</sup> 178 days heat=year � 24 hr=day <sup>¼</sup> <sup>2</sup>:<sup>96</sup> � 108 BTU=hour

Usable hourly waste heat (Eq. 3) was initially applied as needed for climate control. Hourly heat transfer needs were calculated at 80% of available waste heat for building heating, with 80% efficiency being the average efficiency of a standard heat exchanger [59]. Hourly heat transfer needs were calculated at 120% of available waste heat for air-conditioning needs, with 120% being the average efficiency of a two-stage absorption chiller [60]. Remaining heat, on an hourly basis, was then

Infrastructure impact is defined by the degree that the implementation of an integrated development model would relieve strain on the local power distribution grid. As can be seen in Table 7, this is highly correlated to air-conditioning needs as

trigeneration plant to the absorption chillers replaces electrical load for air conditioning. Mumbai and Lagos, cities which essentially require air conditioning yearround, had the highest reduction in load, while London, which essentially requires

The microgrid must be grid-connected for both safety and regulatory reasons in an urban environment. To be effective, the system must not add additional load to the grid but must also be balanced in order to protect the local grid infrastructure; it should not push power onto the distribution grid at any point. In case of emergency, such as a blackout condition, the microgrid should also be able to disconnect from the local power grid and provide all needed services in island mode. Table 7 indicates that the proposed model succeeds in this respect. The incorporated power generating systems produce surplus electricity on an hourly basis between 40% and 60% of the time, depending upon the city ("% hours off-grid"). Excess energy produced in hours of low load can be stored in incorporated batteries to meet demand in hours of high load, producing a system that is completely grid-neutral

shown in Table 6. This is the expected result as heat provided from the

throughout the year. The annual difference between electricity usage and

usage and divided by 8760 hr/year to arrive at an hourly hot water usage of

hour was then applied to each city for each hour (Eqs. 7–8):

Microgrids: Applications, Solutions, Case Studies, and Demonstrations

DOI: http://dx.doi.org/10.5772/intechopen.83560

applied to provide hot water, again at 80% efficiency.

no air conditioning, saw no reduction in load.

in all cities:

5. Results

11

5.1 Infrastructure impact

�3412 BTU=kWh <sup>¼</sup> <sup>2</sup>:<sup>27</sup> � <sup>10</sup><sup>12</sup> BTU=year

(5)

(6)

(8)

$$\text{BTU/hour} \left(\text{gross}\right) \times 0.61 \tag{3}$$

The annual volume of natural gas (NG) required to run the cogeneration system, measured in industry standard cubic feet (ft<sup>3</sup> ), was calculated by summing the hourly energy input, (kWh/hr) divided by hourly efficiency of cogeneration and converted to cubic feet of gas (Eq. 4):

$$\Sigma\left(\left(\text{(kWh/hr)/Efficiency}\right) \times \left(\text{3412 Btu/kWh}\right) \times \left(\text{1 ft}^3/\text{103.7 BTU}\right)\right) \tag{4}$$

#### 4.3.2 Estimated usage

Building performance in terms of ENERGY STAR rating (EGR) was not modified. Usage was normalized to environmental conditions in each of the given cities as follows. Daily average high and low temperatures for obtained for each city [57]. Hourly temperatures were calculated using a linear regression each day of the year in each city, starting from the daily low for the day at 1:00 a.m. up to the daily high for the day at 12:00 noon and going back to the daily low again at 12:00 midnight. It was assumed that, on any given day, air conditioning (AC) would be required at or above a daily high of 80°F, and heat would be needed at a daily low of 50°F (Table 6).


#### Table 6.

Annual climate control needs for cities of interest.

In order to estimate the base electric usage for the proposed development, the hourly base building usage data provided by GridMarket was increased by 30% for each day for each city that air conditioning was assumed to be needed. (Given that the New York City data is actual usage, days that the data indicated that air conditioning would be needed in both New York and any other given city were not modified.) This provided a reference point for the estimated electrical load for the proposed development being connected to the regional/national power grid. Since inclusion of the microgrid would essentially eliminate electricity usage for air conditioning, daily usage data for the development with an included microgrid was reduced by 30% assuming that air conditioning increases daily load by 30% (Table 6).

Hourly heat usage for both hot water and building heat was assumed to remain constant across all cites and climates since the heat capacity of water is constant and the amount of hot water required on a daily basis would be independent of location or climate. Also, as the configuration (and hence the volume) of the buildings was identical in all cities, and the need for heating is temperature dependent, the amount of heat required to provide building heat on an hourly basis would also be constant. Hourly heat requirements for both needs were therefore calculated based upon the New York City data and applied to all cities. The total annual energy use breakdown for New York is available from the United States Energy Information Agency as follows: electricity, 27.2%; heating, 55.8%; and hot water, 17.0% [58].

Total actual electrical usage for the four-tower development was converted to BTUs and divided by 0.272 (27.2%) to give total energy usage (Eq. 5):

Microgrids: Applications, Solutions, Case Studies, and Demonstrations DOI: http://dx.doi.org/10.5772/intechopen.83560

$$\begin{aligned} \text{(1.81 \times 10^8 kWh Annual Electrical/.272)}\\ \times \text{3412 BTU/kWh} = 2.27 \times 10^{12} \text{ BTU/year} \end{aligned} \tag{5}$$

This number was then multiplied by 0.17 (17%) to provide annual hot water usage and divided by 8760 hr/year to arrive at an hourly hot water usage of 4.4 � <sup>10</sup><sup>7</sup> BTU/hr. for hot water (Eq. 6). This was applied to every hour of the year in all cities:

$$(2.27 \times 10^{12} \text{ (BTU/year)} \times 0.17) / 8760 \text{ hr./year} = 4.4 \times 10^7 \text{ BTU/hr.hot} \text{./hr} \cdot \text{water} \tag{6}$$

Hourly structural heating power was calculated at 0.558 (55.8%) of total energy divided by actual hours of heat usage in New York. This number of 2.96 � 108 BTU/ hour was then applied to each city for each hour (Eqs. 7–8):

$$\left(2.27 \times 10^{12} \,\mathrm{(BTU/year)} \times 0.558\right) = 1.27 \times 10^{12} \,\mathrm{BTU/year} \,\mathrm{building heat} \tag{7}$$

$$1.27 \times 10^{12} \,\mathrm{BTU/year} / \text{(178 days heat/year} \times 24 \,\mathrm{hr/day)} = 2.96 \times 10^{8} \,\mathrm{BTU/hour} \tag{8}$$

Usable hourly waste heat (Eq. 3) was initially applied as needed for climate control. Hourly heat transfer needs were calculated at 80% of available waste heat for building heating, with 80% efficiency being the average efficiency of a standard heat exchanger [59]. Hourly heat transfer needs were calculated at 120% of available waste heat for air-conditioning needs, with 120% being the average efficiency of a two-stage absorption chiller [60]. Remaining heat, on an hourly basis, was then applied to provide hot water, again at 80% efficiency.
