Midstream: Integration of Hybrid Renewable Energy Systems in Oil and Gas Transportation
Midstream transportation uses less energy than upstream or downstream operations and correspondingly has the smallest potential for renewable integration . Furthermore, oil transportation by ship, rail, and truck currently has limited opportunities for renewable technologies. That being said, there are still several opportunities for renewable technologies to have a meaningful impact on operation costs and emissions.
A vast network of pipelines transports oil and gas from producers to refineries and end-users. In the United States alone, there are more than 300,000 miles of mainline natural gas pipelines with 1,400 compressor stations  and approximately 55,000 miles of crude oil trunk lines and 95,000 miles of refined products pipelines, which combined transport more than 40 million barrels of liquids per day . Figure 8.5 displays the U.S. natural gas transportation network along with major fields. As can be seen, pipelines can transport supply significant distances from production areas to major demand hubs.
FIGURE 8.5 Natural gas transportation system. (Figure produced using data from Refs. [7,67].
Renewable technologies are already integrated into some pipeline operations, with solar energy powering sensors and providing cathodic corrosion protection (a cathodic protection system uses charged anodes attached to the pipe to prevent the pipe from corroding) . However, as renewable costs decline and the technologies improve, additional opportunities will be available. Several possibilities in this regard as analyzed by Ericson et al.  is presented below.
Compressor Electrification, Heat Recovery, and Use of Turbo Expanders
As most oil pumping stations use electric motors for the prime mover, these systems are automatically using more renewable power as the grid greens . Natural gas pipelines, however, are primarily powered by gas engines or turbines . Replacing gas engines and turbines with electric motors could increase renewable integration and reduce noise, fuel use, and emissions. Electric motors have low operating costs and more efficiently accommodate a wide throughput range than gas engines or turbines . As natural gas plants are ramped to account for increased variable generation, the variability of natural gas demand will increase, meaning pipelines will likely deal with larger swings in throughput. According to Ericson et al. , this trend supports the use of more electric motors to power natural gas compressor stations. An important barrier to using electric motors to power compressor stations is a concern of reliability. While electric motors are highly reliable, the electric grid does not always meet the operation standards of the O&G industry, especially in remote areas where compressor stations are often located . Work on increasing grid reliability, reducing the price of microgrid technologies, and rewarding lower emissions from electric motors could help motivate a shift toward using more electric motors .
Gas turbines used to power natural gas compressor stations generate a considerable amount of heat. In some cases, this heat can be reused to generate power or used in industrial processes. There are currently 12 power generation systems, totaling 64 MW of electricity capacity, installed at natural gas compressor stations in the United States . An estimated 40 MW worth of potential additional projects currently offer expected paybacks of less than 5 years . However, changes in market conditions could significantly increase the economic opportunity for compression station heat recovery. Estimates of the technical potential for electricity generated from compressor station heat recovery range from 500  to 1.1 GW . As economies of scale are present, large systems with high utilization rates are most applicable . A disadvantage of compressor cogeneration is the variability of output. Compressor stations do not always run and therefore w'ould not always generate heat or electricity . If extracted heat is used for industrial processes, then an associated process must be able to operate w'ith variable heat. Similarly, if electricity generated provides off-grid pow'er, then the consumer must be able to handle variable generation .
Compressors raise the gas pressure between 500 and 1,400 pounds per square inch (psi) to transport natural gas long distances. While higher pressures increase the economics of transportation, and w'hile power plants and some industrial customers use high pressure gas, most customers require pressures well under 10 psi . The potential energy created from raising pipeline pressure is lost when the pressure is stepped dowrn at distribution hubs. A turboexpander can capture this potential energy to generate electricity. Turboexpanders are used in a variety of industry applications. Unfortunately, they are generally considered uneconomic for use in natural gas pipelines under current market conditions [17,69]. Primary challenges include the required cost of preheating the gas before expansion and pipeline operations causing variability in output . Turboexpanders have the benefits of being a proven technology that emits significantly less criteria and other gasses per megawatt-hour generated compared to coal and gas pow'er plants. Ericson et al.  indicates that the turboexpanders may therefore see economic opportunities in regions that price or regulate emissions.